Effectiveness of long-lasting insecticidal nets with pyriproxyfen-pyrethroid, chlorfenapyr-pyrethroid, or piperonyl butoxide-pyrethroid versus pyrethroid only against malaria in Tanzania: final-year results of a four-arm, single-blind, cluster-randomised trial

BACKGROUND

New classes of long-lasting insecticidal nets (LLINs) containing two active ingredients have been recently recommended by WHO in areas where malaria vectors are resistant to pyrethroids. This policy was based on evidence generated by the first 2 years of our recently published trial in Tanzania. In this Article, we report the final third-year trial findings, which are necessary for assessing the long-term effectiveness of new classes of LLIN in the community and the replacement intervals required.

METHODS

A third year of follow-up of a four-arm, single-blind, cluster-randomised controlled trial of dual active ingredient LLINs was conducted between July 14, 2021, and Feb 10, 2022, in Misungwi, Tanzania. Restricted randomisation was used to assign 84 clusters to the four LLIN groups (1:1:1:1) to receive either standard pyrethroid (PY) LLINs (reference), chlorfenapyr-PY LLINs, pyriproxyfen-PY LLINs, or piperonyl butoxide (PBO)-PY LLINs. All households received one LLIN for every two people. Data collection was done in consenting households in the cluster core area with at least one child between 6 months and 15 years of age who permanently resided in the selected household. Exclusion criteria were householders absent during the visit, living in the cluster buffer area, no adult caregiver capable of giving informed consent, or eligible children who were severely ill. Field staff and study participants were masked to allocation, and those analysing data were not. The primary 24-month endpoint was reported previously; here, we present the secondary outcome, malaria infection prevalence in children at 36 months post LLIN distribution, reported in the intention-to-treat analysis. The trial was registered with ClinicalTrials.gov (NCT03554616) and is now complete.

FINDINGS

Overall usage of study nets was 1023 (22·3%) of 4587 people at 36 months post distribution. In the standard PY LLIN group, malaria infection was prevalent in 407 (37·4%) of 1088 participants, compared with 261 (22·8%) of 1145 in the chlorfenapyr-PY LLIN group (odds ratio 0·57, 95% CI 0·38-0·86; p=0·0069), 338 (32·2%) of 1048 in the PBO-PY LLIN group (0·95, 0·64-1·42; p=0·80), and 302 (28·8%) of 1050 in the pyriproxyfen-PY LLIN group (0·82, 0·55-1·23; p=0·34). None of the participants or caregivers reported side-effects.

INTERPRETATION

Despite low coverage, the protective efficacy against malaria offered by chlorfenapyr-PY LLINs was superior to that provided by standard PY LLINs over a 3-year LLIN lifespan. Appropriate LLIN replacement strategies to maintain adequate usage of nets will be necessary to maximise the full potential of these nets.

FUNDING

Department for International Development, UK Medical Research Council, Wellcome Trust, Department of Health and Social Care, and Bill & Melinda Gates Foundation via the Innovative Vector Control Consortium.

Effectiveness of dual active ingredient insecticide-treated nets in preventing malaria: A systematic review and meta-analysis

Malaria vectors have demonstrated resistance to pyrethroid-based insecticides used in insecticide-treated nets, diminishing their effectiveness. This systematic review and meta-analysis investigated two forms of dual active-ingredient (DAI) insecticide-treated nets (ITN(s)) for malaria prevention. A comprehensive search was conducted on July 6th 2022. The databases searched included PubMed, Embase, CINAHL, amongst others. Trials were eligible if they were conducted in a region with ongoing malaria transmission. The first DAI ITN investigated were those that combined a pyrethroid with a non-pyrethroid insecticides. The second DAI ITN investigated were that combined a pyrethroid with an insect growth regulator. These interventions were compared against either a pyrethroid-only ITN, or ITNs treated with pyrethroid and piperonyl-butoxide. Assessment of risk of bias was conducted in duplicate using the Cochrane risk of bias 2 tool for cluster-randomised trials. Summary data was extracted using a custom data-extraction instrument. This was conducted by authors THB, JCS and SH. Malaria case incidence was the primary outcome and has been meta-analysed, adverse events were narratively synthesised. The review protocol is registered on PROSPERO (CRD42022333044). From 9494 records, 48 reports were screened and 13 reports for three studies were included. These studies contained data from 186 clusters and all reported a low risk of bias. Compared to pyrethroid-only ITNs, clusters that received pyrethroid-non-pyrethroid DAI ITNs were associated with 305 fewer cases per 1000-person years (from 380 fewer cases to 216 fewer cases) (IRR = 0.55, 95%CI: 0.44-0.68). However, this trend was not observed in clusters that received pyrethroid-insect growth regulator ITNs compared to pyrethroid-only ITNs (from 280 fewer cases to 135 more) (IRR = 0.90, 95%CI: 0.73-1.13). Pyrethroid-non-pyrethroid DAI ITNs demonstrated consistent reductions in malaria case incidence and other outcomes across multiple comparisons. Pyrethroid-non-pyrethroid DAI ITNs may present a novel intervention for the control of malaria.

Muscarinic acetylcholine receptor activation synergizes the knockdown and toxicity of GABA-gated chloride channel insecticides

BACKGROUND

Pest management requires continual identification of new physiological targets and strategies to control pests affecting agriculture and public/animal health. We propose the muscarinic system as a target for agrochemicals because of its physiological importance. Unlike the muscarinic system, gamma-amino butyric acid (GABA) receptors are an established insecticide target. Here, we investigated target-site synergism using small molecule probes (agonist and antagonist) against the muscarinic system and their ability to enhance the toxicity of GABAergic insecticides in Drosophila melanogaster (Meigen).

RESULTS

Oral delivery of pilocarpine (muscarinic agonist) enhanced the toxicity of dieldrin, fipronil, and lindane, resulting in synergist ratios (SRs) between 4-32-fold (orally delivered) or between 2-67-fold when insecticides were topically applied. The synergism between pilocarpine and the GABA-insecticides was greater than the synergism observed with atropine (muscarinic antagonist), and was greater, or comparable, to the synergism observed with the metabolic inhibitor piperonyl butoxide. In addition to lethality, pilocarpine increased the knockdown of lindane. The mechanism of synergism was also investigated in the central nervous system using extracellular electrophysiology, where pilocarpine (3 μmo/L) lowered the half-maximal inhibitory concentration (IC₅₀ ) of lindane from 1.3 (0.86-1.98) μmol/L to 0.17 (0.14-0.21) μmol/L and fipronil's IC₅₀ from 2.2 (1.54-3.29) μmol/L to 0.56 (0.40-0.77) μmol/L.

CONCLUSION

Convergence of the cellular function between the muscarinic and GABAergic systems enhanced the insecticidal activity of GABA receptor blocking insecticides through the modulation of the central nervous system (CNS). The future impact of the findings could be the reduction of the active ingredient needed in a formulation with the development of muscarinic synergists. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Understanding the Intransigence of Malaria in Malawi

Despite the scale-up of interventions against malaria over the past decade, this disease remains a leading threat to health in Malawi. To evaluate the epidemiology of both Plasmodium falciparum infection and malaria disease, the Malawi International Center of Excellence for Malaria Research (ICEMR) has developed and implemented diverse and robust surveillance and research projects. Descriptive studies in ICEMR Phase 1 increased our understanding of the declining effectiveness of long-lasting insecticidal nets (LLINs), the role of school-age children in malaria parasite transmission, and the complexity of host-parasite interactions leading to disease. These findings informed the design of ICEMR Phase 2 to test hypotheses about LLIN use and effectiveness, vector resistance to insecticides, demographic targets of malaria control, patterns and causes of asymptomatic to life-threatening disease, and the impacts of RTS,S vaccination plus piperonyl butoxide-treated LLINs on infection and disease in young children. These investigations are helping us to understand mosquito-to-human and human-to-mosquito transmission in the context of Malawi's intransigent malaria problem.

Advances and future prospects of pyrethroids: Toxicity and microbial degradation

Pyrethroids are a class of insecticides structurally similar to that of natural pyrethrins. The application of pyrethrins in agriculture and pest control lead to many kinds of environmental pollution affecting human health and loss of soil microbial population that affect soil fertility and health. Natural pyrethrins have been used since ancient times as insect repellers, and their synthetic versions especially type 2 pyrethroids could be highly toxic to humans. PBO (Piperonyl butoxide) is known to enhance the toxicity of prallethrin in humans due to the resistance in its metabolic degradation. Pyrethroids are also known to cause plasma biochemical profile changes in humans and they also lead to the production of high levels of reactive oxygen species. Further they are also known to increase SGPT activity in humans. Due to the toxicity of pyrethrins in water bodies, soils, and food products, there is an urgent need to develop sustainable approaches to reduce their levels in the respective fields, which are eco-friendly, economically viable, and socially acceptable for on-site remediation. Keeping this in view, an attempt has been made to analyse the advances and prospects in using pyrethrins and possible technologies to control their harmful effects. The pyrethroid types, composition and biochemistry of necessary pyrethroid insecticides have been discussed in detail, in the research paper, along with their effect on insects and humans. It also covers the impact of pyrethroids on different plants and soil microbial flora. The second part deals with the microbial degradation of the pyrethroids through different modes, i.e., bioaugmentation and biostimulation. Many microbes such as Acremonium, Aspergillus, Microsphaeropsis, Westerdykella, Pseudomonas, Staphylococcus have been used in the individual form for the degradation of pyrethroids, while some of them such as Bacillus are even used in the form of consortia.

Evaluation of pesticide residues in commercial Swiss beeswax collected in 2019 using ultra-high performance liquid chromatographic analysis

The aim of this study was to determine residue levels of pesticides in Swiss commercial beeswax. Foundation samples were collected in 2019 from nine commercial manufacturers for analysis of 21 pesticides using ultra-high performance liquid chromatography. Individual samples showed the variability and residue ranges and pooled samples represented the average annual residue values of the Swiss production. In total, 17 pesticides were identified and 13 pesticides were quantified. They included 13 acaricides and/or insecticides, two fungicides as well as a synergist and a repellent. The means calculated from individual samples were similar to the average annual residue values for most tested pesticides. Mean values of 401, 236, 106 and 3 μg·kg^-1 were obtained for the beekeeping-associated contaminants coumaphos, tau-fluvalinate, bromopropylate and N-(2,4-Dimethylphenyl)-formamide (DMF; breakdown product of amitraz), respectively. For the other pesticides, the mean values were 203 μg·kg^-1 (synergist piperonyl butoxide), 120 μg·kg^-1 (repellent N,N-Diethyl-3-methylbenzamide, DEET), 19 μg·kg^-1 (chlorfenvinphos) and 4 μg·kg^-1 ((E)-fenpyroximate), while the means for acrinathrin, azoxystrobin, bendiocarb, boscalid, chlorpyrifos, flumethrin, permethrin, propoxur and thiacloprid were below the limit of quantification (< LOQ). Individual samples contained from seven to 14 pesticides. The ranges of values for coumaphos and piperonyl butoxide (from 14 to 4270 μg·kg^-1; from 6 to 1555 μg·kg^-1, respectively) were larger as compared to the ranges of values for DEET and tau-fluvalinate (from < LOQ to 585 μg·kg^-1; from 16 to 572 μg·kg^-1, respectively). In conclusion, the most prominent contaminants were the pesticides coumaphos and tau-fluvalinate, which are both acaricides with previous authorization for beekeeping in Switzerland, followed by piperonyl butoxide, a synergist to enhance the effect of insecticides.

