Synergism studies with binary mixtures of pyrethroid, carbamate and organophosphate insecticides on Frankliniella occidentalis (Pergande)

Effect of the Combination of Synthetic Anthelmintics with Carvacryl Acetate in Emulsions with and without a Sodium Alginate Matrix on Haemonchus contortus

The present study aimed to evaluate the effect of nanoemulsions using combined synthetic anthelmintics, thiabendazole (TBZ), levamisole (LEV), and ivermectin (IVM), with carvacryl acetate (CA) against Haemonchus contortus, and also tested the presence and absence of alginate (ALG). The anthelmintic effect of the CA/TBZ nanoemulsion was evaluated in the egg hatch test (EHT). The effects of CA/IVM and CA/LEV nanoemulsions were evaluated in the larval development test (LDT). The emulsions CA/TBZ/ALG and CA/TBZ showed a multimodal profile, with most particles on the nanometric scale. The encapsulation efficiency in CA/TBZ/ALG was 80.25%, and that in CA/LEV/ALG was 89.73%. In the EHT, CA/TBZ and CA/TBZ/ALG showed mean combination indices (CIs) of 0.55 and 0.36, respectively, demonstrating synergism in both. In LDT, CA/IVM had an average CI of 0.75, and CA/LEV and CA/LEV/ALG showed CI values of 0.4 and 0.93, respectively. It was concluded that CA/TBZ showed a synergistic interaction, and CA/TBZ/ALG showed an enhanced effect. In addition, the matrix brought stability to the product, encouraging its improvement to obtain higher efficacy.

Combination of synthetic anthelmintics and monoterpenes: Assessment of efficacy, and ultrastructural and biophysical properties of Haemonchus contortus using atomic force microscopy

The resistance of Haemonchus contortus to synthetic anthelmintics is of increasing concern; and different strategies are being evaluated to improve parasite control. The present study investigated the in vitro effects of combinations of synthetic compounds and monoterpenes. Additionally, the chemical association of the best combinations and their impact on the ultrastructural and biophysical properties of H. contortus eggs was evaluated. We assessed the efficacy of the monoterpenes, carvacrol, thymol, r-carvone, s-carvone, citral, and p-cymene and the anthelmintics, albendazole and levamisole using the egg hatch test (EHT) and the larval migration inhibition test (LMIT), respectively. The minimum effective concentrations of the monoterpenes, according to the EHT (efficacy ranging from 4.4%-11.8%) and LMIT (efficacy ranging from 5.6%-7.4%), were used in combination with different concentrations of synthetic compounds, and the IC₅₀ and synergism rate (SR) were calculated. Fourier-transform infrared spectroscopy (FTIR) was used to analyze the chemical association between the best combinations as revealed by the in vitro tests (albendazole and levamisole with r-carvone or s-carvone). Atomic force microscopy (AFM) was used to assess the ultrastructural and biophysical properties of H. contortus eggs treated with the albendazole and r-carvone combination. Among the monoterpenes, the highest efficacies were exhibited by carvacrol (IC₅₀ = 185.9 μg/mL) and thymol (IC₅₀ = 187.0 μg/mL), according to the EHT, and s-carvone and carvacrol (IC₅₀ = 1526.0 and 1785.3 μg/mL, respectively), according to the LMIT. According to the EHT, albendazole showed a slight statistically significant synergism in combination with r-carvone (SR = 3.8) and s-carvone (SR = 3.0). According to the LMIT, among the monoterpenes, r-carvone (SR = 1.7) and s-carvone (SR = 1.7) showed an increase in efficacy with levamisole; however, this was not statistically significant. The FTIR spectra of albendazole and levamisole, in association with r-carvone and s-carvone, indicated the presence of chemical interactions between the synthetic and natural molecules, contributing to the possible synergistic effects of these associations. Eggs treated with albendazole and r-carvone showed an increase in roughness and a decrease in height, suggesting that the treatment induced damage to the egg surface and an overflow of its internal contents. Overall, the combination of albendazole with r-carvone and s-carvone was efficacious against H. contortus, demonstrating a chemical association between the compounds; the significant changes in the egg ultrastructure justify this efficacy.

