Behavioral insensitivity to DEET in Aedes aegypti is a genetically determined trait residing in changes in sensillum function

Repellency, toxicity, and physiological actions of low molecular weight basic amines in mosquitoes

BACKGROUND

This study investigated the behavioral responses and toxicity of three basic amines: 1-methylpiperazine, 1-methylpyrrolidine, and triethylamine (TEA), compounds suggested previously to be anosmic in vapor exposures to caged mosquitoes.

RESULTS

These compounds showed repellency of Aedes aegypti mosquitoes, followed by flightlessness, knockdown, and paralysis, all increasing with exposure time and dosage. Electrophysiological experiments showed a blocking effect on nerve discharge of the Drosophila melanogaster larval central nervous system (CNS) with little evidence of hyperexcitation. Blockage of voltage-gated (Kv2) potassium channel currents under patch clamp occurred at similar concentrations. Involvement of K+ channels in the action of basic amines was supported by behavior and CNS recordings of a Shaker Kv1 mutant exposed to TEA, where instead of blockage, a hyperexcitation of nerve firing was observed. Experiments on cockroach leg mechanoreceptors demonstrated neuronal excitation and on mosquito antennae strong electroantennogram (EAG) signals with an augmentation of blank air responses after a single puff of basic amine.

CONCLUSIONS

The neurophysiological effects of basic amines are consistent with K+ channel block, whereas the antennal EAG response was not obviously associated with anosmia. The low-dose effects of basic amines appear to be repellency and bradykinesia. Overall, the findings provide key insights into the mechanisms underlying the biological activity of basic amines. © 2024 Society of Chemical Industry.

In the arm-in-cage test, topical repellents activate mosquitoes to disengage upon contact instead of repelling them at distance

Topical repellents provide protection against mosquito bites and their efficacy is often assessed using the arm-in-cage test. The arm-in-cage test estimates the repellent's protection time by exposing a repellent-treated forearm to host-seeking mosquitoes inside a cage at regular intervals until the first confirmed mosquito bite. However, the test does not reveal the repellents' behavioural mode of action. To understand how mosquitoes interact with topical repellents in the arm-in-cage test, we used a 3D infrared video camera system to track individual Aedes aegypti and Anopheles stephensi females during exposure to either a repellent-treated or an untreated forearm. The repellents tested were 20% (m/m) ethanolic solutions of N, N-diethyl-meta-toluamide, p-menthane-3,8-diol, icaridin and ethyl butylacetylaminopropionate. All four repellents substantially reduced the number of bites compared to an untreated forearm, while the flight trajectories indicate that the repellents do not prevent skin contact as the mosquitoes made multiple brief contacts with the treated forearm. We conclude that, in the context of the arm-in-cage test, topical repellents activate mosquitoes to disengage from the forearm with undirected displacements upon contact rather than being repelled at distance by volatile odorants.

Tick chemosensation and implications for novel control strategies

Ticks pose a major threat to the health of humans and animals. The use of synthetic acaricides and repellents has raised the concerns of potential health and environmental risks and increasing resistance in ticks. This article highlights the importance of the research on tick chemosensation in developing novel control agents. It provides a review on our current understanding of tick chemosensory system and proposes using chemosensory receptor (CR) genes as molecular targets to discover novel tick control agents. The releases of high-quality tick genomes provide unprecedented opportunities to explore CR gene repertoires. Further functional characterization is necessary to identify the receptors for key chemical cues and signals and unravel whether tick chemosensation involves ionotropic and/or metabotropic mechanisms.

Why do repellents repel?

Repellents are used by humans and by animals to protect themselves from bites, and they contribute controlling the transmission of vector-borne diseases. There are numerous chemical formulations of both natural and synthetic origins, as well as electronic devices, that are commercially available worldwide alleged to provide personal protection against insect bites. Not all insect repellents are effective enough to provide real protection, nor are they considered safe for humans, animals or the environment. There is an urgent need to find novel, nontoxic and environmentally friendly alternatives. However, current efforts in this direction face a major obstacle: our understanding of how they keep away mosquitoes and other biting arthropods remains incomplete. This is also true for DEET, the 'gold standard' of repellents and the most widely used since its discovery in the 1940s. This article aims to guide the search for new repellents by discussing our current knowledge and future needs.