Optimising the deployment of vector control tools against malaria: a data-informed modelling study

BACKGROUND

Concern that insecticide resistant mosquitoes are threatening malaria control has driven the development of new types of insecticide treated nets (ITNs) and indoor residual spraying (IRS) of insecticide. Malaria control programmes have a choice of vector control interventions although it is unclear which controls should be used to combat the disease. The study aimed at producing a framework to easily compare the public health impact and cost-effectiveness of different malaria prevention measures currently in widespread use.

METHODS

We used published data from experimental hut trials conducted across Africa to characterise the entomological effect of pyrethroid-only ITNs versus ITNs combining a pyrethroid insecticide with the synergist piperonyl butoxide (PBO). We use these estimates to parameterise a dynamic mathematical model of Plasmodium falciparum malaria which is validated for two sites by comparing simulated results to empirical data from randomised control trials (RCTs) in Tanzania and Uganda. We extrapolated model simulations for a series of potential scenarios likely across the sub-Saharan African region and include results in an online tool (Malaria INtervention Tool [MINT]) that aims to identify optimum vector control intervention packages for scenarios with varying budget, price, entomological and epidemiological factors.

FINDINGS

Our model indicates that switching from pyrethroid-only to pyrethroid-PBO ITNs could averted up to twice as many cases, although the additional benefit is highly variable and depends on the setting conditions. We project that annual delivery of long-lasting, non-pyrethroid IRS would prevent substantially more cases over 3-years, while pyrethroid-PBO ITNs tend to be the most cost-effective intervention per case averted. The model was able to predict prevalence and efficacy against prevalence in both RCTs for the intervention types tested. MINT is applicable to regions of sub-Saharan Africa with endemic malaria and provides users with a method of designing intervention packages given their setting and budget.

INTERPRETATION

The most cost-effective vector control package will vary locally. Models able to recreate results of RCTs can be used to extrapolate outcomes elsewhere to support evidence-based decision making for investment in vector control.

FUNDING

Medical Research Council, IVCC, Wellcome Trust.

TRANSLATION

For the French translation of the abstract see Supplementary Materials section.

Multiple Mechanisms Conferring Broad-Spectrum Insecticide Resistance in the Tropical Bed Bug (Hemiptera: Cimicidae)

The modern resurgence of the common (Cimex lectularius L.) and tropical bed bugs (C. hemipterus [F.]) is thought to be primarily due to insecticide resistance. While there are many reports on insecticide resistance mechanisms in C. lectularius, such information in C. hemipterus is limited. We examined dichloro-diphenyl-trichloroethane (DDT), malathion, deltamethrin, permethrin, lambda-cyhalothrin resistance, and the underlying mechanisms in several C. hemipterus strains (Australia: Queensland [QLD-AU]; Malaysia: Kuala Lumpur [KL-MY], Tanjung Tokong [TT-MY], Christian [CH-MY], and Green Lane [GL-MY]). We used a surface contact method, synergism studies (utilizing piperonyl butoxide [PBO], S,S,S-tributyl phosphorotrithioate [DEF], and diethyl maleate [DEM]), and molecular detection of kdr mutations. Results demonstrated that all C. hemipterus strains possessed high resistance to DDT and the pyrethroids and moderate to high resistance to malathion. Synergism studies showed that deltamethrin resistance in all strains was significantly (P < 0.05) inhibited by PBO. In contrast, deltamethrin resistance was not affected in DEF or DEM. Similar findings were found with lambda-cyhalothrin resistance. Malathion resistance was significantly (P < 0.05) reduced by DEF in all strains. Resistance to DDT was not affected by DEM in all strains. Multiple kdr mutations (M918I, D953G, and L1014F) were detected by molecular analyses. TT-MY strain was found with individuals possessing three kdr mutation combinations; D953G + L1014F (homozygous susceptible: M918), M918I + D953G + L1014F (heterozygous resistant: I918), and M918I + D953G + L1014F (homozygous resistant: I918). Individuals with M918I + D953G + L1014F (homozygous resistant: I918) survived longer on deltamethrin (>12 h) than those (≤1 h) with other combinations. M918I + L1014F mutations most likely conferred super-kdr characteristic toward pyrethroids and DDT in C. hemipterus.

Evidence supporting deployment of next generation insecticide treated nets in Burkina Faso: bioassays with either chlorfenapyr or piperonyl butoxide increase mortality of pyrethroid-resistant Anopheles gambiae

BACKGROUND

Pyrethroid resistance poses a major threat to the efficacy of insecticide-treated nets (ITNs) in Burkina Faso and throughout sub-Saharan Africa, particularly where resistance is present at high intensity. For such areas, there are alternative ITNs available, including the synergist piperonyl butoxide (PBO)-based ITNs and dual active ingredient ITNs such as Interceptor G2 (treated with chlorfenapyr and alpha-cypermethrin). Before deploying alternative ITNs on a large scale it is crucial to characterize the resistance profiles of primary malaria vector species for evidence-based decision making.

METHODS

Larvae from the predominant vector, Anopheles gambiae sensu lato (s.l.) were collected from 15 sites located throughout Burkina Faso and reared to adults for bioassays to assess insecticide resistance status. Resistance intensity assays were conducted using WHO tube tests to determine the level of resistance to pyrethroids commonly used on ITNs at 1×, 5 × and 10 × times the diagnostic dose. WHO tube tests were also used for PBO synergist bioassays with deltamethrin and permethrin. Bottle bioassays were conducted to determine susceptibility to chlorfenapyr at a dose of 100 µg/bottle.

RESULTS

WHO tube tests revealed high intensity resistance in An. gambiae s.l. to deltamethrin and alpha-cypermethrin in all sites tested. Resistance intensity to permethrin was either moderate or high in 13 sites. PBO pre-exposure followed by deltamethrin restored full susceptibility in one site and partially restored susceptibility in all but one of the remaining sites (often reaching mortality greater than 80%). PBO pre-exposure followed by permethrin partially restored susceptibility in 12 sites. There was no significant increase in permethrin mortality after PBO pre-exposure in Kampti, Karangasso-Vigué or Mangodara; while in Seguenega, Orodara and Bobo-Dioulasso there was a significant increase in mortality, but rates remained below 50%. Susceptibility to chlorfenapyr was confirmed in 14 sites.

CONCLUSION

High pyrethroid resistance intensity in An. gambiae s.l. is widespread across Burkina Faso and may be a predictor of reduced pyrethroid ITN effectiveness. PBO + deltamethrin ITNs would likely provide greater control than pyrethroid nets. However, since susceptibility in bioassays was not restored in most sites following pre-exposure to PBO, Interceptor G2 may be a better long-term solution as susceptibility was recorded to chlorfenapyr in nearly all sites. This study provides evidence supporting the introduction of both Interceptor G2 nets and PBO nets, which were distributed in Burkina Faso in 2019 as part of a mass campaign.

When vector control and organic farming intersect: Pesticide residues on rice plants from aerial mosquito sprays

Conflicts often exist between the use of pesticides for public health protection and organic farming. A prominent example is the use of insecticides for mosquito control in rice fields designated for organic farming. Rice fields, with static water and other conducive conditions, are favorable mosquito habitats. Best management practices are urgently needed to ensure the integrity of organic farming while addressing the need for public health protection. In this study, we evaluated aerial ultra-low-volume (ULV) applications of two classes of mosquito adulticides, pyrethrins and organophosphates, and their deposition and residues on rice plants throughout an active growing season in the Sacramento Valley of California. Frequent applications of pyrethrin synergized with piperonyl butoxide (PBO) and rotating applications of synergized pyrethrins and naled, an organophosphate, were carried out on two large blocks of rice fields. Aerial ULV application of either synergized pyrethrins or naled was able to generate uniform droplets above the fields with high efficacy for mosquito control. Rice leaf samples were collected before and after a subset of applications, and rice grains were sampled at harvest. Frequent applications of synergized pyrethrins resulted in some accumulation of the synergist PBO on rice leaves, but pyrethrins and naled dissipated rapidly from the leaves after each application with no noticeable accumulation over repeated applications. At harvest, no detectable residues of the pesticides or PBO were found in the rice grains. The absence of pesticide residues in the rice grains at harvest suggested that the ULV aerial application led to deposition of only very low levels of residues on rice plants during the growing season. When coupled with the short persistence and/or poor mobility of the insecticides, such applications resulted in negligible pesticide residues in rice grains.

Piperonyl butoxide (PBO) combined with pyrethroids in insecticide-treated nets to prevent malaria in Africa

BACKGROUND

Pyrethroid long-lasting insecticidal nets (LLINs) have been important in the large reductions in malaria cases in Africa, but insecticide resistance in Anopheles mosquitoes threatens their impact. Insecticide synergists may help control insecticide-resistant populations. Piperonyl butoxide (PBO) is such a synergist; it has been incorporated into pyrethroid-LLINs to form pyrethroid-PBO nets, which are currently produced by five LLIN manufacturers and, following a recommendation from the World Health Organization (WHO) in 2017, are being included in distribution campaigns. This review examines epidemiological and entomological evidence on the addition of PBO to pyrethroid nets on their efficacy.

OBJECTIVES

To compare effects of pyrethroid-PBO nets currently in commercial development or on the market with effects of their non-PBO equivalent in relation to: 1. malaria parasite infection (prevalence or incidence); and 2. entomological outcomes.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group (CIDG) Specialized Register, CENTRAL, MEDLINE, Embase, Web of Science, CAB Abstracts, and two clinical trial registers (ClinicalTrials.gov and WHO International Clinical Trials Registry Platform) up to 25 September 2020. We contacted organizations for unpublished data. We checked the reference lists of trials identified by these methods.

SELECTION CRITERIA

We included experimental hut trials, village trials, and randomized controlled trials (RCTs) with mosquitoes from the Anopheles gambiae complex or the Anopheles funestus group.