Effects of predator cues and pesticide resistance on the toxicity of a (bio)pesticide mixture

BACKGROUND

Populations of target species are typically exposed to pesticide mixtures and natural stressors such as predator cues, and are increasingly developing resistance to single pesticides. Nevertheless, we have poor knowledge whether natural stressors and the presence of pesticide resistance shape mixture toxicity. We tested the single and combined effects of the pesticide chlorpyrifos and the biopesticide Bacillus thuringiensis israelensis (Bti) on the survival of the Southern house mosquito (Culex quinquefasciatus, Say) and whether these effects were magnified by synthetic predator cues of Notonecta water bugs and differed between a chlorpyrifos-resistant (Ace-1R) and non-resistant (S-Lab) strain.

RESULTS

Single exposure to Bti caused mortality in both strains (S-Lab ∼27%, Ace-1R ∼41%) and single exposure to chlorpyrifos caused only mortality in the S-Lab strain (∼33%), while predator cues did not induce mortality. The chlorpyrifos-resistant strain was 1.5-fold more sensitive to Bti, indicating a cost of resistance. The interaction types between chlorpyrifos and Bti (additive), between chlorpyrifos and predator cues (additive), and between Bti and predator cues (synergistic) were consistent in both strains. Despite predator cues making Bti approximately 8% more lethal, they did not change the additive interaction between Bti and chlorpyrifos in their mixture in either strain.

CONCLUSION

These results indicate that the resistance against chlorpyrifos was not partly lifted when chlorpyrifos exposure was combined with Bti and predator cues. Identifying the interaction type within pesticide mixtures and how this depends on natural stressors is important to select control strategies that give a disadvantage to resistant individuals compared to non-resistant individuals. © 2019 Society of Chemical Industry.

Lethal toxicity and gene expression changes in embryonic zebrafish upon exposure to individual and mixture of malathion, chlorpyrifos and lambda-cyhalothrin

Pesticides are usually present as mixtures in water environments. Evaluating the toxic effects of individual pesticide may not be enough for protecting ecological environment due to interactions among substances. In this study, we aimed to examine the lethal doses and gene expression changes in zebrafish (Danio rerio) upon exposure to individual and mixture pesticides [malathion (MAL), chlorpyrifos (CHL) and lambda-cyhalothrin (LCY)]. Individual pesticide toxicity evaluation manifested that the toxicity of the three pesticides to D. rerio at various developmental stages (embryonic, larval, juvenile and adult stages) followed the order of LCY > CHL > MAL. On the contrary, the least toxicity to the animals was discovered from MAL. Most of the tested pesticides displayed lower toxicities to the embryonic stage compared with other life stages of zebrafish. Synergistic effects were monitored from two binary mixtures of LCY in combination with MAL or CHL and ternary mixture of MAL + CHL + LCY. The expressions of 16 genes involved in oxidative stress, immunity system, cell apoptosis and endocrine disruption at the mRNA level revealed that embryonic zebrafish were influenced by the individual or mixture pesticides. The expressions of Tnf, P53, TRα, Crh and Cyp19a exerted greater variations upon exposure to pesticide mixtures compared with their individual compounds. Collectively, the transcriptional responses of these genes might afford early warning biomarkers for identifying pollutant exposure, and the data acquired from this study provided valuable insights into the comprehensive toxicity of pesticide mixtures to zebrafish.

Resistance to Thrips in Peanut and Implications for Management of Thrips and Thrips-Transmitted Orthotospoviruses in Peanut