Development and laboratory validation of a plant-derived repellent blend, effective against Aedes aegypti [Diptera: Culicidae], Anopheles gambiae [Diptera: Culicidae] and Culex quinquefasciatus [Diptera: Culicidae]

Mosquitoes of the genera Aedes, Anopheles and Culex vector a wide range of pathogens seriously affecting humans and livestock on a global scale. Over-reliance on insecticides and repellents has driven research into alternative, naturally-derived compounds to fulfil the same objectives. Steam distilled extracts of four plants with strong, yet attractive, volatile profiles were initially assessed for repellency in a dual-port olfactometer using Aedes aegypti as the model species. Picea sitchensis was found to be the most repellent, proving comparable to leading products when applied at 100% (p = 1.000). Key components of conifer-derived volatile profiles were then screened via electroantennography before those components eliciting an electrophysiological response were assayed individually in the olfactometer; according to WHO protocol. The most promising 5 were selected for reductive analyses to produce an optimised semiochemical blend. This combination, and a further two variations of the blend, were then progressed to a multi-species analysis using the BG-test whereby bite-attempt frequency on hands was assessed under different repellent treatments; assays were compared between Aedes aegypti, Anopheles gambiae and Culex quinquefasciatus. Efficacy was found against all three species, although it was found that Ae. aegypti was the most susceptible to the repellent, with An. gambiae being the least. Here, a novel, naturally-derived blend is presented with weak spatial repellency, as confirmed in laboratory assays. Further work will be required to assess the full extent of the potential of the products, both in terms of field application and species screening; however, the success of the products developed demonstrate that plant metabolites have great capacity for use in the repellent sector; both to improve upon known compounds and to reduce the usage of toxic products currently on the market.

Structural Exploration of Novel Pyrethroid Esters and Amides for Repellent and Insecticidal Activity against Mosquitoes

The emergence of pyrethroid-resistant mosquitoes is a worldwide problem that necessitates further research into the development of new repellents and insecticides. This study explored the modification of existing pyrethroid acids to identify structural motifs that might not be affected by kdr active site mutations that elicit pyrethroid resistance. Because synthetic pyrethroids almost always contain activity-dependent chiral centers, we chose to focus our efforts on exploring alkoxy moieties of esters obtained with 1R-trans-permethrinic and related acids, which we showed in previous studies to have repellent and/or repellent synergistic properties. To this end, compounds were synthesized and screened for spatially acting repellency and insecticidal activity against the susceptible, Orlando, and pyrethroid-resistant, Puerto Rico, strains of Aedes aegypti mosquito. Screening utilized a high-throughput benchtop glass tube assay, and the compounds screened included a mixture of branched, unbranched, aliphatic, halogenated, cyclic, non-cyclic, and heteroatom-containing esters. Structure-activity relationships indicate that n-propyl, n-butyl, n-pentyl, cyclobutyl, and cyclopentyl substituents exhibited the most promising repellent activity with minimal kdr cross resistance. Preliminary testing showed that these small alcohol esters can be synergistic with phenyl amides and pyrethroid acids. Further derivatization of pyrethroid acids offer an interesting route to future active compounds, and while mosquitoes were the focus of this work, pyrethroid acids and esters have potential for use in reducing pest populations and damage in cropping systems as well.

Repellent Screening of Selected Plant Essential Oils Against Dengue Fever Mosquitoes Using Behavior Bioassays

Among the efforts to reduce mosquito-transmitted diseases, such as malaria and dengue fever, essential oils (EOs) have become increasingly popular as natural replacements for the repellant DEET. In this study, seven commercially available plant EOs against Aedes species mosquitoes were evaluated for their complete protection time (CPT, min) in vivo using human-hand in cage tests (GB2009/China and WHO2009). Among the EOs with the highest efficacy in repelling mosquitoes, Aedes albopictus (Skuse) were clove bud oil and patchouli oil. Both were further assessed according to the in vivo method recommended by the WHO, to determine their minimum effective dose and CPT. A comparison of the ED₅₀ values (dose yielding a 50% repellent response) of these two EOs against Aedes aegypti(L.) showed that the ED₅₀ (2.496 µg/cm²) of patchouli oil was 1248 times higher than that of clove bud oil (0.002 µg/cm²), thus demonstrating them greater efficacy of the latter in repelling Ae. aegypti mosquitoes. For the 2 EOs, eugenol was the major component with higher than 80% in relative amount of the clove bud oil. The patchouli oil had more than 30% of character chemical patchouli alcohol along with α-bulnesene (10.962%), α-guaiene (9.227%), and seychellene (7.566%). Clove bud oil was found to confer longer complete protection than patchouli oil against a common species of mosquito. These results suggest use of EOs as safe, highly potent repellents for use in daily life and against mosquito-transmitted diseases, such as malaria and dengue fever.