DATA COLLECTION AND ANALYSIS

Two review authors assessed each trial for eligibility, extracted data, and determined the risk of bias for included trials. We resolved disagreements through discussion with a third review author. We analysed data using Review Manager 5 and assessed the certainty of evidence using the GRADE approach.

MAIN RESULTS

Sixteen trials met the inclusion criteria: 10 experimental hut trials, four village trials, and two cluster-RCTs (cRCTs). Three trials are awaiting classification, and four trials are ongoing.  Two cRCTs examined the effects of pyrethroid-PBO nets on parasite prevalence in people living in areas with highly pyrethroid-resistant mosquitoes (< 30% mosquito mortality in discriminating dose assays). At 21 to 25 months post intervention, parasite prevalence was lower in the intervention arm (odds ratio (OR) 0.79, 95% confidence interval (CI) 0.67 to 0.95; 2 trials, 2 comparisons; moderate-certainty evidence). In highly pyrethroid-resistant areas, unwashed pyrethroid-PBO nets led to higher mosquito mortality compared to unwashed standard-LLINs (risk ratio (RR) 1.84, 95% CI 1.60 to 2.11; 14,620 mosquitoes, 5 trials, 9 comparisons; high-certainty evidence) and lower blood feeding success (RR 0.60, 95% CI 0.50 to 0.71; 14,000 mosquitoes, 4 trials, 8 comparisons; high-certainty evidence). However, in comparisons of washed pyrethroid-PBO nets to washed LLINs, we do not know if PBO nets had a greater effect on mosquito mortality (RR 1.20, 95% CI 0.88 to 1.63; 10,268 mosquitoes, 4 trials, 5 comparisons; very low-certainty evidence), although the washed pyrethroid-PBO nets did decrease blood-feeding success compared to standard-LLINs (RR 0.81, 95% CI 0.72 to 0.92; 9674 mosquitoes, 3 trials, 4 comparisons; high-certainty evidence). In areas where pyrethroid resistance is moderate (31% to 60% mosquito mortality), mosquito mortality was higher with unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs (RR 1.68, 95% CI 1.33 to 2.11; 751 mosquitoes, 2 trials, 3 comparisons; moderate-certainty evidence), but there was little to no difference in effects on blood-feeding success (RR 0.90, 95% CI 0.72 to 1.11; 652 mosquitoes, 2 trials, 3 comparisons; moderate-certainty evidence). For washed pyrethroid-PBO nets compared to washed standard-LLINs, we found little to no evidence for higher mosquito mortality or reduced blood feeding (mortality: RR 1.07, 95% CI 0.74 to 1.54; 329 mosquitoes, 1 trial, 1 comparison, low-certainty evidence; blood feeding success: RR 0.91, 95% CI 0.74 to 1.13; 329 mosquitoes, 1 trial, 1 comparison; low-certainty evidence). In areas where pyrethroid resistance is low (61% to 90% mosquito mortality), studies reported little to no difference in the effects of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.25, 95% CI 0.99 to 1.57; 948 mosquitoes, 2 trials, 3 comparisons; moderate-certainty evidence), and we do not know if there was any effect on blood-feeding success (RR 0.75, 95% CI 0.27 to 2.11; 948 mosquitoes, 2 trials, 3 comparisons; very low-certainty evidence). For washed pyrethroid-PBO nets compared to washed standard-LLINs, we do not know if there was any difference in mosquito mortality (RR 1.39, 95% CI 0.95 to 2.04; 1022 mosquitoes, 2 trials, 3 comparisons; very low-certainty evidence) or on blood feeding (RR 1.07, 95% CI 0.49 to 2.33; 1022 mosquitoes, 2 trials, 3 comparisons; low-certainty evidence). In areas where mosquito populations are susceptible to insecticides (> 90% mosquito mortality), there may be little to no difference in the effects of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.20, 95% CI 0.64 to 2.26; 2791 mosquitoes, 2 trials, 2 comparisons; low-certainty evidence). This is similar for washed nets (RR 1.07, 95% CI 0.92 to 1.25; 2644 mosquitoes, 2 trials, 2 comparisons; low-certainty evidence). We do not know if unwashed pyrethroid-PBO nets had any effect on the blood-feeding success of susceptible mosquitoes (RR 0.52, 95% CI 0.12 to 2.22; 2791 mosquitoes, 2 trials, 2 comparisons; very low-certainty evidence). The same applies to washed nets (RR 1.25, 95% CI 0.82 to 1.91; 2644 mosquitoes, 2 trials, 2 comparisons; low-certainty evidence). In village trials comparing pyrethroid-PBO nets to LLINs, there was no difference in sporozoite rate (4 trials, 5 comparisons) nor in mosquito parity (3 trials, 4 comparisons).

AUTHORS' CONCLUSIONS

In areas of high insecticide resistance, pyrethroid-PBO nets have greater entomological and epidemiological efficacy compared to standard LLINs, with sustained reduction in parasite prevalence, higher mosquito mortality and reduction in mosquito blood feeding rates 21 to 25 months post intervention. Questions remain about the durability of PBO on nets, as the impact of pyrethroid-PBO nets on mosquito mortality was not sustained over 20 washes in experimental hut trials, and epidemiological data on pyrethroid-PBO nets for the full intended three-year life span of the nets is not available. Little evidence is available to support greater entomological efficacy of pyrethroid-PBO nets in areas where mosquitoes show lower levels of resistance to pyrethroids.

Toxicological Evaluation of Novel Butenolide Pesticide Flupyradifurone Against Culex quinquefasciatus (Diptera: Culicidae) Mosquitoes

The impact of increasing resistance of mosquitoes to conventional pesticides has led to investigate various unique tools and pest control strategies. Herein, we assessed the potency of flupyradifurone, a novel pesticide, on fourth instar larvae of Culex quinquefasciatus Say. Further, we evaluated the synergistic action of piperonyl butoxide (PBO) and the octopamine receptor agonists (OR agonists) chlordimeform (CDM) and amitraz (AMZ) on the toxicity of flupyradifurone in comparison with sulfoxaflor and nitenpyram to increase their toxicity on Cx. quinquefasciatus. Results demonstrated that flupyradifurone was the most potent pesticide followed by sulfoxaflor and nitenpyram. Further, the synergetic effect of PBO, CDM, and AMZ was significant for all selected pesticides especially flupyradifurone. However, AMZ had the most significant effect in combination with the selected pesticides followed by CDM and PBO. The toxicity of the pesticides was time-dependent and increased over time from 24, 48, to 72 h of exposure in all experiments. The results indicate that flupyradifurone is a promising component in future mosquito control programs.

Susceptibility to indoxacarb and synergism by enzyme inhibitors in laboratory and field strains of five major stored product insects in Pakistan

Resistance to commonly used grain protectants and fumigants in stored product insect pests necessitates the need to explore alternative substances. Indoxacarb is a reduced-risk oxadiazine insecticide that is generally used in field crops, but there are limited reports of its susceptibility in stored insect pests. The objective of this study was to determine susceptibility to indoxacarb in laboratory and field strains of five major stored product insects: Rhyzopertha dominica (Fabricius), Sitophilus zeamais (Motschulsky), S. oryzae (Linnaeus), Tribolium castaneum (Herbst), and Oryzaephilus surinamensis (L.), using dose-mortality bioassays on wheat grains. In most of the cases, the susceptibility of laboratory strains of all the studied pests were significantly higher than the corresponding field strains. The LD₅₀ and LD₉₉ values (mg a.i./kg of grains) of field strains ranged from 0.13 to 0.38, and 3.44 to 24.76, respectively (for R. dominica), 0.26 to 0.55, and 5.26 to 19.37, respectively (for S. oryzae), 0.41 to 1.01, and 13.11 to 22.46, respectively (for S. zeamais), 0.67 to 1.37, and 15.43 to 43.44, respectively (for T. castaneum), and 0.52 to 0.92, and 18.06 to 61.63, respectively (for O. surinamensis). Synergism bioassays implementing piperonyl butoxide or S,S,S-tributyl phosphorotrithioate along with indoxacarb on selected field strains revealed enhanced susceptibility to indoxacarb. The study demonstrates relative susceptibility to indoxacarb in major stored product insects. Synergism results support the probability of metabolic-based mechanisms responsible for mitigating indoxacarb toxicity. The results might be helpful for monitoring future variation in susceptibility to indoxacarb in the selected insect species and for setting field rates.

Potential of Outdoor Ultra-Low-Volume Aerosol and Thermal Fog to Suppress the Dengue Vector, Aedes aegypti, Inside Dwellings

A field study investigated penetration of outdoor ground ultra-low-volume (ULV) aerosol and thermal fog adulticide applications into a dwelling to control the dengue vector Aedes aegypti (L). Four applications of Kontrol 4-4 (4.6% permethrin active ingredient [AI], 4.6% piperonyl butoxide) at the maximum label rate were made at 25-30 m in front of a house at Camp Blanding Joint Training Center, Starke, FL, during summer 2016. The ULV sprayer and thermal fogger nozzles were oriented horizontally, and vehicle travel speeds were 16 and 24 km/h, respectively. All doors and windows of the house were left open. Spray efficacy was assessed using caged female mosquitoes positioned 30 cm above ground, outside and inside of the house. Interior cages were placed in open areas and cryptic sites (i.e., in a closet or cardboard box). A spinner holding 2 rods sized 3 mm × 75 mm was deployed next to each cage (except cryptic sites) to sample droplets and to quantify AI deposition. Thirty minutes after application, cages were removed, slides collected, and mosquitoes transferred to clean cages in the laboratory where mortality was assessed at 24 h posttreatment. The ULV application to the south side of the house produced 100% mortality in outdoor and indoor cages and 24% mortality at cryptic sites. Similarly applied thermal fog resulted in 85% mortality outdoors, 34% indoors, and only 4% in cages at cryptic sites. Application of either method from the west resulted in 19-61% mortality outdoors and 0.5-6.5% indoors. Droplet volume median diameter (Dv0.5) on rods from the ULV application was significantly larger compared with the thermal fogger outdoors, but similar indoors. Outdoors and indoors, the AI deposition from ULV was significantly higher than from thermal fog. Our results show the potential for controlling dengue vectors inside houses with outdoor ground ULV applications in areas where doors and windows are left open for ventilation.