Thrips are major pests of peanut (Arachis hypogaea L.) worldwide, and they serve as vectors of devastating orthotospoviruses such as Tomato spotted wilt virus (TSWV) and Groundnut bud necrosis virus (GBNV). A tremendous effort has been devoted to developing peanut cultivars with resistance to orthotospoviruses. Consequently, cultivars with moderate field resistance to viruses exist, but not much is known about host resistance to thrips. Integrating host plant resistance to thrips in peanut could suppress thrips feeding damage and reduce virus transmission, will decrease insecticide usage, and enhance sustainability in the production system. This review focuses on details of thrips resistance in peanut and identifies future directions for incorporating thrips resistance in peanut cultivars. Research on thrips-host interactions in peanut is predominantly limited to field evaluations of feeding damage, though, laboratory studies have revealed that peanut cultivars could differentially affect thrips feeding and thrips biology. Many runner type cultivars, field resistant to TSWV, representing diverse pedigrees evaluated against thrips in the greenhouse revealed that thrips preferred some cultivars over others, suggesting that antixenosis "non-preference" could contribute to thrips resistance in peanut. In other crops, morphological traits such as leaf architecture and waxiness and spectral reflectance have been associated with thrips non-preference. It is not clear if foliar morphological traits in peanut are associated with reduced preference or non-preference of thrips and need to be evaluated. Besides thrips non-preference, thrips larval survival to adulthood and median developmental time were negatively affected in some peanut cultivars and in a diploid peanut species Arachis diogoi (Hoehne) and its hybrids with a Virginia type cultivar, indicating that antibiosis (negative effects on biology) could also be a factor influencing thrips resistance in peanut. Available field resistance to orthotospoviruses in peanut is not complete, and cultivars can suffer substantial yield loss under high thrips and virus pressure. Integrating thrips resistance with available virus resistance would be ideal to limit losses. A discussion of modern technologies such as transgenic resistance, marker assisted selection and RNA interference, and future directions that could be undertaken to integrate resistance to thrips and to orthotospoviruses in peanut cultivars is included in this article.

Toxicity of Bifenthrin and Mixtures of Bifenthrin Plus Acephate, Imidacloprid, Thiamethoxam, or Dicrotophos to Adults of Tarnished Plant Bug (Hemiptera: Miridae)

To assess the toxicity of bifenthrin and four mixtures of insecticides to tarnished plant bug, we used an insecticide dip method of green bean to treat adults of a laboratory colony; mortality was assessed after 48 h. LC50s for imidacloprid, bifenthrin, acephate, thiamethoxam, and dicrotophos were 0.12, 0.39, 0.62, 0.67, and 3.96 ppm, respectively. LC75s for imidacloprid, bifenthrin, acephate, thiamethoxam, and dicrotophos were 0.61, 4.22, 5.10, 2.65, and 7.86 ppm, respectively. Based on the LC50s and LC75s, dicrotophos was much less toxic than the other chemicals tested. PoloMix software was used to determine syngerism, antagonism, or addition effects of the mixtures. Three out of four analyses of the joint action of bifenthrin plus imidacloprid or acephate or dicrotophos showed that toxicity was not independent and not correlated. For bifenthrin plus dicrotophos, observed mortality was greater than expected mortality at most concentrations suggesting synergism. Mixtures of bifenthrin plus imidacloprid and bifenthrin plus acephate showed observed mortality significantly less than expected, suggesting antagonism. LC50s for bifenthrin plus dicrotophos, acephate, imidacloprid, and thiamethoxam were 0.38, 1.06, 0.17, and 0.26 ppm, respectively. LC75s for bifenthrin plus dicrotophos, acephate, imidacloprid, and thiamethoxam were 13.61, 13.18, 0.67, and 0.80 ppm, respectively. Based on the LC50s and LC75s, bifenthrin plus acephate was 3- to 10-fold less toxic than the other chemicals tested. Bifenthrin plus acephate is frequently used in tank mixes to control tarnished plant bug and other cotton pests, and the effectiveness of each individual chemical appears to be reduced in one to one ratio mixtures.

Single and joint toxic effects of five selected pesticides on the early life stages of zebrafish (Denio rerio)