The structure of AgamOBP5 in complex with the natural insect repellents Carvacrol and Thymol: Crystallographic, fluorescence and thermodynamic binding studies

Among several proteins participating in the olfactory perception process of insects, Odorant Binding Proteins (OBPs) are today considered valid targets for the discovery of compounds that interfere with their host-detection behavior. The 3D structures of Anopheles gambiae mosquito AgamOBP1 in complex with the known synthetic repellents DEET and Icaridin have provided valuable information on the structural characteristics that govern their selective binding. However, no structure of a plant-derived repellent bound to an OBP has been available until now. Herein, we present the novel three-dimensional crystal structures of AgamOBP5 in complex with two natural phenolic monoterpenoid repellents, Carvacrol and Thymol, and the MPD molecule. Structural analysis revealed that both monoterpenoids occupy a binding site (Site-1) by adopting two alternative conformations. An additional Carvacrol was also bound to a secondary site (Site-2) near the central cavity entrance. A protein-ligand hydrogen-bond network supplemented by van der Waals interactions spans the entire binding cavity, bridging α4, α6, and α3 helices and stabilizing the overall structure. Fluorescence competition and Differential Scanning Calorimetry experiments verified the presence of two binding sites and the stabilization effect on AgamOBP5. While Carvacrol and Thymol bind to Site-1 with equal affinity in the submicromolar range, they exhibit a significantly lower and distinct binding capacity for Site-2 with Kd's of ~7 μΜ and ~18 μΜ, respectively. Finally, a comparison of AgamOBP5 complexes with the AgamOBP4-Indole structure revealed that variations of ligand-interacting aminoacids such as A109T, I72M, A112L, and A105T cause two structurally similar and homologous proteins to display different binding specificities.

Development of a New Sex Attractant via the Peripheral Coding of Pheromones in Mythimna loreyi

Sex pheromones play an essential role when moths are searching for mates. Male olfactory receptor neurons (ORNs) are the primary determinant during peripheral pheromone recognition. Here, we identified the sex pheromones of a global agricultural pest, Mythimna loreyi, using gas chromatography coupled with mass spectrometry and electroantennographic detection. Nine pheromone components were identified, including (Z)-9-tetradecen-1-yl acetate (Z9-14:OAc), (Z)-7-dodecen-1-yl acetate (Z7-12:OAc), and (Z)-11-hexadecen-1-yl acetate (Z11-16:OAc), and the first two elicited electrophysiological activities in the male antennae. Trichoid sensilla were classified into four functional types on the basis of neuronal responses to pheromones by single sensillum recording. Five functional ORNs were involved in recognizing pheromones and pheromone analogues. Finally, a field bioassay revealed that a blend of Z9-14:OAc, Z7-12:OAc, and Z11-16:OAc at a ratio of 100:8.8:19.7 was highly efficient for trapping males. Our results uncover the pheromone recognition mechanism in M. loreyi and provide a novel angle for developing efficient sex attractants of pests on the basis of screening the peripheral olfactory neurons.

Neuromodulation and differential learning across mosquito species

Mosquitoes can change their feeding behaviours based on past experiences, such as shifting from biting animals to biting humans or avoiding defensive hosts (Wolff & Riffell 2018 J. Exp. Biol. 221, jeb157131. (doi:10.1242/jeb.157131)). Dopamine is a critical neuromodulator for insects, allowing flexibility in their feeding preferences, but its role in the primary olfactory centre, the antennal lobe (AL), remains unclear (Vinauger et al. 2018 Curr. Biol. 28, 333-344.e8. (doi:10.1016/j.cub.2017.12.015)). It is also unknown whether mosquitoes can learn some odours and not others, or whether different species learn the same odour cues. We assayed aversive olfactory learning in four mosquito species with different host preferences, and found that they differentially learn odours salient to their preferred host. Mosquitoes that prefer humans learned odours found in mammalian skin, but not a flower odour, and a nectar-feeding species only learned a floral odour. Comparing the brains of these four species revealed significantly different innervation patterns in the AL by dopaminergic neurons. Calcium imaging in the Aedes aegypti AL and three-dimensional image analyses of dopaminergic innervation show that glomeruli tuned to learnable odours have significantly higher dopaminergic innervation. Changes in dopamine expression in the insect AL may be an evolutionary mechanism to adapt olfactory learning circuitry without changing brain structure and confer to mosquitoes an ability to adapt to new hosts.