Insecticide Resistance, and Its Effects on Bait Performance in Field-Collected German Cockroaches (Blattodea: Ectobiidae) From Taiwan

Insecticide resistance in the German cockroach, Blattella germanica (L.), is a significant challenge to the pest management professionals worldwide. We collected 24 field populations of B. germanica from different localities in Taiwan island, reared them for one to two generations, and evaluated them for their resistance to deltamethrin, propoxur, and fipronil using the surface-contact method. Results showed that deltamethrin resistance ratio ranged from 1.5 to 817.5×. Among the strains, TC Supermarket, TC Sanshang Logistics, TC THSR, and TC 1Taichungsteak strains showed very high resistance to deltamethrin, which mortality ranged between 0 and 33% at 7-d post-treatment. On the other hand, resistance to propoxur and fipronil RR were 0.70-7.13× and 1.67-3.72×, respectively. Synergism studies using piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) suggested the major involvement of cytochrome P450 monooxygenase and minor involvement of esterases. However, deltamethrin resistance in two strains (i.e., TC Supermarket and TC THSR) was not affected by both PBO and DEF, indicating that other mechanisms are involved in the resistance, including kdr resistance. Evaluation of the field strains using commercial gel baits containing fipronil, imidacloprid, hydramethylnon, and indoxacarb for up to 7 d resulted in 24.4-100%, 11.3-78.5%, 15.8-75.5%, and 63.3-100% mortality, respectively. We found that high deltamethrin resistance in some strains could affect the performance of fipronil, imidacloprid, and indoxacarb baits, indicating the potential involvement of cytochrome P450 monooxygenase in reducing the effectiveness of the bait toxicants.

Anopheles gambiae (s.l.) exhibit high intensity pyrethroid resistance throughout Southern and Central Mali (2016-2018): PBO or next generation LLINs may provide greater control

BACKGROUND

Millions of pyrethroid LLINs have been distributed in Mali during the past 20 years which, along with agricultural use, has increased the selection pressure on malaria vector populations. This study investigated pyrethroid resistance intensity and susceptible status of malaria vectors to alternative insecticides to guide choice of insecticides for LLINs and IRS for effective control of malaria vectors.

METHODS

For 3 years between 2016 and 2018, susceptibility testing was conducted annually in 14-16 sites covering southern and central Mali. Anopheles gambiae (s.l.) were collected from larval sites and adult mosquitoes exposed in WHO tube tests to diagnostic doses of bendiocarb (0.1%) and pirimiphos-methyl (0.25%). Resistance intensity tests were conducted using CDC bottle bioassays (2016-2017) and WHO tube tests (2018) at 1×, 2×, 5×, and 10× the diagnostic concentration of permethrin, deltamethrin and alpha-cypermethrin. WHO tube tests were conducted with pre-exposure to the synergist PBO followed by permethrin or deltamethrin. Chlorfenapyr was tested in CDC bottle bioassays at 100 µg active ingredient per bottle and clothianidin at 2% in WHO tube tests. PCR was performed to identify species within the An. gambiae complex.

RESULTS

In all sites An. gambiae (s.l.) showed high intensity resistance to permethrin and deltamethrin in CDC bottle bioassay tests in 2016 and 2017. In 2018, the WHO intensity tests resulted in survivors at all sites for permethrin, deltamethrin and alpha-cypermethrin when tested at 10× the diagnostic dose. Across all sites mean mortality was 33.7% with permethrin (0.75%) compared with 71.8% when pre-exposed to PBO (4%), representing a 2.13-fold increase in mortality. A similar trend was recorded for deltamethrin. There was susceptibility to pirimiphos-methyl, chlorfenapyr and clothianidin in all surveyed sites, including current IRS sites in Mopti Region. An. coluzzii was the primary species in 4 of 6 regions.

CONCLUSIONS

Widespread high intensity pyrethroid resistance was recorded during 2016-2018 and is likely to compromise the effectiveness of pyrethroid LLINs in Mali. PBO or chlorfenapyr LLINs should provide improved control of An. gambiae (s.l.). Clothianidin and pirimiphos-methyl insecticides are currently being used for IRS as part of a rotation strategy based on susceptibility being confirmed in this study.

Topical Toxicity Profiles of Some Aliphatic and Aromatic Essential Oil Components Against Insecticide-Susceptible and Resistant Strains of German Cockroach (Blattodea: Ectobiidae)

Toxicity profiles of four aliphatic (α-pinene, cyclononanone, limonene, nerolidol), four aromatic (β-thujaplicin, carvacrol, eugenol, tropolone) essential oil components (EOCs), and permethrin were investigated against three strains of German cockroach, Blattella germanica (L.). The strains include a susceptible strain (S), and two multi-resistant strains - strains D and E. Also, a synergism bioassay, using piperonyl butoxide (PBO) was conducted. The most toxic EOCs were aromatic EOCs carvacrol, eugenol, and tropolone, followed by aliphatic EOC limonene; all had LD50 values of <0.7 mg/µl. Four of the EOCs were equally toxic against all the strains, with carvacrol being the most toxic, followed by eugenol, tropolone, and α-pinene. The other four EOCs were more toxic against strain S than against the two resistant strains. Permethrin was significantly more toxic to strain S (LD50 = 0.056 µg/µl) compared with the resistant strains (D = 2.138 µg/µl, E = 1.730 µg/µl). Toxicity of aliphatic EOCs correlated positively with their molecular weight against strain E only, whereas both molecular weight and vapor pressure of aromatic EOCs correlated significantly with toxicity in all strains. Strain D exhibited the greatest resistance (RR of 6.7) to EOCs, and synergism to the aliphatic EOC cyclononanone. Clear synergism with PBO was observed in permethrin against resistant strains, but not in all of the EOCs, suggesting multiple resistance mechanisms in the resistant cockroaches. These findings give insight on the potential of EOCs to be incorporated as parts of an IPM approach to managing insecticide resistant German cockroaches.

Gravid Culex pipiens Exhibit A Reduced Susceptibility to Ultra-Low Volume Adult Control Treatments Under Field Conditions

In July and August of 2018, a field trial was conducted to examine the effectiveness of the North Shore Mosquito Abatement District's operational ultra-low volume (ULV) adulticide program. Two study sites were selected in Skokie, IL, and treated by truck-based ULV with d-phenothrin and prallethrin synergized with piperonyl butoxide over the course of a month. Natural mosquito populations were sampled via Biogents (BG)-counter baited with CO₂ or Alfalfa infusion. The results from this study demonstrate that host-seeking mosquitoes were reduced by 65.3% after ULV treatment while gravid mosquitoes were reduced by only 29.2%. In addition, host-seeking mosquitoes rebounded dramatically (303.1%) 3 days posttreatment while gravid mosquitoes did not (5.7%). Based on the differential effect between gravid and host-seeking mosquitoes, we concluded that the gonotrophic cycle and timing of ULV adulticide operations are important factors affecting the resistance of West Nile virus vectors to pyrethroid exposures.

Monitoring, Cross-Resistance, Inheritance, and Synergism of Plutella xylostella (Lepidoptera: Plutellidae) Resistance to Pyridalyl in China

Pyridalyl is an insecticide that shows significant efficacy against Plutella xylostella, a notorious pest insect worldwide. In this study, we monitored resistance of P. xylostella to pyridalyl in China from 2016 to 2017, determined cross-resistance, inheritance, and synergism of pyridalyl resistance in two pyridalyl-resistant populations, one field-evolved resistant population (ZL-PR) and one laboratory-selected resistant population (XY-PR). We found that variation in susceptibility among 15 field populations in China from 2016 to 2017 was high, with mean LC50 values ranging from 1.839 to 1,652 mg/liter. The laboratory-selected XY-PR strain showed 31.3-fold resistance to pyridalyl and moderate cross-resistance to fipronil. The ZL-PR displayed 1,050.2-fold resistance to pyridalyl and high resistance to all tested insecticides. Genetic analysis illustrated that pyridalyl resistance in ZL-PR was autosomally inherited and incompletely recessive. However, pyridalyl resistance in the XY-PR strain was autosomally inherited but incompletely dominant. Moreover, piperonyl butoxide significantly inhibited pyridalyl resistance in the XY-PR strain. In conclusion, P. xylostella field populations from South China have high levels of resistance to pyridalyl and different modes of inheritance of resistance were found in XY-PR and ZL-PR. Moreover, enhanced oxidative metabolism is possibly involved in resistance of the XY-PR strain but not in the ZL-PR strain.

Metabolic Activity of Cytochrome P450s Towards Four Pyrethroids in Midgut Tissue From Locusta migratoria (Orthoptera: Acrididae)

Cytochrome P450 monooxygenases (P450s) play important roles in metabolizing various insecticides and often contribute to the development of insecticide resistance in insects and other arthropod species. The objective of this study was to compare the metabolism of four commonly used pyrethroids including deltamethrin, fluvalinate, fenvalerate, and permethrin in the midgut tissue of Locusta migratoria Linnaeus (Orthoptera: Acrididae) by using synergism bioassay and ultra-performance liquid chromatography (UPLC)-mass spectrometer (MS) analyses. Our study showed that piperonyl butoxide (PBO, P450 enzyme inhibitor) can significantly synergize the toxicity of deltamethrin, fluvalinate, and fenvalerate with synergism ratios ranging from 1.30 to 1.70 folds. Preincubations of the midgut tissue with PBO followed by incubations with each of the four pyrethroids resulted in significantly higher amounts of unmetabolized deltamethrin and fluvalinate than those in the control (preincubation without PBO) as well as preincubations with other two detoxification enzyme inhibitors. These results indicate that P450s play important roles in metabolizing deltamethrin and fluvalinate in the midgut tissue. Our further study using deltamethrin as a representative pyrethroid and UPLC-MS techniques confirmed that the reduced amount of deltamethrin in the control (preincubation without PBO) was due to the metabolism of deltamethrin to yield hydroxydeltamethrin which is a major metabolite produced by P450-mediated aromatic hydroxylation of deltamethrim. These results provide new insights into differential metabolic activity of P450s towards different pyrethroids in the midgut tissue of L. migratoria.

that are highly resistant to pyrethroids? An experimental hut evaluation in Burkina Fasov

Malaria control is dependent on the use of longlasting insecticidal nets (LLINs) containing pyrethroids. A new generation of LLINs containing both pyrethroids and the synergist piperonyl butoxide (PBO) has been developed in response to increasing pyrethroid resistance in African malaria vectors, but questions remain about the performance of these nets in areas where levels of pyrethroid resistance are very high. This study was conducted in two settings in southwest Burkina Faso, Vallée du Kou 5 and Tengrela, where Anopheles gambiae s.l. (Diptera: Culicidae) mortality rates in World Health Organization (WHO) discriminating dose assays were < 14% for permethrin and < 33% for deltamethrin. When mosquitoes were pre-exposed to PBO in WHO tube assays, mortality rates increased substantially but full susceptibility was not restored. Molecular characterization revealed high levels of kdr alleles and elevated levels of P450s previously implicated in pyrethroid resistance. In cone bioassays and experimental huts, PBO LLINs outperformed the pyrethroid-only equivalents from the same manufacturers. Blood feeding rates were 1.6-2.2-fold lower and mortality rates were 1.69-1.78-fold greater in huts with PBO LLINs vs. non-PBO LLINs. This study indicates that PBO LLINs provide greater personal and community-level protection than standard LLINs against highly pyrethroid-resistant mosquito populations.