Instead of individual ones, pesticides are usually detected in water environment as mixtures of contaminants. Laboratory tests were conducted in order to investigate the effects of individual and joint pesticides (phoxim, atrazine, chlorpyrifos, butachlor and λ-cyhalothrin) on zebrafish (Denio rerio). Results from 96-h semi-static toxicity test indicated that λ-cyhalothrin had the greatest toxicity to the three life stages (embryonic, larval and juvenile stages) of D. rerio with LC₅₀ values ranging from 0.0031 (0.0017-0.0042) to 0.38 (0.21-0.53) mg a.i. L^-1, followed by butachlor and chlorpyrifos with LC₅₀ values ranging from 0.45 (0.31-0.59) to 1.93 (1.37-3.55) and from 0.28 (0.13-0.38) to 13.03 (7.54-19.71) mg a.i. L^-1, respectively. In contrast, atrazine showed the least toxicity with LC₅₀ values ranging from 6.09 (3.34-8.35) to 34.19 (24.42-51.9) mg a.i. L^-1. The larval stage of D. rerio was a vulnerable period to most of the selected pesticides in the multiple life stages tested. Pesticide mixtures containing phoxim and λ-cyhalothrin exerted synergistic effects on the larvae of D. rerio. Moreover, the binary mixture of phoxim-atrazine also displayed synergistic response to zebrafish. It has been assumed that most chemicals are additive in toxicity. Therefore, it is crucial to clarify the synergistic interaction for pesticide regulators and environment managers. In the present study, our data provided a clear picture on ecological risk of these pesticide mixtures to aquatic organisms. Moreover, joint effects play a more important role than individual ones, which require more attention when defining standard for water environment quality and risk assessment protocols.

The Joint Toxicity of Different Temperature Coefficient Insecticides on Apolygus lucorum (Hemiptera: Miridae)

The effect of temperature on the cotoxicity coefficient (CTC) value was used to evaluate mixture efficacy of different temperature coefficient chemicals from 15 to 35°C by exposing third-instar Apolygus lucorum (Meyer-Dür) to dip-treated asparagus bean pods. The results indicated the joint toxicity of same temperature coefficient insecticide (TCI) types were unaffected by temperature. This means that even when temperatures change, the mixture ratios of the highest CTC values remained the same, and the effect of temperature on the joint toxicity of same TCI types was only on the CTC values. However, the effect of temperature was variable when considering the joint toxicity of different TCI types. The effect of temperature on the joint toxicity of both strong positive and strong negative TCI types was clear, and the highest CTC values of mixture ratios changed with temperature regularly. When comparing the influence of temperature between strong/slight positive/negative insecticides, the results indicated a greater influence of the strong TCI. Paradoxically, the highest CTC value of the imidacloprid and methomyl mixture did not change with temperature changes consistently, even with the variance of imidacloprid ratios, a strong TCI. These results will guide pest managers in choosing the most effective insecticide mixtures for A. lucorum control under given environmental conditions.

Joint acute toxicity of the herbicide butachlor and three insecticides to the terrestrial earthworm, Eisenia fetida

The herbicide butachlor and three insecticides phoxim, chlorpyrifos, and lambda-cyhalotrhin are widely used pesticides with different modes of action. As most previous laboratory bioassays for these pesticides have been conducted solely based on acute tests with a single compound, only limited information is available on the possible combined toxicity of these common chemicals to soil organisms. In this study, we evaluated their mixture toxicity on the terrestrial earthworm, Eisenia fetida, with binary, ternary, and quaternary mixtures. Two different types of bioassays were employed in our work, including a contact filter paper toxicity test and a soil toxicity test. Mixture toxicity effects were assessed using the additive index method. For all of the tested binary mixtures (butachlor-phoxim, butachlor-chlorpyrifos, and butachlor-lambda-cyhalothrin), significant synergistic interactions were observed after 14 days in the soil toxicity assay. However, greater additive toxicity was found after 48 h in the contact toxicity bioassay. Most of the ternary and quaternary mixtures exhibited significant synergistic effects on the worms in both bioassay systems. Our findings would be helpful in assessing the ecological risk of these pesticide mixtures to soil invertebrates. The observed synergistic interactions underline the necessity to review soil quality guidelines, which are likely underestimating the adverse combined effects of these compounds.

Incidence and characterization of resistance to pyrethroid and organophosphorus insecticides in Thrips tabaci (Thysanoptera: Thripidae) in onion fields in Isfahan, Iran

Onion thrips, Thrips tabaci Lindeman, is the main pest of onion-growing fields in Isfahan and is mainly controlled by frequently spraying several insecticides. To investigate the resistance status and mechanisms, the susceptibility of ten field populations collected from Isfahan onion-growing regions were tested to several currently used pesticides. Resistance to the tested insecticides was observed in most populations when compared with the susceptible reference population. Enhanced detoxification, implicated by the use of inhibitors of major metabolic detoxification enzymes, was observed in the populations resistant to profenofos and chlorpyrifos. In the deltamethrin resistant populations, the amino acid substitution T929I was detected in the voltage gated sodium channel, which is known to confer pyrethroid resistance. These data are a first step towards more efficient resistance management tactics through early detection of resistant onion thrips in Iran.