High-throughput behavioral phenotyping of tiny arthropods: Chemosensory traits in a mesostigmatic hematophagous mite

Pest management using attractive and/or repellent semiochemicals is a key alternative to synthetic insecticides. Its implementation requires a good understanding of the intra- and interspecific chemical interactions of arthropod pests, their interactions with their abiotic environment, as well as their evolutionary dynamics. Although mites include many pest species and biocontrol agents of economic importance in agriculture, their chemical ecology is largely understudied compared to insects. We developed a high-throughput ethomics system to analyze these small arthropods and conducted a study on Dermanyssus gallinae, a problematic poultry parasite in the egg industry. Our purpose was to elucidate the role played by host-derived odorants (synthetic kairomone) and conspecific odorants (mite body odors) in D. gallinae. After validating our nanocomputer controlled olfactometric system with volatile semiochemicals of known biological activity, we characterized response traits to kairomonal and/or pheromonal volatile blends in mites from different populations. We were able to accurately characterize the repulsion or attraction behaviors in >1000 individual specimens in a standardized way. Our results confirm the presence of a volatile aggregation pheromone emitted by D. gallinae and bring new elements to the effect of odor source presentation. Our results also confirm the attractive effect on Dermanyssus gallinae of a blend of volatile compounds contained in hen odor, while highlighting a repellent effect at high concentration. Significant interindividual and interpopulation variation was noted particularly in responses to synthetic kairomone. This information lays a valuable foundation for further exploring the emergence risk of resistance to semiochemicals.

Hierarchical Virtual Screening and Binding Free Energy Prediction of Potential Modulators of Aedes Aegypti Odorant-Binding Protein 1

The Aedes aegypti mosquito is the main hematophagous vector responsible for arbovirus transmission in Brazil. The disruption of A. aegypti hematophagy remains one of the most efficient and least toxic methods against these diseases and, therefore, efforts in the research of new chemical entities with repellent activity have advanced due to the elucidation of the functionality of the olfactory receptors and the behavior of mosquitoes. With the growing interest of the pharmaceutical and cosmetic industries in the development of chemical entities with repellent activity, computational studies (e.g., virtual screening and molecular modeling) are a way to prioritize potential modulators with stereoelectronic characteristics (e.g., pharmacophore models) and binding affinity to the AaegOBP1 binding site (e.g., molecular docking) at a lower computational cost. Thus, pharmacophore- and docking-based virtual screening was employed to prioritize compounds from Sigma-Aldrich® (n = 126,851) and biogenic databases (n = 8766). In addition, molecular dynamics (MD) was performed to prioritize the most potential potent compounds compared to DEET according to free binding energy calculations. Two compounds showed adequate stereoelectronic requirements (QFIT > 81.53), AaegOBP1 binding site score (Score > 42.0), volatility and non-toxic properties and better binding free energy value (∆G < −24.13 kcal/mol) compared to DEET ((N,N-diethyl-meta-toluamide)) (∆G = −24.13 kcal/mol).

A rapid method for screening mosquito repellents on Anopheles pseudopunctipennis and Aedes aegypti

One of the main vectors for malaria in Latin America is Anopheles pseudopunctipennis (Theobald), whereas Aedes aegypti (L.) is the primary vector of dengue, yellow fever, Zika and chikungunya viruses. The use of repellents is recommended as a personal protection method against these mosquitoes. However there are very few studies evaluating the effect of repellents on An. pseudopunctipennis. The use of a Petri dish to study repellence has been applied by several authors on flies, cockroaches, kissing bugs and mosquitoes, being a valuable technique for species that are difficult to breed under laboratory conditions, such as An. pseudopunctipennis. In the present study, we evaluated the repellence of the essential oil of the Eucalyptus nitens (Shining gum), its main component (1,8-cineole) and the commercial repellent DEET on An. pseudopunctipennis and Ae. aegypti adult females using the plaque repellency method coupled to EthoVision XT10.1 video-tracking software. Repellent effect and locomotor activity were studied through a repellence index (RI) together with an axis constructed from the behavioural variables obtained using the tracking software. DEET repellent effect was observed at 0.01 mg/mL for Ae. aegypti and 0.01 and 0.1 mg/mL for An. pseudopunctipennis. In addition, the essential oil showed significant repellence at 1 and 10 mg/mL for Ae. aegypti, and 1, 5, 10 and 25 mg/mL for An. pseudopunctipennis. Neither of these species were repelled at any concentration of 1,8-cineole. This is the first study that evaluates these compounds on An. pseudopunctipennis females and quantifies their effects on the activity of both species.