Piperonyl butoxide (PBO) combined with pyrethroids in insecticide-treated nets to prevent malaria in Africa

BACKGROUND

Public health strategies that target mosquito vectors, particularly pyrethroid long-lasting insecticidal nets (LLINs), have been largely responsible for the substantial reduction in the number of people in Africa developing malaria. The spread of insecticide resistance in Anopheles mosquitoes threatens these impacts. One way to control insecticide-resistant populations is by using insecticide synergists. Piperonyl butoxide (PBO) is a synergist that inhibits specific metabolic enzymes within mosquitoes and has been incorporated into pyrethroid-LLINs to form pyrethroid-PBO nets. Pyrethroid-PBO nets are currently produced by four LLIN manufacturers and, following a recommendation from the World Health Organization (WHO) in 2017, are being included in distribution campaigns in countries. This review examines epidemiological and entomological evidence on whether the addition of PBO to LLINs improves their efficacy.

OBJECTIVES

1. Evaluate whether adding PBO to pyrethroid LLINs increases the epidemiological and entomological effectiveness of the nets.2. Compare the effects of pyrethroid-PBO nets currently in commercial development or on the market with their non-PBO equivalent in relation to:a. malaria infection (prevalence or incidence);b. entomological outcomes.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group (CIDG) Specialized Register; CENTRAL, MEDLINE, Embase, Web of Science, CAB Abstracts, and two clinical trial registers (ClinicalTrials.gov and WHO International Clinical Trials Registry Platform) up to 24 August 2018. We contacted organizations for unpublished data. We checked the reference lists of trials identified by the above methods.

SELECTION CRITERIA

We included laboratory trials, experimental hut trials, village trials, and randomized clinical trials with mosquitoes from the Anopheles gambiae complex or Anopheles funestus group.

DATA COLLECTION AND ANALYSIS

Two review authors assessed each trial for eligibility, extracted data, and determined the risk of bias for included trials. We resolved disagreements through discussion with a third review author. We analysed the data using Review Manager 5 and assessed the certainty of the evidence using the GRADE approach.

MAIN RESULTS

Fifteen trials met the inclusion criteria: two laboratory trials, eight experimental hut trials, and five cluster-randomized controlled village trials.One village trial examined the effect of pyrethroid-PBO nets on malaria infection prevalence in an area with highly pyrethroid-resistant mosquitoes. The latest endpoint at 21 months post-intervention showed that malaria prevalence probably decreased in the intervention arm (OR 0.40, 95% CI 0.20 to 0.80; 1 trial, 1 comparison, moderate-certainty evidence).In highly pyrethroid-resistant areas (< 30% mosquito mortality), in comparisons of unwashed pyrethroid-PBO nets to unwashed standard-LLINs, PBO nets resulted in higher mosquito mortality (risk ratio (RR) 1.84, 95% CI 1.60 to 2.11; 14,620 mosquitoes, 5 trials, 9 comparisons, high-certainty evidence) and lower blood feeding success (RR 0.60, 95% CI 0.50 to 0.71; 14,000 mosquitoes, 4 trials, 8 comparisons, high-certainty evidence). However, in comparisons of washed pyrethroid-PBO nets to washed LLINs we do not know if PBO nets have a greater effect on mosquito mortality (RR 1.20, 95% CI 0.88 to 1.63; 10,268 mosquitoes, 4 trials, 5 comparisons, very low-certainty evidence), although the washed pyrethroid-PBO nets do decrease blood feeding success compared to standard-LLINs (RR 0.81, 95% CI 0.72 to 0.92; 9674 mosquitoes, 3 trials, 4 comparisons, high-certainty evidence).In areas where pyrethroid resistance is considered moderate (31% to 60% mosquito mortality), there may be little or no difference in effects of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.16, 95% CI 0.88 to 1.54; 242 mosquitoes, 1 trial, 1 comparison, low-certainty evidence), and there may be little or no difference in the effects on blood feeding success (RR 0.87, 95% CI 0.67 to 1.13; 242 mosquitoes, 1 trial, 1 comparison, low-certainty evidence). The same pattern is apparent for washed pyrethroid-PBO nets compared to washed standard-LLINs (mortality: RR 1.07, 95% CI 0.74 to 1.54; 329 mosquitoes, 1 trial, 1 comparison, low-certainty evidence; blood feeding success: RR 0.91, 95% CI 0.74 to 1.13; 329 mosquitoes, 1 trial, 1 comparison, low-certainty evidence).In areas where pyrethroid resistance is low (61% to 90% mosquito mortality), there is probably little or no difference in the effect of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.10, 95% CI 1.05 to 1.16; 708 mosquitoes, 1 trial, 2 comparisons, moderate-certainty evidence), but there is no evidence for an effect on blood feeding success (RR 0.67, 95% CI 0.06 to 7.37; 708 mosquitoes, 1 trial, 2 comparisons, very low-certainty evidence). For washed pyrethroid-PBO nets compared to washed standard-LLINs we do not know if there is any difference in mosquito mortality (RR 1.16, 96% CI 0.83 to 1.63; 878 mosquitoes, 1 trial, 2 comparisons, very low-certainty evidence), but blood feeding may decrease (RR 1.50, 95% CI 0.89 to 2.54; 878 mosquitoes, 1 trial, 2 comparisons, low-certainty evidence).In areas were mosquito populations are susceptible to insecticides (> 90% mosquito mortality), there may be little or no difference in the effect of unwashed pyrethroid-PBO nets compared to unwashed standard-LLINs on mosquito mortality (RR 1.20, 95% CI 0.64 to 2.26; 2791 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence). This is similar for washed nets (RR 1.07, 95% CI 0.92 to 1.25; 2644 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence). We do not know if unwashed pyrethroid-PBO nets have any effect on blood feeding success of susceptible mosquitoes (RR 0.50, 95% CI 0.11 to 2.32; 2791 mosquitoes, 2 trials, 2 comparisons, very low-certainty evidence). The same applies to washed nets (RR 1.28, 95% CI 0.81 to 2.04; 2644 mosquitoes, 2 trials, 2 comparisons, low-certainty evidence).In village trials comparing pyrethroid-PBO nets to LLINs, there was no difference in sporozoite rate (4 trials, 5 comparison) and mosquito parity (3 trials, 4 comparisons).

AUTHORS' CONCLUSIONS

In areas of high insecticide resistance, pyrethroid-PBO nets reduce mosquito mortality and blood feeding rates, and results from a single clinical trial demonstrate that this leads to lower malaria prevalence. Questions remain about the durability of PBO on nets, as the impact of pyrethroid-PBO LLINs on mosquito mortality was not sustained over 20 washes in experimental hut trials. There is little evidence to support higher entomological efficacy of pyrethroid-PBO nets in areas where the mosquitoes show lower levels of resistance to pyrethroids.

Nighttime Applications of Two Formulations of Pyrethroids are Effective Against Diurnal Aedes albopictus

The successful control of Aedes albopictus requires a multifaceted approach using a variety of integrated pest management techniques. Because this species is diurnal, nighttime ultra-low volume adulticide applications seem likely to miss resting mosquitoes and, therefore, are often met with skepticism. The goal of this study was to compare the efficacy of nighttime applications of pyrethroids with and without prallethrin to control caged and field populations of Ae. albopictus. During August and September of 2015, 2 adulticide applications were performed, treating 4 urban sites in the city of Trenton. We compared Anvil®, which contains sumithrin and piperonyl butoxide (PBO), to Duet™, which contains sumithrin, prallethrin, and PBO. Because prallethrin excites resting mosquitoes to flight, we hypothesized that Duet would kill more mosquitoes, especially those resting in cryptic harborages. Comparing pretreatment and posttreatment adult mosquito numbers, Biogents Sentinel trap collections revealed twice as many mosquitoes were killed by Duet than by Anvil. For caged Ae. albopictus, both products performed comparably, with Duet achieving a slightly higher mortality in front yards and Anvil achieving a slightly higher mortality in backyards. It is clear that nighttime adulticide applications are effective against Ae. albopictus, and the need to continue efficacy data collection is important because adulticiding is a key component of disease control response.

Effect of Travel Speed on Dispersion of Aqualuer 20-20 Sprayed by a Truck-Mounted Ultra-Low-Volume Sprayer Against Caged Aedes aegypti1

The effect of travel speed of a truck-mounted ultra-low-volume (ULV) sprayer on its application efficacy was studied at St. Johns County Fairground, Elkton, FL, during summer 2015. The efficacy was assessed by spray deposition, droplet size spectrum, and 24-h mortality of caged adult Aedes aegypti, using 2 rows of sampling locations, 15 m apart and spread up to 122 m from the spray. Each location had a bioassay cage and an impinger droplet sampler, 1 m apart from each other, at 1.5 m off the ground. Aqualuer^® 20-20 (20.6% permethrin AI and 20% piperonyl butoxide) was applied at the maximum label rate, travelling at 8, 16, and 32 km/h. Three replications were completed on 3 days at least a week apart, with 1 replication of each travel speed per day. On each application day the travel speeds were rotated. Overall, a travel speed of 32 km/h achieved the highest efficacy of Aqualuer^® 20-20, followed by 16 km/h, and then 8 km/h, in an open field. In general, droplet size, deposition, and mosquito mortality increased with increasing travel speed. The increased travel speed will also enhance the work rate of a sprayer and operator, thus reducing the cost of ULV applications.

Effect of Nozzle Orientation on Dispersion of Aqualuer 20-20 Sprayed by a Truck-Mounted Ultra-Low Volume Sprayer Against Caged Aedes aegypti

Inconsistencies in efficacy of ultra-low volume (ULV) ground applications in the literature are linked to the lack of adjustments in sprayer parameters. To investigate the effect of nozzle orientation of a truck-mounted ULV sprayer on application efficacy, a study was conducted at St. Johns County Fairground, Elkton, FL, during the summer of 2014. The efficacy was assessed by mortality of caged adult Aedes aegypti, spray deposition, and droplet size spectrum up to 122 m from the spray line. Aqualuer 20-20 (20.6% permethrin active ingredient [AI] and 20% piperonyl butoxide [PBO]) was applied at the maximum label rate with the nozzle pointed 45° upward, horizontal (0°), or 30° downward. Mortality was recorded after 24 h, deposition was determined with fluorometry, and droplets were measured with DropVision. Overall, horizontal nozzle angle spraying Aqualuer 20-20 achieved the highest efficacy followed by a 30° downward angle, while a 45° angle showed the least efficacy in open field tests. The mortality data showed complete mortality from a 0° nozzle up to 122 m from the spray line except for 1 location at 122 m in 1 replication. The mortality from a 30° downward orientation was lower beyond 30 m from the spray line, while the mortality from a 45° upward orientation was low close to the spray line and beyond 30 m. Horizontal orientation had higher deposition than other orientations, but the differences were not significant. There was also no significant difference in droplet spectrum from all orientations.