Cyantraniliprole: a valuable tool for Frankliniella occidentalis (Pergande) management

BACKGROUND

Frankliniella occidentalis is a worldwide economically important pest. Scarcity of effective products and cross-resistance issues make resistance to existing insecticides a recurring problem that requires the development of new control tools, such as incorporating novel compounds. Lethal effects of cyantraniliprole on adults and larvae from field and insecticide-resistant populations were evaluated. In addition, the sublethal effects on biological features such as fecundity, fertility, feeding, oviposition and mating were studied.

RESULTS

Results obtained for larvae produced LC50 values ranging from 33.4 to 109.2 mg L(-1) , with a low natural variability (3.3-fold) and a LC50 composite value of 52.2 mg L(-1) . The susceptibility for adults was 23-fold lower than for larvae. No evidence of cross-resistance between cyantraniliprole and established insecticides used against thrips was evident. Relevant sublethal effects of cyantraniliprole were demonstrated, including reduced fecundity, fertility, feeding, oviposition and mating success.

CONCLUSION

Low variation in susceptibility across contemporary populations of F. occidentalis and a lack of cross-resistance to other insecticides indicate that cyantraniliprole is a potential candidate in rotation programmes within an insecticide resistance management strategy. The combined sublethal effect on reproduction will have an important impact on population reduction. Available data indicate that cyantraniliprole is likely to be a valuable tool for managing thrips populations.

Abamectin resistance in strains of vegetable leafminer, Liriomyza sativae (Diptera: Agromyzidae) is linked to elevated glutathione S-transferase activity

Abamectin resistance was selected in the vegetable leafminer, Liriomyza sativae (Blanchard) (Diptera: Agromyzidae) under laboratory conditions, and cross-resistance patterns and possible resistance mechanisms in the abamectin-resistant strains (AL-R, AF-R) were investigated. Compared with the susceptible strain (SS), strain AL-R displayed 39-fold resistance to abamectin after 20 selection cycles during 25 generations, and strain AF-R exhibited 59-fold resistance to abamectin after 16 selection cycles during 22 generations. No cross-resistance to cyromazine was found in both abamectin-resistant strains. However, we failed to select for cyromazine resistance in L. sativae under laboratory conditions by conducting 17 selection cycles during 22 generations. However, moderate levels of cross-resistance to abamectin (6-9 fold) were observed in strains which received cyromazine treatments. Biochemical analysis showed that glutathione S-transferase (GST) activity in both abamectin-resistant strains (AL-R, AF-R) was significantly higher than in the susceptible strain (SS), suggesting metabolically driven resistance to abamectinin L. sativae. Recommendations of mixtures or rotation of cyromazine and abamectin should be considered carefully, as consecutive cyromazine treatments may select for low-level cross-resistance to abamectin.

Soluble and membrane-bound Drosophila melanogaster CYP6G1 expressed in Escherichia coli: purification, activity, and binding properties toward multiple pesticides

Cytochrome P450 CYP6G1 has been implicated in the resistance of Drosophila melanogaster to numerous pesticides. While in vivo and in vitro studies have provided insight to the diverse functions of this enzyme, direct studies on the isolated CYP6G1 enzyme have not been possible due to the need for a source of recombinant enzyme. In the current study, the Cyp6g1 gene was isolated from D. melanogaster and re-engineered for heterologous expression in Escherichia coli. Approximately 460 nmol L⁻¹ of P450 holoenzyme were obtained in 500 mL cultures. The recombinant enzyme was located predominantly within the bacterial cytosol. A two-step purification protocol using Ni-chelate affinity chromatography followed by removal of detergent on a hydroxyapatite column produced essentially homogenous enzyme from both soluble and membrane fractions. Recombinant CYP6G1 exhibited p-nitroanisole O-dealkylation activity but was not active against eleven other typical P450 marker substrates. Substrate-induced binding spectra and IC₅₀ values for inhibition of p-nitroanisole O-dealkylation were obtained for a wide selection of pesticides, namely DDT, imidacloprid, chlorfenvinphos, malathion, endosulfan, dieldrin, dicyclanil, lufenuron and carbaryl, supporting previous in vivo and in vitro studies on Drosophila that have suggested that the enzyme is involved in multi-pesticide resistance in insects.