Increased Repellent Effect of DEET on Aedes aegypti (Diptera: Culicidae) Field Population

Insecticides and repellents are routinely used in Brazil because of the high rates of arbovirus transmission and the nuisance caused by mosquitoes. However, few studies have assessed the effectiveness of repellents against mosquito populations that have been under exposure to xenobiotics, mainly insecticides and repellents. This study investigated the sensitivity of a field population of Aedes aegypti (Linnaeus, 1762) from a dengue-endemic area under high insecticide pressure to N,N-diethylmethylbenzamide (DEET), the active ingredient in common repellent products. The field (Laranjeiras, Sergipe, Brazil) and laboratory (Rockefeller) populations were characterized for the presence of the Val1016Ile kdr mutation, associated with pyrethroid resistance, and locomotor activity. Repellency bioassays were performed to assess the response of the mosquitoes to human odor by exposing them to 10% DEET applied to the skin in ethanol. Samples from the field population showed higher frequency of the kdr mutation, 21.9% homozygous and 21.9% heterozygous, greater locomotor activity and greater sensitivity to DEET than the laboratory population. These results suggest increased sensitivity to DEET in field populations and a possible interaction between insecticide exposure and sensitivity to DEET.

Host-Guest Inclusion Complexes of Natural Products and Nanosystems: Applications in the Development of Repellents

Repellents are compounds that prevent direct contact between the hosts and the arthropods that are vectors of diseases. Several studies have described the repellent activities of natural compounds obtained from essential oils. In addition, these chemical constituents have been pointed out as alternatives to conventional synthetic repellents due to their interesting residual protection and low toxicity to the environment. However, these compounds have been reported with short shelf life, in part, due to their volatile nature. Nanoencapsulation provides protection, stability, conservation, and controlled release for several compounds. Here, we review the most commonly used polymeric/lipid nanosystems applied in the encapsulation of small organic molecules obtained from essential oils that possess repellent activity, and we also explore the theoretical aspects related to the intermolecular interactions, thermal stability, and controlled release of the nanoencapsulated bioactive compounds.

Cellulose Hydrogels Containing Geraniol and Icaridin Encapsulated in Zein Nanoparticles for Arbovirus Control

The most important arboviruses are those that cause dengue, yellow fever, chikungunya, and Zika, for which the main vector is the Aedes aegypti mosquito. The use of repellents is an important way to combat mosquito-borne pathogens. In this work, a safe method of protection employing a repellent was developed based on a slow release system composed of zein nanoparticles containing the active agents icaridin and geraniol incorporated in a cellulose gel matrix. Analyses were performed to characterize the nanoparticles and the gel formulation. The nanoparticles containing the repellents presented a hydrodynamic diameter of 229 ± 9 nm, polydispersity index of 0.38 ± 0.10, and zeta potential of +29.4 ± 0.8 mV. The efficiencies of encapsulation in the zein nanoparticles exceeded 85% for icaridin and 98% for geraniol. Rheological characterization of the gels containing nanoparticles and repellents showed that the viscoelastic characteristic of hydroxypropylmethylcellulose gel was preserved. Release tests demonstrated that the use of nanoparticles in combination with the gel matrix led to improved performance of the formulations. Atomic force microscopy analyses enabled visualization of the gel network containing the nanoparticles. Cytotoxicity assays using 3T3 and HaCaT cell cultures showed low toxicity profiles for the active agents and the nanoparticles. The results demonstrated the potential of these repellent systems to provide prolonged protection while decreasing toxicity.

Species-Specificity in Thermopreference and CO2-Gated Heat-Seeking in Culex Mosquitoes

Combining thermopreference (Tp) and CO2-gated heat-seeking assays, we studied the thermal preferendum and response to thermal cues in three Culex mosquito species exhibiting differences in native habitat and host preference (e.g., biting cold and/or warm-blooded animals). Results show that these species differ in both Tp and heat-seeking behavior. In particular, we found that Culex territans, which feed primarily on cold-blood hosts, did not respond to heat during heat-seeking assays, regardless of the CO2 concentration, but exhibited an intermediate Tp during resting. In contrast, Cx. quinquefasciatus, which feeds on warm blooded hosts, sought the coolest locations on a thermal gradient and responded only moderately to thermal stimuli when paired with CO2 at higher concentrations. The third species, Cx. tarsalis, which has been shown to feed on a wide range of hosts, responded to heat when paired with high CO2 levels and exhibited a high Tp. This study provides the first insights into the role of heat and CO2 in the host seeking behavior of three disease vectors in the Culex genus and highlights differences in preferred resting temperatures.

Identification of a Novel Series of Potent Organosilicon Mosquito Repellents

Mosquito control by personal protection is one of the most efficient ways of curtailing deadly diseases such as malaria and dengue with the potential to save millions of lives per year. DEET (N,N-diethyl-3-methyl benzamide) is currently considered as the gold standard for mosquito repellents, being used for the past several decades. Control by DEET, however, is being threatened by emerging resistance among mosquitoes. To address this concern and also to improve protection times, we synthesized a novel series of 25 silicon-containing acyl piperidines using acid-amine coupling protocol and tested their activity against Aedes aegypti in mosquito-repellent assays. Several compounds from this series appear to possess good mosquito-repellent properties. Most notably, at 0.5 mg/cm² concentrations, the mean protection time for NDS100100 was 756 min, which was higher than that of DEET (616 min). The details of design, synthesis, and biological evaluation are discussed herein.