Cross-resistance and synergism bioassays suggest multiple mechanisms of pyrethroid resistance in western corn rootworm populations

Recently, resistance to the pyrethroid bifenthrin was detected and confirmed in field populations of western corn rootworm, Diabrotica virgifera virgifera LeConte from southwestern areas of Nebraska and Kansas. As a first step to understand potential mechanisms of resistance, the objectives of this study were i) to assess adult mortality at diagnostic concentration-LC99 to the pyrethroids bifenthrin and tefluthrin as well as DDT, ii) estimate adult and larval susceptibility to the same compounds as well as the organophosphate methyl-parathion, and iii) perform synergism experiments with piperonyl butoxide (PBO) (P450 inhibitor) and S,S,S-tributyl-phosphorotrithioate (DEF) (esterase inhibitor) in field populations. Most of the adult field populations exhibiting some level of bifenthrin resistance exhibited significantly lower mortality to both pyrethroids and DDT than susceptible control populations at the estimated LC99 of susceptible populations. Results of adult dose-mortality bioassays also revealed elevated LC50 values for bifenthrin resistant populations compared to the susceptible control population with resistance ratios ranging from 2.5 to 5.5-fold for bifenthrin, 28 to 54.8-fold for tefluthrin, and 16.3 to 33.0 for DDT. These bioassay results collectively suggest some level of cross-resistance between the pyrethroids and DDT. In addition, both PBO and DEF reduced the resistance ratios for resistant populations although there was a higher reduction in susceptibility of adults exposed to PBO versus DEF. Susceptibility in larvae varied among insecticides and did not correlate with adult susceptibility to tefluthrin and DDT, as most resistance ratios were < 5-fold when compared to the susceptible population. These results suggest that both detoxifying enzymes and target site insensitivity might be involved as resistance mechanisms.

Impact of Topical Application Site On the Efficacy of Permethrin and Malathion To Culex quinquefasciatus

Although insecticide spray droplets will potentially impinge on many exoskeletal body regions, traditional mosquito topical bioassays focus insecticide application to the mesothoracic pleural or dorsal area. Concentrations of permethrin and malathion found in droplets from ultra-low volume and low-volume sprays were evaluated for efficacy against adult Culex quinquefasciatus using a topical application bioassay. Results document nonuniform insecticide sensitivity across body regions, which has not been previously assessed in mosquitoes. Insecticide contact with appendages, such as the leg and the wing, returned much lower mortality from both insecticides than exposure to the primary body (i.e., head, thorax, and abdomen). No difference was observed in percent mortality 24 h after exposure to different insecticides to the same body region. Sublethal behaviors were also observed and discussed for both insecticides. Our findings provide valuable information for those performing topical bioassays, and may help explain insecticide effectiveness wherever droplets impinge upon the mosquito body during laboratory or field applications.

Evaluation of a New Thermal Fog Machine for Control of Adult Aedes albopictus in a Large Enclosed Space

Testing of the PSO BASDKA-AC1200 multifunction ultrafine particle atomization machine, a thermal fog machine, with Aqualuer 20-20(®) (permethrin 20.6%, piperonyl butoxide 20.6%) was conducted against Aedes albopictus. The machine was set at a 40 sec maximum burst interval dispersing 36 ml of chemical with an average droplet volume of 50%. Female adult Ae. albopictus were placed into cylindrical paper cages and adhered to poles at 5, 8, 10, 15, and 25 m from the center point of the machine. Control cages consisted of 1 cage placed at 5, 10, and 25 m. Control and treatment groups were left in the experiment area for 15 min. Initial knockdown after 15 min and 24 h mortality were documented. At 15 min post-treatment, Ae. albopictus displayed less than 50% knockdown. After 24 h, all treatment cages displayed greater than 90% mortality. Further bottle bioassays were conducted to determine the lowest chemical dose possible to achieve a lethal dose of 90%. A 1% dilution (10 ml Aqualuer 20-20 to 1,000 ml of polyether) of Aqualuer showed high mortality in the laboratory. However, after running 3 repetitions of a 1% dilution, there was no significant difference between the mortality of the mosquitoes at any of the distances 24 h post-treatment. This study indicates that the test machine would be an applicable and suitable machine for control of Ae. albopictus in enclosed spaces.

Evaluation of Three Commercial Handheld Ultra-Low-Volume Foggers with Aqualure® 20-20 Against Adult Aedes albopictus

The Anastasia Mosquito Control District (AMCD) tests all equipment before field use to determine if machines are suitable for the needs of the district. Three handheld ultra-low-volume (ULV) foggers--the American LongRay (ULV) Fogger Model 3600B with rechargeable lithium battery (DC model), American LongRay ULV Fogger Model 3600E with 110V or 220V AC power (AC model), and Boston Fog Battery Motorized Fogger (Boston Fogger)--were compared to determine which fogger would be most suitable for use by AMCD. Mortality of caged Aedes albopictus was analyzed after 24 h to determine the success of a single application. All 3 foggers resulted in 100% mortality after 24 h using the insecticide Aqualuer 20-20 (active ingredients permethrin 20.6% and piperonyl butoxide 20.6%) 1:5 dilution with reverse osmosis water. Based on operator safety, robustness, and operational performance, the American LongRay DC model was found to be the most suitable at administering Aqualuer 20-20 against caged adult Ae. albopictus.

Effect of Piperonyl Butoxide on the Toxicity of Four Classes of Insecticides to Navel Orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae)

Amyelois transitella (Walker) (Lepidoptera: Pyralidae), the navel orangeworm, is a highly polyphagous economic pest of almond, pistachio, and walnut crops in California. Increasing demand for these crops and their rising economic value has resulted in substantial increases of insecticide applications to reduce damage to acceptable levels. The effects of piperonyl butoxide (PBO), a methylenedioxyphenyl compound that can act as a synergist by inhibiting cytochrome P450-mediated detoxification on insecticide metabolism by A. transitella, were examined in a series of feeding bioassays with first-instar A. transitella larvae from a laboratory strain. PBO, however, can have a variety of effects on metabolism, including inhibition of glutathione-S-transferases and esterases and induction of P450s. In our study, PBO synergized the toxicity of acetamiprid, λ-cyhalothrin, and spinosad, suggesting possible involvement of P450s in their detoxification. In contrast, PBO interacted antagonistically with the organophosphate insecticide chlorpyrifos, reducing its toxicity, an effect consistent with inhibition of P450-mediated bioactivation of this pesticide. The toxicity of the anthranilic diamide insecticide chlorantraniliprole was not altered by PBO, suggestive of little or no involvement of P450-mediated metabolism in its detoxification. Because a population of navel orangeworm in Kern County, CA, has already acquired resistance to the pyrethroid insecticide bifenthrin through enhanced P450 activity, determining the effect of adding a synergist such as PBO on detoxification of all insecticide classes registered for use in navel orangeworm management can help to develop rotation practices that may delay resistance acquisition or to implement alternative management practices where resistance is likely to evolve.

Resistance to lambda-cyhalothrin in Spanish field populations of Ceratitis capitata and metabolic resistance mediated by P450 in a resistant strain

BACKGROUND

The withdrawal of malathion in the European Union in 2009 resulted in a large increase in lambda-cyhalothrin applications for the control of the Mediterranean fruit fly, Ceratitis capitata, in Spanish citrus crops.

RESULTS

Spanish field populations of C. capitata have developed resistance to lambda-cyhalothrin (6-14-fold), achieving LC50 values (129-287 ppm) higher than the recommended concentration for field treatments (125 ppm). These results contrast with the high susceptibility to lambda-cyhalothrin found in three Tunisian field populations. We have studied the mechanism of resistance in the laboratory-selected resistant strain W-1Kλ (205-fold resistance). Bioassays with synergists showed that resistance was almost completely suppressed by the P450 inhibitor PBO. The study of the expression of 53 P450 genes belonging to the CYP4, CYP6, CYP9 and CYP12 families in C. capitata revealed that CYP6A51 was overexpressed (13-18-fold) in the resistant strain. The W-1Kλ strain also showed high levels of cross-resistance to etofenprox (240-fold) and deltamethrin (150-fold).

CONCLUSION

Field-evolved resistance to lambda-cyhalothrin has been found in C. capitata. Metabolic resistance mediated by P450 appears to be the main resistance mechanism in the resistant strain W-1Kλ. The levels of cross-resistance found may compromise the effectiveness of other pyrethroids for the control of this species. © 2014 Society of Chemical Industry.

A collection of cytochrome P450 monooxygenase genes involved in modification and detoxification of herbicide atrazine in rice (Oryza sativa) plants

Plant cytochrome P450 monooxygenases constitute one of the largest families of protein genes involved in plant growth, development and acclimation to biotic and abiotic stresses. However, whether these genes respond to organic toxic compounds and their biological functions for detoxifying toxic compounds such as herbicides in rice are poorly understood. The present study identified 201 genes encoding cytochrome P450s from an atrazine-exposed rice transcriptome through high-throughput sequencing. Of these, 69 cytochrome P450 genes were validated by microarray and some of them were confirmed by real time PCR. Activities of NADPH-cytochrome P450 reductase (CPR) and p-nitroanisole O-demethylase (PNOD) related to toxicity were determined and significantly induced by atrazine exposure. To dissect the mechanism underlying atrazine modification and detoxification by P450, metabolites (or derivatives) of atrazine in plants were analyzed by ultra performance liquid chromatography mass spectrometry (UPLC/MS). Major metabolites comprised desmethylatrazine (DMA), desethylatrazine (DEA), desisopropylatrazine (DIA), hydroxyatrazine (HA), hydroxyethylatrazine (HEA) and hydroxyisopropylatrazine (HIA). All of them were chemically modified by P450s. Furthermore, two specific inhibitors of piperonyl butoxide (PBO) and malathion (MAL) were used to assess the correlation between the P450s activity and rice responses including accumulation of atrazine in tissues, shoot and root growth and detoxification.