Thiamethoxam acts as a target-site synergist of spinosad in resistant strains of Frankliniella occidentalis

BACKGROUND

Previous studies have suggested that the resistance mechanism towards spinosad in Frankliniella occidentalis (Pergande) is an altered target site. Like the neonicotinoids, the spinosyns act on nicotinic acetylcholine receptors (nAChRs) in insects, but at a distinct site. The changes in nAChRs related to spinosad resistance in thrips might involve interaction with neonicotinoids. In this study, the efficacy of spinosad and neonicotinoids, alone and in combination, was evaluated in susceptible and spinosad-resistant thrips strains.

RESULTS

The neonicotinoids tested were imidacloprid, thiacloprid, acetamiprid, thiamethoxam and clothianidin. No cross-resistance was shown between spinosad and any of the neonicotinoids. However, an increased toxicity was observed when a mixture of spinosad with thiamethoxam or clothianidin was tested. No synergism was found in the susceptible strains. The more spinosad-resistant the thrips strain, the stronger was the synergism.

CONCLUSION

Data suggest that spinosad and thiamethoxam may interact at the nAChRs in spinosad-resistant thrips, facilitating enhanced insecticidal action.

Toxicity of thiamethoxam and mixtures of chlorantraniliprole plus acetamiprid, esfenvalerate, or thiamethoxam to neonates of oriental fruit moth (Lepidoptera: Tortricidae)

To assess the toxicity ofthiamethoxam and three mixtures of insecticides to oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae), we added the insecticides to diet and fed it to neonates of two laboratory colonies; mortality was assessed after 96 h. Thiamethoxam was much less toxic than insecticides previously tested. Five of six analyses of the joint action of chlorantraniliprole plus acetamiprid, esfenvalerate, or thiamethoxam indicated that toxicity was not independent and not correlated. For chlorantraniliprole plus acetamiprid, mortality was slightly lower than expected at low concentrations and greater than expected at high concentrations. For chlorantraniliprole plus esfenvalerate, mortality was less than expected at nearly all concentrations, suggesting antagonism despite the two compounds' different modes of action. For chlorantraniliprole plus thiamethoxam, observed mortality exceeded expected mortality at low concentrations, but this trend did not continue at higher concentrations. Although the null hypothesis of independent and uncorrelated toxicity was rejected for chlorantraniliprole plus acetamiprid and chlorantraniliprole plus thiamethoxam in three of four analyses, differences between observed and expected mortality were minor and inconsistent over the range of concentrations tested. We do not expect these mixtures to exhibit significant synergism or antagonism in the field. Apparent antagonism between chlorantraniliprole and esfenvalerate is particularly relevant because these insecticides (or chlorantraniliprole plus a different pyrethroid) may be used together in apples or peaches for control of oriental fruit moth and hemipteran pests. The effectiveness of each insecticide against oriental fruit moth might be reduced in such applications.

Western flower thrips resistance to insecticides: detection, mechanisms and management strategies

Insecticide resistance continues to be one of the most important issues facing agricultural production. The challenges in insecticide resistance and its management are exemplified by the situation with the western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). This highly invasive pest has a great propensity for developing insecticide resistance because of its biological attributes, and cases of resistance to most classes of insecticides used for its management have been detected. To combat insecticide resistance in the western flower thrips, several insecticide resistance management (IRM) programs have been developed around the world, and these are discussed. Successful programs rely on non-insecticidal tactics, such as biological and cultural controls and host plant resistance, to reduce population pressures, rotations among insecticides of different mode of action classes to conserve insecticide efficacy, resistance monitoring, sampling to determine the need for insecticide applications and education to assure proper implementation. More judicious insecticide use is possible with the development of well-founded economic thresholds for more cropping systems. While growers will continue to rely on insecticides as part of western-flower-thrips- and thrips-transmitted virus management, more effective management of these pests will be achieved by considering their management in the context of overall integrated pest management, with IRM being a key component of those comprehensive programs.