Engineering Yeast for De Novo Synthesis of the Insect Repellent Nepetalactone

While nepetalactone, the active ingredient in catnip, is a potent insect repellent, its low in planta accumulation limits its commercial viability as an alternative repellent. Here we describe for the first time de novo nepetalactone synthesis in Saccharomyces cerevisiae, enabling sustainable and scalable production. Nepetalactone production required introducing eight exogenous genes including the cytochrome P450 geraniol-8-hydroxylase, the bottleneck of the heterologous pathway. Combinatorial assessment of geraniol-8-hydroxylase and cytochrome P450 reductase variants, and copy-number variations were used to overcome this bottleneck. We found that several reductases improved hydroxylation activity and increasing geraniol-8-hydroxylase gene copy number improved 8-hydroxygeraniol titers. The accumulation of an unwanted metabolite implied inefficient channeling of carbon through the pathway. With the native yeast old yellow enzymes previously shown to use monoterpene intermediates as substrates, both homologues were deleted. These deletions increased 8-hydroxygeraniol yield, resulting in 3.10 mg/L/OD₆₀₀ of nepetalactone from simple sugar in microtiter plates. This optimized pathway will benefit the development of high yielding strains for the scale up production of nepetalactone.

Bioactivities and modes of action of VUAA1

BACKGROUND

The compound 2-((4-ethyl-5-(pyridin-3-yl)-4H-1,2,4-triazol-3-yl)thio)-N-(4-ethylphenyl) acetamide (VUAA1) is reported to be an odorant receptor co-receptor (Orco) agonist in insects with potential use as an insect repellent. For this study, the biological activity of VUAA1 was investigated in several bioassays with Aedes aegypti, including adult contact, spatial repellency, and larval repellency assays, as well as topical, injection, and feeding toxicity assays. Neurophysiological action was further explored by analysis of fruit fly central nervous system firing, cockroach axon recordings, patch clamp analysis of Kv2 potassium channel, and acetylcholinesterase inhibition studies. Finally, the metabolic impact on the toxicity of VUAA1 was explored by applying it in combination with established metabolic synergists.

RESULTS

In repellency and bite protection screens, VUAA1 showed little activity against adult mosquitoes, apparently due to its low volatility, since its effectiveness was increased by heating or mixing with transfluthrin acid and citronella oil. It did produce measurable repellency of mosquito larvae that was more potent than N,N-diethyl-m-toluamide (DEET). Overall, VUAA1 showed low acute toxicity to both insects and mice, and it was weakly synergized by triphenyl phosphate. There was no observed cross-resistance in a pyrethroid-resistant strain of Anopheles gambiae. VUAA1 showed a two-phase effect on the central nervous system, with neuroexcitation at 1 μmol L^-1 and an inhibitory effect at 100 μmol L^-1 that may relate to block of Kv2 potassium channels.

CONCLUSIONS

VUAA1 presented low toxicity, similar to other insect repellents. Its limited solubility, low volatility, and resulting poor adult repellency without additional adjuvants may restrict the utility of VUAA1 in typical public health applications. © 2020 Society of Chemical Industry.

Sodium channel activation underlies transfluthrin repellency in Aedes aegypti

Background

Volatile pyrethroid insecticides, such as transfluthrin, have received increasing attention for their potent repellent activities in recent years for controlling human disease vectors. It has been long understood that pyrethroids kill insects by promoting activation and inhibiting inactivation of voltage-gated sodium channels. However, the mechanism of pyrethroid repellency remains poorly understood and controversial.

Methodology/Principal findings

Here, we show that transfluthrin repels Aedes aegypti in a hand-in-cage assay at nonlethal concentrations as low as 1 ppm. Contrary to a previous report, transfluthrin does not elicit any electroantennogram (EAG) responses, indicating that it does not activate olfactory receptor neurons (ORNs). The 1S-cis isomer of transfluthrin, which does not activate sodium channels, does not elicit repellency. Mutations in the sodium channel gene that reduce the potency of transfluthrin on sodium channels decrease transfluthrin repellency but do not affect repellency by DEET. Furthermore, transfluthrin enhances DEET repellency.

Conclusions/Significance

These results provide a surprising example that sodium channel activation alone is sufficient to potently repel mosquitoes. Our findings of sodium channel activation as the principal mechanism of transfluthrin repellency and potentiation of DEET repellency have broad implications in future development of a new generation of dual-target repellent formulations to more effectively repel a variety of human disease vectors.