Resistance of the house fly Musca domestica (Diptera: Muscidae) to lambda-cyhalothrin: mode of inheritance, realized heritability, and cross-resistance to other insecticides

Lambda-cyhalothrin, a pyrethroid insecticide, has been used frequently for the control of house flies, Musca domestica L., worldwide including Pakistan. To assess the resistance risk and design a resistance management strategy, a house fly population was exposed to lambda-cyhalothrin in the laboratory to assess inheritance and heritability, and cross-resistance to other insecticides, including different chemical classes. After 11 generations of selection, the population developed 113.57-fold resistance to lambda-cyhalothrin compared to the susceptible population. There was no cross-resistance to bifenthrin and methomyl, but very low cross-resistance to abamectin and indoxacarb in the lambda-cyhalothrin selected population compared to the field population. Synergism bioassay with piperonyl butoxide and S,S,S-tributylphosphorotrithioate indicated that lambda-cyhalothrin resistance was associated with microsomal oxidases and esterases. The LC50 values of F1 (Lambda-SEL ♀ × Susceptible ♂) and F'1 (Lambda-SEL ♂ × Susceptible ♀) populations were not significantly different and dominance (DLC) values were 0.68 and 0.62. The resistance to lambda-cyhalothrin was completely recessive (DML = 0.00) at highest dose and completely dominant at lowest dose (DML = 0.95). The monogenic model of inheritance showed that lambda-cyhalothrin resistance was controlled by multiple factors. The heritability values were 0.20, 0.04, 0.003, 0.07 and 0.08 for lambda-cyhalothrin, bifenthrin, methomyl, indoxacarb and abamectin resistance, respectively. It was concluded that lambda-cyhalothrin resistance in house flies was autosomally inherited, incompletely dominant and controlled by multiple factors. These findings would be helpful to improve the management of house flies.

Hyperforin and deoxycohumulone as a larvicidal agent against Culex pipiens (Diptera: Culicidae)

The larvicidal effect of hyperforin (1), a bioactive compound of Hypericum perforatum, and deoxycohumulone (2) (biosynthetic precursor of hyperforin) were evaluated against Culex pipiens (Diptera: Culicidae) for the first time. All the acetate analogues (3-6) of hyperforin (1) and deoxycohumulone (2) were also synthesized and bioassayed to provide information on structural requirements for the tested compounds. Larvicidal results revealed that hyperforin (1) and deoxycohumulone (2) exhibited potent activity with LC50 value of 26.72 and 51.03 mg L(-1), respectively. The monoacetyl-deoxycohumulone (4) displayed lower activity with LC50 value of 135.92 mg L(-1), while all other acetate analogues were inactive at concentrations even as high as 150 mg L(-1), indicating that the free hydroxyl groups are essential for the larvicidal activity. The mortality values were increased, more than 80%, when 10 mg L(-1) piperonyl butoxide were added in hyperforin (1) or deoxycohumulone (2) bioassays. Finally, sub-lethal survival analysis is conducted for three doses of hyperforin (1) and deoxycohumulone (2) and results are discussed.

The importance of oxidases in the tolerance of deciduous leaf infusions by Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae)

Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) larvae rely on oxidases to reduce toxicity of water soluble toxins from some senescent tree leaf infusions. The mortality of third instar Ae. aegypti larvae in live oak and pin oak leaf infusions increased significantly in the presence of piperonyl butoxide (PBO), a broad inhibitor of cytochrome P450s (CYPs). In contrast, PBO treatment did not increase mortality in water controls or infusions of northern red oak or sugar maple leaf infusions for Ae. aegypti larvae. A similar pattern was observed for Ae. albopictus larvae, that is, an increase in mortality when CYPs were inhibited in live oak leaf infusions and no increase in sugar maple leaf infusions or water controls. However, the fresh live oak leaf infusion (5 d old) was the most toxic infusion to Ae. aegypti, but appeared less toxic to Ae. albopictus than the older infusions. A direct comparison of survival between the two Aedes species revealed Ae. aegypti exhibited a greater mortality than Ae. albopictus in PBO-treated live oak leaf infusions. These findings suggest that toxic components of some leaf litter in larval habitats may impose cryptic energy costs (detoxification).

[Pyrethroid resistance mechanisms in the body lice Pediculus humanus humanus L.: detoxification enzyme systems]

Synergists (piperonyl butoxide, MGK-264 (N-octyl bicycloheptene dicarboximide), DEF (S, S, S, tributyl phosphorotrithioate), and diethylmaleate) were used to investigate the metabolic resistance of body lice to permethrin. Instances of the synergistic activity ofpermethrin when combined with enzyme system inhibitors have been identified. The synergic ratios were 3.3 to 52.0 for monooxygenase inhibitors; 5.2 to 7.4 for esterases, and about 4 for glutathione-S-transferases. Since the use of the synergists did not lead to full inhibition of resistance, it was concluded that the lice had also a kdr-like type of resistance.

The synergistic effects of insecticidal essential oils and piperonyl butoxide on biotransformational enzyme activities in Aedes aegypti (Diptera: Culicidae)

The biochemical mechanisms underlying the increased toxicity of several plant essential oils (thymol, eugenol, pulegone, terpineol, and citronellal) against fourth instar of Aedes aegypti L. when exposed simultaneously with piperonyl butoxide (PBO) were examined. Whole body biotransformational enzyme activities including cytochrome P450-mediated oxidation (ethoxyresorufin O-dethylase [EROD]), glutathione S-transferase (GST), and beta-esterase activity were measured in control, essential oil-exposed only (single chemical), and essential oil + PBO (10 mg/liter) exposed larvae. At high concentrations, thymol, eugenol, pulegone, and citronellal alone reduced EROD activity by 5-25% 16 h postexposure. Terpineol at 10 mg/liter increased EROD activity by 5 +/- 1.8% over controls. The essential oils alone reduced GST activity by 3-20% but PBO exposure alone did not significantly affect the activity of any of the measured enzymes. All essential oils in combination with PBO reduced EROD activity by 58-76% and reduced GST activity by 3-85% at 16 h postexposure. This study indicates a synergistic interaction between essential oils and PBO in inhibiting the cytochrome P450 and GST detoxification enzymes in Ae. aegypti.

The involvement of cytochrome P450 monooxygenases in methanol elimination in Drosophila melanogaster larvae

Methanol is one of the most common short-chain alcohols in fermenting fruits, the natural food of the fruit fly, Drosophila melanogaster. The larvae cope continuously with methanol at various concentrations in order to survive and develop. In the present article, we found toxicities of dietary methanol and formaldehyde were enhanced by piperonyl butoxide, but not by 3-amino-1, 2, 4-triazole, 4-methylpyrazole, diethylmeleate, and triphenyl phosphate, when assessing by the combination index method. These results reveal that cytochrome P450 monooxygenases (CYPs), rather than catalases, alcohol dehydrogenases, glutathione S-transferases, and esterases, participate in methanol metabolism. Moreover, methanol exposure dramatically increased CYP activity. The ratios of the CYP activities in treated larvae to those in control reached, respectively, up to 3.0-, 3.9-, and 2.7-fold, at methanol concentrations of 22.6, 27.9, and 34.5 mg/g diet. In addition, methanol exposure greatly up-regulated the mRNA expression level of five Cyp genes, which were Cyp304a1, Cyp9f2, Cyp28a5, Cyp4d2, and Cyp4e2. Their resulting proteins were suggested as the candidate enzymes for methanol metabolism in D. melanogaster larvae.

Investigating the potential of selected natural compounds to increase the potency of pyrethrum against houseflies Musca domestica (Diptera: Muscidae)

BACKGROUND

A study was undertaken to determine the efficacy of seven natural compounds compared with piperonyl butoxide (PBO) in synergising pyrethrum, with the intention of formulating an effective natural synergist with pyrethrum for use in the organic crop market.

RESULTS

Discriminating dose bioassays showed PBO to be significantly more effective at synergising pyrethrum in houseflies than the seven natural compounds tested, causing 100% mortality in insecticide-susceptible WHO and resistant 381zb strains of housefly. The most effective natural synergists against WHO houseflies were dillapiole oil, grapefruit oil and parsley seed oil, with 59, 50 and 41% mortality respectively, compared with 18% mortality with unsynergised pyrethrum. Against 381zb houseflies, the most effective natural synergists were parsley seed oil and dillapiole oil. Esterase inhibition by the natural compounds and PBO in vitro showed no correlation with pyrethrum synergism in vivo, whereas the inhibition of oxidases in vitro more closely correlated with pyrethrum synergism in vivo.

CONCLUSION

Dillapiole oil and parsley seed oil showed the greatest potential as pyrethrum synergists. PBO remained the most effective synergist, possibly owing to its surfactant properties, enhancing penetration of pyrethrins. The results suggest the involvement of oxidases in pyrethroid resistance in houseflies, with the efficacy of synergists showing a high correlation with inhibition of oxidases.

Absorption, translocation and metabolism of metamitron in Chenopodium album

BACKGROUND

In recent years, common lambsquarters (Chenopodium album L.) populations from sugar beet fields in different European countries have responded as resistant to the as-triazinone metamitron. The populations have been found to have the same D1 point mutation as known for atrazine-resistant biotypes (Ser264 to Gly). However, pot experiments revealed that metamitron resistance is not as clear-cut as observed with triazine resistance in the past. The objectives of this study were to clarify the absorption, translocation and metabolic fate of metamitron in C. album.

RESULTS

Root absorption and foliar absorption experiments showed minor differences in absorption, translocation and metabolism of metamitron between the susceptible and resistant C. album populations. A rapid metabolism in the C. album populations was observed when metamitron was absorbed by the roots. The primary products of metamitron metabolism were identified as deamino-metamitron and metamitron-N-glucoside. PABA, known to inhibit the deamination of metribuzin, did not alter the metabolism of metamitron, and nor did the cytochrome P450 inhibitor PBO. However, inhibition of metamitron metabolism in the presence of the cytochrome P450 inhibitor ABT was demonstrated.

CONCLUSION

Metamitron metabolism in C. album may act as a basic tolerance mechanism, which can be important in circumstances favouring this degradation pathway.

Thermal fog efficacy tests against mosquitoes in storm drains in Harris County, Texas 2008-09

In 2008 and 2009, our current thermal fogging methods and materials were evaluated in underground systems against feral and laboratory strains of adult Culex quinquefasciatus. Culex quinquefasciatus adults collected from storm drain systems the night before treatment were tested concurrently with adults from a susceptible laboratory strain (Sebring) in 10 separate tests. During 2008, there were no significant differences in the low mean percent control obtained between any of the feral populations (29.39%) and susceptible (Sebring) populations (56.04%) tested, whereas in 2009, application of mineral oil alone yielded extremely low, but significantly different mean percent control at 0.99% for ferals and 0.01% for Sebrings. During 2009 mineral oil evaluations, 45,677 droplets were collected in storm drains at distances of 99.1, 50.6, 57.9, 67.7, and 109.7 m from the application site, with 99% of the droplets below 3 microm in diam; additionally, we found no significant differences between mean percent control of Sebring and feral mosquito populations using the higher (3.2x) 1:10 application rate of Pyrocide. However, mean percent control between the feral and susceptible strain (Sebring) during 2009 was lower than in 2008 at 16.55% for ferals and 24.43% in Sebrings. Results indicated that control methodologies and/or chemicals used were ineffective at controlling Cx. quinquefasciatus in storm drains using the chosen experimental design. Based upon this information, thermal fog operations were discontinued due to lack of effectiveness.