Baseline toxicity and field efficacy of metaflumizone on Colorado potato beetle (Coleoptera: Chrysomelidae)

Baseline toxicity levels to a novel semicarbazone insecticide, metaflumizone were established for 25 field populations of Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae),from North America. Excluding the susceptible laboratory strain, 50% lethal concentrations of metaflumizone ranged from 0.57 to 1.31 ppm, while response slopes ranged from 1.92 to 4.24 (average = 2.93), and were unrelated to the 50% lethal concentration (r = 0.06; P = 0.76). Beetle populations with known resistance to the neonicotinoid imidacloprid also exhibited the highest LC50 levels to metaflumizone suggesting at least the possibility of cross-resistance. Additional experiments using a potato leaf-dip bioassay as well as field efficacy evaluations confirmed the high level of toxicity of metaflumizone to L. decemlineata and demonstrated a potential benefit of tank mixing a low rate of the pyrethroid esfenvalerate with metaflumizone at one-tenth the recommended field rate. These research findings confirm that metaflumizone is highly active against L. decemlineata larvae and adults and could provide an effective alternative insecticide for potato pest management.

Esterase inhibition by synergists in the western flower thrips Frankliniella occidentalis

BACKGROUND

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is among the most important crop pests in the south-eastern region of Spain. Its increasing resistance to insecticides constitutes a serious problem, and understanding the mechanisms involved is therefore of great interest. Use of synergists to inhibit the enzymes involved in insecticide detoxification is widely used to determine their responsibility for insecticide resistance. However, they do not always act as intended or expected, and caution must be exercised when interpreting synergist results.

RESULTS

Laboratory-selected strains of WFT were used to analyse the effects of the synergists piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and methiocarb on total esterase activity. Significant differences were found, indicating esterase activity inhibition by DEF, a lower effect for methiocarb and a small inhibition of the activity by PBO. Esterase isoenzyme inhibition by these compounds showed a similar result; this assay revealed an extreme sensitivity of Triplet A (resistance-associated esterases) to DEF. In an in vivo assay carried out with these compounds at different incubation times, only DEF caused posterior in vitro esterase activity inhibition, with a maximum effect 1 h after treatment.

CONCLUSION

In this work, only DEF shows true synergistic inhibition of WFT esterases.

English field samples of Thrips tabaci show strong and ubiquitous resistance to deltamethrin

BACKGROUND

Onion thrips, Thrips tabaci Lindeman, is a prevalent pest of glasshouse and field vegetables, such as leeks and salad onions. Pesticides are the main option for control, but this approach risks the evolution and spread of insecticide resistance. In the light of anecdotal reports in the United Kingdom of poor efficacy of pyrethroids against T. tabaci, thought to be primarily a consequence of resistance, the responses to deltamethrin of ten English samples of T. tabaci, collected from field leeks and salad onions in 2006, were measured to establish the level and prevalence of potential resistance to this compound.

RESULTS

All the samples showed significant deltamethrin resistance that was comparable or slightly lower than the maximum level reported previously for T. tabaci from New Zealand.

CONCLUSION

The mechanism(s) conferring deltamethrin resistance remain unknown at present but are likely to extend to other synthetic pyrethroids approved for use against this species. The findings suggest that these compounds are unlikely to give satisfactory control of T. tabaci in England and have stimulated the recommendation to use alternatives.

Insecticide resistance of Triatoma infestans (Hemiptera, Reduviidae) vector of Chagas disease in Bolivia

OBJECTIVE

To define the insecticide resistance status of Triatoma infestans to deltamethrin (pyrethroid), malathion (organophosphate) and bendiocarb (carbamate) in Bolivia.

METHODS

Fifty populations of T. infestans were sampled in Bolivian human dwellings. Quantal response data were obtained by topical applications of 0.2 μl of insecticide-acetone solutions on nymphs N1 of the F1 generations. For most populations, dose-mortality relationships and resistance ratios (RR) were analysed. Discriminating concentrations were established for each insecticide with a susceptible reference strain and used on the other field populations. A tarsal-contact diagnostic test using insecticide impregnated papers was designed to rapidly identify deltamethrin-resistant populations in the field.

