Insecticidal and Behavioral Avoidance Responses of Anopheles minimus and Culex quinquefasciatus (Diptera: Culicidae) to Three Synthetic Repellents

Developmental and cardiac toxicity assessment of Ethyl 3-(N-butylacetamido) propanoate (EBAAP) in zebrafish embryos

Ethyl 3-(N-butylacetamido) propanoate (EBAAP) is one of the most widely used mosquito repellents worldwide, and is also commonly used to produce cosmetics. Residues have recently been detected in surface and groundwater in many countries, and their potential to harm the environment is unknown. Therefore, more studies are needed to fully assess the toxicity of EBAAP. This is the first investigation into the developmental toxicity and cardiotoxicity of EBAAP on zebrafish embryos. EBAAP was toxic to zebrafish, with a lethal concentration 50 (LC50) of 140 mg/L at 72 hours post fertilization (hpf). EBAAP exposure also reduced body length, slowed the yolk absorption rate, induced spinal curvature and pericardial edema, decreased heart rate, promoted linear lengthening of the heart, and diminished cardiac pumping ability. The expression of heart developmental-related genes (nkx2.5, myh6, tbx5a, vmhc, gata4, tbx2b) was dysregulated, intracellular oxidative stress increased significantly, the activities of catalase (CAT) and superoxide dismutase (SOD) decreased, and malondialdehyde (MDA) content increased significantly. The expression of apoptosis-related genes (bax/bcl2, p53, caspase9, caspase3) was significantly upregulated. In conclusion, EBAAP induced abnormal morphology and heart defects during the early stages of zebrafish embryo development by potentially inducing the generation and accumulation of reactive oxygen species (ROS) in vivo and activating the oxidative stress response. These events dysregulate the expression of several genes and activate endogenous apoptosis pathways, eventually leading to developmental disorders and heart defects.

The effect of transfluthrin-treated jute and cotton emanator vests on human landing and fecundity of Anopheles minimus in Thailand

Complementary approaches to tackle outdoor and early evening biting mosquitoes are urgently required. Transfluthrin (TFT) is a volatile pyrethroid capable of altering mosquito feeding behavior. This study investigated the efficacy of TFT-treated jute (TFT-J) and cotton (TFT-C) fabrics on human landing activity, feeding and fecundity of Anopheles minimus in Thailand. Jute and cotton fabrics each measuring 1024 cm² were impregnated with 7.34 × 10^-4 g/cm² TFT (20%, w/v), and evaluated in a semi-field screen house system. Two collectors, wearing an untreated control or TFT-treated vests, conducted human-landing collections of released 100 laboratory-reared adult females of An. minimus from 18:00-00:00 h for 16 consecutive nights. Recaptured mosquitoes were given a blood meal for 30 min. with a membrane feeding system for assessment of blood feeding and fecundity. TFT-J, relative to control, significantly reduced human landings (Odds Ratio (OR) =0.27 (95% Confidence Interval (CI) [0.10-0.74], p = 0.011)), however no significant reduction was observed for TFT-C (OR=0.67 [95% CI 0.24-1.82], p = 0.43). Blood feeding was significantly lower among mosquitoes exposed to TFT-J (12.45% [95% CI, 2.04-22.85], p = 0.029) and TFT-C (13% [95% CI, 0.99-26.84], p = 0.016) relative to control. Impregnated fabrics had no effect on the mean number of egg oviposition. However, egg hatchability was reduced in TFT-J (49.5% [95% CI, 21.74-77.26], p = 0.029) and TFT-C (40.2% [95% CI, 17.21-63.19], p = 0.008) relative to control. TFT-J significantly reduced the landing, blood feeding, and fertility of An. minimus. Further studies are needed to evaluate different treatment methods on fabrics and their incorporation in integrated mosquito management.