Vector-transmitted human diseases, such as dengue fever, represent serious global health burdens. Pyrethroids, including transfluthrin, are widely used as insecticides and repellents due to their low mammalian toxicity and relatively benign environmental impact. Pyrethroids target voltage-gated sodium channels for their insecticidal action. However, the mechanism of pyrethroid repellency remains unclear and controversial. Insect repellency is traditionally thought to be mediated by olfactory receptors. We made two important discoveries in this study, showing that transfluthrin repellency is via activation of sodium channels and transfluthrin enhances DEET repellency. Discovery of sodium channel activation as a major mechanism of pyrethroid repellency has broad significance in insect olfaction study, repellents development, and control of human disease vectors.

A Survey of Chemoreceptive Responses on Different Mosquito Appendages

Research on the functions of insect chemoreceptors have primarily focused on antennae (olfactory receptors) and mouthparts (gustatory receptors). However, chemoreceptive sensilla are also present on other appendages, such as the leg tarsi and the anterior wing margin, and their specific roles in chemoreception and mosquito behavior remain largely unknown. In this study, electrophysiological analyses in an electroantennogram recording format were performed on Aedes aegypti (L., Diptera: Culicidae) antennae, mouthparts, tarsi, and wings during exposure to a variety of insect repellent and attractant compounds. The results provide evidence that the tarsi and wings can sense chemicals in a gaseous form, and that the odors produce differing responses on different appendages. The most consistent and strongest response occurred when exposed to triethylamine (TEA). Antennae and mouthparts showed nearly identical responses pattern to all tested compounds, and their rank orders of effectiveness were similar to those of fore- and mid-leg tarsi. Hindleg tarsi only responded to TEA, indicating that the hind legs are not as chemoreceptive. Wings responded to a range of odorants, but with a different rank order and voltage amplitude. Insights gleaned into the function of these appendages in insect chemoreception are discussed.

Vapor phase repellency and insecticidal activity of pyridinyl amides against anopheline mosquitoes

It is important to identify repellents that can provide reliable protection from arthropod biting and prevent arthropod-borne diseases, such as malaria. In the present study, the spatial repellent activity and toxicity of two novel pyridinyl amides (1 and 2) were evaluated against Anopheles albimanus, Anopheles quadrimaculatus, and Anopheles gambiae. In vapor repellency bioassays, compound 2 was generally more effective than DEET and 2-undecanone, while compound 1 was about as active as these standards. Overall, transfluthrin was the most active compound for inducing anopheline mosquito repellency, knockdown, and lethality. Although they were not the most active repellents, the two experimental amides produced the largest electroantennographic responses in female antennae. They also displayed modest toxicity to anopheline mosquitoes. Significant synergism of repellency was observed for the mixture of a pyrethroid-derived acid and the repellent 2-undecanone against anopheline mosquitoes, similar to that observed previously in Aedes aegypti. Overall, this study provides insight for further synthesis of alternative amide compounds for use as spatial treatments.

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Two experimental pyridyl amides were synthesized.

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They were more repellent than DEET, equal to 2-undecanone and less than transfluthrin.

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They were about as toxic as DEET and 2-undecanone, but less than transfluthrin.

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Experimental amides performed about the same across all anopheline species.

Carboxylic acid responses by a conserved odorant receptor in culicine vector mosquitoes

Many mosquito behaviours that are critical for survival and reproduction depend upon timely responses to chemical cues. Of interest are the effects of volatile organic compounds like carboxylic acids (CAs) that are released by potential blood meal hosts. Short chain CAs are among the primary attractants for host-seeking females and influence host selection in vector species. Although the behavioural relevance of CA's has been established, less is known about the molecular receptive events that evoke responses to specific compounds, with the Ir family of chemoreceptors being broadly implicated in their detection. In this study, we demonstrate that Or orthologs from two vector species, Aedes aegypti (L.) and Aedes albopictus (Skuse), are selectively activated by straight chain carboxylic acids and that these responses are attenuated by the commercial insect repellant N,N-Diethyl-meta-toluamide. Our results suggest that multiple chemoreceptors, representing diverse families, are able to mediate molecular responses to CAs and may therefore underlie important behaviours that directly impact disease-transmission cycles.