Biochemical basis of permethrin resistance in Anopheles arabiensis from Lower Moshi, north-eastern Tanzania

BACKGROUND

Development of resistance to different classes of insecticides is a potential threat to malaria control. With the increasing coverage of long-lasting insecticide-treated nets in Tanzania, the continued monitoring of resistance in vector populations is crucial. It may facilitate the development of novel strategies to prevent or minimize the spread of resistance. In this study, metabolic-based mechanisms conferring permethrin (pyrethroid) resistance were investigated in Anopheles arabiensis of Lower Moshi, Kilimanjaro region of north-eastern Tanzania.

METHODS

WHO susceptibility test kits were used to detect resistance to permethrin in An. arabiensis. The levels and mechanisms of permethrin resistance were determined using CDC bottle bioassays and microplate (biochemical) assays. In bottle bioassays, piperonyl butoxide (PBO) and s,s,s-tributyl phosphorotrithioate (DEF) were used as synergists to inhibit mixed function oxidases and non-specific esterases respectively. Biochemical assays were carried out in individual mosquitoes to detect any increase in the activity of enzymes typically involved in insecticide metabolism (mixed function oxidases, alpha- and beta-esterases).

RESULTS

Anopheles arabiensis from the study area was found to be partially resistant to permethrin, giving only 87% mortality in WHO test kits. Resistance ratios at KT50 and KT95 were 4.0 and 4.3 respectively. The permethrin resistance was partially synergized by DEF and by PBO when these were mixed with permethrin in bottle bioassays and was fully synergized when DEF and PBO were used together. The levels of oxidase and beta-esterase activity were significantly higher in An. arabiensis from Lower Moshi than in the laboratory susceptible strain. There was no difference in alpha-esterase activity between the two strains.

CONCLUSION

Elevated levels of mixed function oxidases and beta-esterases play a role in detoxification of permethrin in the resistant An. arabiensis population of Lower Moshi.

Metabolic enzyme(s) confer imidacloprid resistance in a clone of Myzus persicae (Sulzer) (Hemiptera: Aphididae) from Greece

BACKGROUND

Previous studies have reported varying levels of resistance against imidacloprid in several insect species, including populations of the peach-potato aphid, Myzus persicae (Sulzer). These cases of resistance have been attributed to either target-site resistance or enhanced detoxification. In this study, a clone of M. persicae originating from Greece revealed a 60-fold resistance factor to imidacloprid.

RESULTS

The Greek clone is compared in terms of metabolic enzyme activity and synergism profiles with other M. persicae clones showing lower imidacloprid resistance.

CONCLUSION

A combination of in vitro biochemical assays and in vivo differential synergism studies using PBO and a close analogue EN 16/5-1 suggests that the mechanism conferring increased resistance in this clone is primarily due to enhanced oxidase activity.

Evaluation of piperonyl butoxide as a deltamethrin synergist for pyrethroid-resistant bed bugs

An understanding of the mechanisms of insecticide resistance in the bed bug, Cimex lectularius L., has the potential to lead to new approaches for the control of resistant populations. We used the cytochrome P450 monooxygenase (P450) inhibitor piperonyl butoxide (PBO) to assess the role of P450s in deltamethrin resistance in three field-collected bed bug strains, LA-1, CIN-1 and WOR-1. In addition, we exposed two highly resistant strains, CIN-1 and WOR-1 (resistance ratio [RR] >2,500-fold), to dry residues of piperonyl butoxide-synergized pyrethroid formulations to determine the utility of synergism by PBO. Piperonyl butoxide synergized deltamethrin in all three strains, but its impact was variable. The synergistic ratio varied from 40 in CIN-1 to 176 in WOR-1. Because the resistance ratio for each strain after piperonyl butoxide treatment was 174 and 39, respectively, our results suggest that P450s have some involvement in deltamethrin resistance, but other resistance mechanisms must be involved as well. No significant synergistic effect of formulated deltamethrin was observed with the addition of synergized pyrethrins or formulated piperonyl butoxide in the CIN-1 strain, but synergism occurred in the WOR-1 strain. Addition of PBO to pyrethroids is not a comprehensive solution to pyrethroid resistance because strains vary in both overall resistance level and the proportion of that resistance attributable to P450s.

Diminished reproductive fitness associated with the deltamethrin resistance in an Indian strain of dengue vector mosquito, Aedes aegypti L

The susceptible (SS) and resistant (DLR) strains of Aedes aegypti selected with deltamethrin and combination of deltamethrin and PBO (1:5) at the larval/adult stage were studied in the laboratory for their reproductive fitness in terms of fecundity, hatchability and longevity of gonotrophic cycles. The DLR strains exhibited 73-88% reduction in the duration of gonotrophic cycles as compared to their SS counterparts. There was a considerable decrease in egg production and hatchability rates in the selected strains of Ae. aegypti, as compared to that of the SS strain. Data indicate deltamethrin being an effective insecticide against Ae. aegypti and a possible correlation between the deltamethrin resistance and disadvantages during reproduction. The most drastic and significant effect was observed in DLR1b strains exhibiting 36.7% decrease in fecundity and 32.4% reduction in hatchability. Another important observation was diminished reproductive fitness in DLR2 strains. This suggests the usefulness of synergized deltamethrin selections in reducing the frequency of resistant individuals. A significant finding was to observe the reproductive disadvantage in adult-selected strains having negligible resistance to deltamethrin implicating the efficacy of deltamethrin as an adulticide rather than as a larvicide. Various probable reasons for the reduction in the reproductive potential and the possible resistance-management strategies of Ae. aegypti are discussed.

Efficacy of various pyrethroid structures against a highly metabolically resistant isogenic strain of Helicoverpa armigera (Lepidoptera: Noctuidae) from China

BACKGROUND

Resistance to pyrethroids and other types of insecticides in Helicoverpa armigera (Hübner) has been documented in many countries. The isolation of specific resistance mechanisms in isogenic strains is an optimal approach to investigate cross-resistance pattern, and to validate resistance breaking pyrethroids. In this study an isogenic metabolic resistance CMR strain was successfully isolated from a field pyrethroid-resistant population of H. armigera. With this strain, cross-resistance among 19 pyrethroid insecticides with varying chemical structures was analysed.

RESULTS

Resistance to pyrethroids in the CMR strain was likely to be due to enhanced oxidative metabolism. The most significant cross-resistance in the CMR strain was between pyrethroids such as fenvalerate, tau-fluvalinate and flumethrin characterised by having both phenoxybenzyl and aromatic acid moieties. Substitution of the phenoxybenzyl group with a polyfluorobenzyl group, as in tefluthrin, benfluthrin and transfluthrin, overcame most of this resistance.

CONCLUSION

The findings in this study support the assertion that it is possible to find pyrethroids that are active against resistant populations. Such pyrethroids could be considered as possible partners or resistance breaking pyrethroids in a pyrethroid resistance management programme for H. armigera in China and in other Asian countries where the oxidative metabolism resistance is a dominant mechanism.

Genetics and mechanism of resistance to deltamethrin in a field population of Spodoptera litura (Lepidoptera: Noctuidae)

BACKGROUND

Spodoptera litura (F.) causes enormous losses in many economically important crops. The genetics of insecticide resistance has been extensively studied in several insect pests, but there is a lack of information on S. litura. Therefore, the genetics and mechanisms of the resistance of S. litura to deltamethrin were investigated.

RESULTS

Bioassays at generation G1 gave resistance ratios of 9, 5, 41, 52 and 49 for deltamethrin, cypermethrin, profenofos, chlorpyrifos and triazofos respectively, when compared with the susceptible Lab-PK strain. Bioassays at G4 with a deltamethrin-selected population (Delta-SEL) showed that selection gave resistance ratios of 63 and 7 for deltamethrin when compared with the Lab-PK and UNSEL strains respectively. Cross-resistance to other insecticides tested was observed in the selected population. A notable feature of the Delta-SEL strain was that resistance to deltamethrin, cypermethrin, profenofos and chlorpyrifos did not decline over the course of five generations. Synergism tests with microsomal oxidase (MO) and esterase-specific inhibitors indicated that the deltamethrin resistance was associated with MO and, possibly, esterase activity. Reciprocal crosses between the Delta-SEL and Lab-PK strains indicated that resistance was autosomal and incompletely dominant. A direct test of monogenic inheritance suggested that resistance to deltamethrin was controlled by more than one locus.

CONCLUSION

Stability and dominance of resistance and cross-resistance suggest that insecticides with different modes of action should be recommended to reduce pyrethroid selection pressure.

Possible involvement of oxidative stress in piperonyl butoxide induced hepatocarcinogenesis in rats

To clarify the possible mechanism of non-genotoxic hepatocarcinogenesis induced by piperonyl butoxide (PBO), male F344 rats were administered an i.p. injection of N-diethylnitrosamine (DEN) to initiate hepatocarcinogenesis. Two weeks later, the rats were administered a PBO-containing (0, 1, or 2%) diet for 6 weeks and subjected to a two-third partial hepatectomy 1 week later. After sacrificing them on week 8, their livers were histopathologically examined and analyzed for gene expression using a microarray and real-time RT-PCR. Reactive oxygen species (ROS) products were also measured using liver microsomes. Hepatocytes exhibited centrilobular hypertrophy and increased glutathione S-transferase placental form (GST-P) positive foci formation. ROS products increased significantly in liver microsomes. In the microarray analysis, the expressions of genes related to metabolism and oxidative stress - NAD(P)H dehydrogenase, quinone 1 (Nqo1), UDP-glucuronosyltransferase (UDPGTR-2), glutathione peroxidase 2 (Gpx2), glutathione reductase (GRx) - multidrug resistance associated protein 3 (Abcc3), and solute carrier family 7 (cationic amino acid transporter, y+ system) member 5 (Slc7a5) were up-regulated in the PBO group in comparison to the 0% PBO group; this was confirmed by real-time RT-PCR. Additionally, a significant up-regulation of stress response related genes such as CYP1A1 was observed in PBO-treated groups in real-time RT-PCR. HPLC analysis revealed that the level of 8-OHdG in the 2% PBO group was significantly higher than that in the 0% PBO group. This suggests that PBO has the potential to generate ROS via metabolic pathways and induce oxidative stress, including oxidative DNA damage, resulting in the induction of hepatocellular tumors in rats.