Semi-field evaluation of human landing catches versus human double net trap for estimating human biting rate of Anopheles minimus and Anopheles harrisoni in Thailand

Background

Whilst the human landing catch (HLC) technique is considered the 'gold standard' for estimating human-biting rates, it is labor-intensive and fraught with potential risk of exposure to infectious mosquito bites. This study evaluated the feasibility and performance of an alternative method, the human double net trap (HDNT) relative to HLC for monitoring host-seeking malaria vectors of the Anopheles minimus complex in a semi-field system (SFS).

Methods

HDNT and HLC were positioned in two rooms, 30 m apart at both ends of the SFS. Two human volunteers were rotated between both traps and collected released mosquitoes (n = 100) from 6:00 pm till 6:00 am. Differences in Anopheles mosquito densities among the trapping methods were compared using a generalized linear model based on a negative binomial distribution.

Results

There were 82.80% (2,136/2,580) of recaptures of wild-caught and 94.50% (2,835/3,000) of laboratory-reared mosquitoes that were molecularly identified as An. harrisoni and An. minimus, respectively. Mean density of An. harrisoni was significantly lower in HNDT (15.50 per night, 95% CI [12.48-18.52]) relative to HLC (25.32 per night (95% CI [22.28-28.36]), p < 0.001). Similarly, the mean density of a laboratory strain of An. minimus recaptured in HDNT was significantly lower (37.87 per night, 95% CI [34.62-41.11]) relative to HLC (56.40 per night, 95% CI [55.37-57.43]), p < 0.001. Relative sampling efficiency analysis showed that HLC was the more efficient trap in collecting the An. minimus complex in the SFS.

Conclusion

HDNT caught proportionately fewer An. minimus complex than HLC. HDNT was not sensitive nor significantly correlated with HLC, suggesting that it is not an alternative method to HLC.

Behavioral avoidance and biological safety of vetiver oil and its constituents against Aedes aegypti (L.), Aedes albopictus (Skuse) and Culex quinquefasciatus Say

Numerous plant-based repellents are widely used for personal protection against host-seeking mosquitoes. Vitiveria zizanioides (L.) Nash essential oil and its constituents have demonstrated various mosquito repellent activities. In this study, three chemical actions of vetiver oil and five constituents (terpinen-4-ol, α-terpineol, valencene, vetiverol and vetivone) were characterized against Aedes aegypti, Aedes albopictus and Culex quinquefasciatus by using the high-throughput screening assay system (HITSS). Significant contact escape responses in Ae. aegypti and Ae. albopictus to all test compounds at concentrations between 2.5 and 5% were observed. Spatial repellency responses were also observed in some tested mosquito populations depending upon concentrations. The most significant toxic response on mosquitoes was found at the highest concentration, except for vetivone which had no toxic effect on Ae. aegypti and Ae. albopictus. Results on phototoxic and genotoxic hazard revealed that vetiver oil and their constituents showed no phototoxic potential or any significant genotoxic response. In conclusion, vetiver oil and two constituents, valencene and vetiverol, are potentials as active ingredients for mosquito repellency and present no toxicity.

Enhanced Excito-Repellency of Binary Mixtures of Plant-Based Mosquito Repellents Against Culex quinquefasciatus Say (Diptera: Culicidae), a Night Biting Mosquito Species

Efficacies of essential oils (EOs) of Vetiveria zizanioides (L.) Nash. (Poales: Poaceae) (VZ EO), Cananga odorata (Lam) Hook. F. & Thomson (Magnoliales: Annonaceae) (CO EO), and crude extract (CE) of Andrographis paniculata (Burm.F.) Wall ex. Nees (Lamiales: Acanthaceae) (AP CE), against laboratory (lab) and field strains of Culex quinquefasciatus Say were investigated. Irritant and repellent activities of individual and binary mixtures of plant extracts were compared with N,N-diethyl-m-toluamide (DEET) using an excito-repellency system. The irritant activity (direct tarsal contact), the mean percent escape response of VZ EO (91.67%, 83.33%), and CO EO (80%, 88.33%) were not significantly different compared with DEET (88.33%, 95%) against lab and field strains, respectively. Similarly, irritant responses in combinations (1:1 and 1:2, v:v) of either VZ EO or CO EO with AP CE were not significantly different from DEET against both strains (P > 0.001). The repellent activity (no tarsal contact), the mean percent escape response of VZ EO (68.33%), CO EO (61.67%), and VZ EO+AP CE (1:1, v:v) (81.67%) against lab strain and CO EO (85%) against field strain were not significantly different from that of DEET (P > 0.001). Interestingly, the greatest contact irritancy of VZ EO+AP CE (1:1, v:v) (96.67%) (P = 0.0026) and a stronger repellency response of CO EO (85%) (P = 0.0055) produced significantly different patterns of escape response compared with DEET against both lab and field strains, respectively. The EOs of VZ EO and CO EO or their mixture with AP CE showed potential as plant-based active ingredients for mosquito repellents. In addition, the major chemical constituents of VZ EO were β-vetivone (6.4%), khusimol (2.96%), and α-vetivone (2.94%) by gas chromatograpy-mass spectrometry.