RESULTS

Discriminating concentrations for topical applications were 5, 70 and 120 ng active ingredient per insect for deltamethrin, bendiocarb and malathion, respectively. The diagnostic concentration for deltamethrin was 0.30% for the 1-h exposure by tarsal contact. All populations sampled in human dwellings exhibited significant levels of resistance to deltamethrin, from 6 to 491 and varied among regions. Resistant populations did not recover complete susceptibility to deltamethrin when the synergist piperonyl butoxide (PBO) was used. None of the sampled populations exhibited significant resistance to bendiocarb (all RR(50) < 1.8) or malathion (all RR(50) < 2.2).

CONCLUSION

In Bolivia, most 'domestic' T. infestans populations are resistant to deltamethrin. Because insecticide vector control is the only selection pressure, resistance likely originates from it. Switching from pyrethroids to organophosphates or carbamates could be a short-term solution to control this vector, but other alternative integrated control strategies should also be considered in the long term.

RNAi of ace1 and ace2 in Blattella germanica reveals their differential contribution to acetylcholinesterase activity and sensitivity to insecticides

Cyclorrhapha insect genomes contain a single acetylcholinesterase (AChE) gene while other insects contain at least two ace genes (ace1 and ace2). In this study we tested the hypothesis that the two ace paralogous from Blattella germanica have different contributions to AChE activity, using RNA interference (RNAi) to knockdown each one individually. Paralogous-specific depletion of Bgace transcripts was evident in ganglia of injected cockroaches, although the effects at the protein level were less pronounced. Using spectrophotometric and zymogram measurements, we obtained evidence that BgAChE1 represents 65-75% of the total AChE activity in nerve tissue demonstrating that ace1 encodes a predominant AChE. A significant increase in sensitivity of Bgace1-interfered cockroaches was observed after 48 h of exposure to chlorpyrifos. In contrast, Bgace2 knockdown had a negligible effect on mortality to this organophosphate. These results point out a key role, qualitative and/or quantitative, of AChE1 as target of organophosphate insecticides in this species. Silencing the expression of Bgace1 but not Bgace2 also produced an increased mortality in insects when synergized with lambda-cyhalothrin, a situation which resembles the synergistic effects observed between organophosphates and pyrethroids. Gene silencing of ace genes by RNAi offers an exciting approach for examining a possible functional differentiation in ace paralogous.

Insecticide resistance management strategies against the western flower thrips, Frankliniella occidentalis

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an economically important pest of a wide range of crops grown throughout the world. Insecticide resistance has been documented in many populations of WFT. Biological and behavioural characteristics and pest management practices that promote insecticide resistance are discussed. In addition, an overview is provided of the development of insecticide resistance in F. occidentalis populations and the resistance mechanisms involved. Owing to widespread resistance to most conventional insecticides, a new approach to insecticide resistance management (IRM) of F. occidentalis is needed. The IRM strategy proposed consists of two parts. Firstly, a general strategy to minimise the use of insecticides in order to reduce selection pressure. Secondly, a strategy designed to avoid selection of resistance mechanisms, considering cross-resistance patterns and resistance mechanisms.

Inheritance of resistance to acrinathrin in Frankliniella occidentalis (Thysanoptera: Thripidae)

BACKGROUND

The western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an economically important pest. The genetic basis of acrinathrin resistance was investigated in WFT.

RESULTS

The resistant strain, selected in the laboratory for acrinathrin resistance from a pool of thrips populations collected in Almeria (south-eastern Spain), showed a high resistance to acrinathrin (43-fold based on LC(50) values) compared with the laboratory susceptible strain. Mortality data from reciprocal crosses of resistant and susceptible thrips indicated that resistance was autosomal and not influenced by maternal effects. Analysis of probit lines from the parental strains and reciprocal crosses showed that resistance was expressed as a codominant trait. To determine the number of genes involved, a direct test of monogenic inheritance based on the backcrosses suggested that resistance to acrinathrin was probably controlled by one locus. Another approach, which was based on phenotypic variances, showed n(E), or the minimum number of freely segregating genetic factors for the resistant strain, to be 0.79.

CONCLUSION

The results showed that acrinathrin resistance in WFT was autosomal and not influenced by maternal effects, and was expressed as a codominant trait, probably controlled by one locus.