Time of Test Periods Influence the Behavioral Responses of Anopheles minimus and Anopheles dirus (Diptera: Culicidae) to DEET

Information on factors influencing the behavioral responses of mosquitoes to repellents is lacking and poorly understood, especially in the Anopheles species, night-biting mosquitoes. Our goal was to investigate the impact of different time periods on circadian activity and behavioral responses of two malaria vectors, Anopheles minimus and An. dirus, to 5% DEET using an excito-repellency test system. Each mosquito species was exposed to the repellent during the daytime (06.00-18.00) and nighttime (18.00-06.00), and time of observation was further divided into four 3-h intervals. Significant escape responses were observed between daytime and nighttime for An. minimus in both noncontact and contact tests. An. dirus showed statistical differences in contact irritancy escape response, whereas no significant difference was found in noncontact repellency tests. Both mosquito species showed more significantly higher escape responses when exposed to DEET during the afternoon and late in the night. This finding indicates that the time of testing may affect the behavioral responses of mosquitoes to repellents, especially in An. minimus and An. dirus. A better understanding of nocturnally active mosquito behavioral responses spanning from dusk to dawn would assist in optimizing product development, screening, and effective evaluation.

Behavioral responses to transfluthrin by Aedes aegypti, Anopheles minimus, Anopheles harrisoni, and Anopheles dirus (Diptera: Culicidae)

Airborne spatial repellency (SR) is characterized and distinguished from other chemical actions including contact locomotor excitation and toxicity. The use of volatile spatial repellents is a potential new intervention class for combatting mosquito-borne pathogen transmission; therefore, continuing investigations on the actions of these chemicals that modify mosquito host‐seeking behavior (i.e., bite prevention) is needed. The objective of this study is to characterize the key behavioral avoidance actions of transfluthrin (TFT) to advance spatial repellent development into practical products. Behavioral avoidance responses were observed for adult laboratory strains of Aedes aegypti, Anopheles minimus and An. dirus, and two field populations of An. harrisoni and Ae. aegypti, respectively. Established TFT sublethal (LC₅₀ and LC₇₅), lethal concentrations (LC₉₉) and discriminating concentrations (DCs) were selected corresponding to each mosquito test species. Spatial repellency and contact excitation (‘irritancy’) responses on adult mosquitoes to TFT were assessed using an excito-repellency assay system. At LC₅₀, TFT exhibited strong avoidance with An. minimus (60.1% escape) and An. dirus (80% escape) laboratory strains, showing between 12 and 16x greater escape response than Ae. aegypti (5% escape). Repellency responses for field collected Ae. aegypti and An. harrisoni were 54.9 and 47.1% escape, respectively. After adjusting the initial contact escape response (a measure of combined irritancy and repellency) to estimate only escape due to contact, the LC₅₀ and LC₉₉ showed moderate escape irritancy with laboratory Ae. aegypti (41.4% escape) and no contact activity against the field population. Adjustment showed only weak contact activity (16.1% escape) in laboratory An. minimus at LC₅₀. Spatial repellency is the predominant mode of action of TFT among colonized and field mosquitoes used in this study. Established baseline (susceptible) dose-response curves assist in optimizing SR products for mosquito control and pathogen transmission prevention.

Excito-repellency Activity of Andrographis paniculata (Lamiales: Acanthaceae) Against Colonized Mosquitoes

Excito-repellency activity of plant extracts have been increasingly studied as mosquito repellents. In this study, the crude extract of Andrographis paniculata was evaluated for its noncontact repellency, contact excitation (irritancy + repellency), and knockdown/toxicity response against five colonized mosquitoes; Aedes aegypti (L.), Aedes albopictus (Skuse), Anopheles dirus Peyton & Harrison, Anopheles epiroticus Linton & Harbach, and Culex quinquefasciatus Say (Diptera: Culicidae) using an excito-repellency assay system under laboratory-controlled conditions. The escape responses were observed at four different concentrations (0.5-5.0% w/v) with A. paniculata showing strong spatial repellency against Ae. albopictus (96.7% escape) and Ae. aegypti (71.7% escape) at the 2.5% and 0.5% concentrations, respectively. At 0.5% and 5.0% concentrations, the greatest repellency was seen for An. dirus (48.2% escape) and Cx. quinquefasciatus (59.7% escape), respectively. Comparatively, low repellency action was observed against An. epiroticus (1.6-15.0% escape). Escape in contact assays (before adjustment) was generally less pronounced compared to noncontact spatial repellency, with Ae. albopictus showing highest percent escape (71.4% escape) in the contact assay at 1.0% concentration. After adjusting for spatial repellency, escape due to contact irritancy alone was either not present or an insignificant contribution to the overall avoidance response for all species. No knockdown or mortality at 24-h postexposure was observed in any trials. These findings indicate that the A. paniculata crude extract is more active against day-biting mosquitoes; however, this may be a reflection of the time of testing. This study demonstrates compelling evidence that A. paniculata extract performs primarily as a spatial repellent. Further investigations exploring the use A. paniculata as a potential active ingredient in repellent products are needed.

Entomology in the 21st Century: Tackling Insect Invasions, Promoting Advancements in Technology, and Using Effective Science Communication-2018 Student Debates

The 2018 student debates of the Entomological Society of America were held at the Joint Annual Meeting for the Entomological Societies of America, Canada, and British Columbia in Vancouver, BC. Three unbiased introductory speakers and six debate teams discussed and debated topics under the theme 'Entomology in the 21st Century: Tackling Insect Invasions, Promoting Advancements in Technology, and Using Effective Science Communication'. This year's debate topics included: 1) What is the most harmful invasive insect species in the world? 2) How can scientists diffuse the stigma or scare factor surrounding issues that become controversial such as genetically modified organisms, agricultural biotechnological developments, or pesticide chemicals? 3) What new/emerging technologies have the potential to revolutionize entomology (other than Clustered Regularly Interspaced Short Palindromic Repeats)? Introductory speakers and debate teams spent approximately 9 mo preparing their statements and arguments and had the opportunity to share this at the Joint Annual Meeting with an engaged audience.

Oxidative stress and neuromodulatory effects of deltamethrin and its combination with insect repellents in rats

Deltamethrin (DEL) and other synthetic repellents like N,N-diethyl benzamide (DEB), N,N-diethyl phenylacetamide (DEPA), and N,N-diethyl meta toluamide (DEET) are widely used due their high efficacy against mosquitoes. We evaluated the safe dose on exposure to insecticide and insect repellent individually and in combination in Wistar rats. Rats were administrated with individual chemicals DEL, DEB, DEPA, and DEET and in combination of DEL along DEB, DEPA, and DEET. The reduced glutathione and its detoxifying enzymes were significantly increased in DEL treated rats. Serotonin, dopamine, and noradrenaline were increased in DEL alone and its combination groups. Histopathology revealed that DEL and along with other insect repellants can cause lung toxicities. This study suggests that DEL in combination with other insect repellents showed antagonistic effect against oxidative stress, and there was no significant toxicological effect in the combination groups. The insect repellents with DEL cause less toxic effect and is safe for use.