Structure-Activity Relationship Analysis of Potential New Vapor-Active Insect Repellents

A total of 115 aryl amides were synthesized and screened for vapor repellency against the Orlando (OR) strain of Aedes aegypti mosquitoes. Of these compounds, 29 had 1 h repellency EC₅₀ values comparable to or better than N,N-diethyl-meta-toluamide (DEET, 1 h EC₅₀ value of 35 μg/cm²), with 2,2,3,3,3-pentafluoro-N-(4-fluorophenyl)propenamide (53) and 2,2,3,3,4,4,4-heptafluoro-N-(3,4,5-trifluorophenyl)butanamide (101) exhibiting the most potent EC₅₀ values of 4.5 and 2.9 μg/cm², respectively. The cross-resistance of select, highly potent, derivatives against the pyrethroid-resistant Puerto Rico (PR) strain of A. aegypti was also investigated, and little to no resistance was observed. When synergized with 1R-trans-permethrinic acid (TFA), compound 101 had a 1 h EC₅₀ value 6 times lower than metofluthrin against OR and 40 times lower against PR mosquitoes. Additionally, preliminary mammalian oral toxicity was screened for compounds 69 and 101, and both exhibited LD₅₀ values of >2000 mg/kg. The structure-activity relationship analysis, which guided the synthesis of these derivatives, is given, and key trends are highlighted to inform future analogue design.

Natural insensitivity and the effects of concentration on the repellency and survival of American dog ticks (Dermacentor variabilis) by DEET

N,N-Diethyl-m-toluamide (DEET) is by far the most used repellent worldwide. When applied topically to the skin, the active ingredient has been shown to provide protection from a variety of hematophagous insects, including mosquitoes and flies. DEET's effectiveness against ticks is influenced by a variety of factors (e.g., duration and concentration of application, drying time, route of exposure, tick species and developmental stage), and may differ from insects due to their unique chemosensory system that primarily involves the Haller's organ. We therefore used several approaches to investigate DEET's efficacy to repel Dermacentor variabilis at different concentrations (5, 30 or 75%), as well as explore its toxicological properties and natural variability in DEET insensitivity across populations from Manitoba, Canada. Climbing bioassays indicated that higher concentrations of DEET were more effective at repelling D. variabilis, and that ticks from some sampling localities were more sensitive to lower concentrations than others. Petri dish arena assays revealed ticks exposed to high concentrations of the repellent lose their ability to discriminate lower concentrations, perhaps due to overstimulation or habituation. Finally, our tactile assays demonstrated reduced tick survival after contact with high DEET concentrations, with mortality occurring more rapidly with increased concentration. Dermacentor variabilis from these tactile assays displayed a multitude of physiological and neurological symptoms, such as 'hot foot' and various bodily secretions. Overall, our study shows a strong association between repellency, concentration and the acaricidal effects of DEET on D. variabilis.

BiteOscope, an open platform to study mosquito biting behavior

Female mosquitoes need a blood meal to reproduce, and in obtaining this essential nutrient they transmit deadly pathogens. Although crucial for the spread of mosquito-borne diseases, blood feeding remains poorly understood due to technological limitations. Indeed, studies often expose human subjects to assess biting behavior. Here, we present the biteOscope, a device that attracts mosquitoes to a host mimic which they bite to obtain an artificial blood meal. The host mimic is transparent, allowing high-resolution imaging of the feeding mosquito. Using machine learning, we extract detailed behavioral statistics describing the locomotion, pose, biting, and feeding dynamics of Aedes aegypti, Aedes albopictus, Anopheles stephensi, and Anopheles coluzzii. In addition to characterizing behavioral patterns, we discover that the common insect repellent DEET repels Anopheles coluzzii upon contact with their legs. The biteOscope provides a new perspective on mosquito blood feeding, enabling the high-throughput quantitative characterization of this lethal behavior.

Cellular and Molecular Targets of Waterbuck Repellent Blend Odors in Antennae of Glossina fuscipes fuscipes Newstead, 1910

Insects that transmit many of the world's deadliest animal diseases, for instance trypanosomosis, find their suitable hosts and avoid non-preferred hosts mostly through olfactory cues. The waterbuck repellent blend (WRB) comprising geranylacetone, guaiacol, pentanoic acid, and δ-octalactone derived from waterbuck skin odor is a repellent to some savannah-adapted tsetse flies and reduces trap catches of riverine species. However, the cellular and molecular mechanisms associated with detection and coding of the repellent odors remain to be elucidated. Here, we demonstrated that WRB inhibited blood feeding in both Glossina pallidipes Austen, 1903 and Glossina fuscipes fuscipes Newstead, 1910. Using the DREAM (Deorphanization of Receptors based on Expression Alterations in odorant receptor mRNA levels) technique, combined with ortholog comparison and molecular docking, we predicted the putative odorant receptors (ORs) for the WRB in G. f. fuscipes, a non-model insect. We show that exposure of G. f. fuscipes in vivo to WRB odorant resulted in up- and downregulation of mRNA transcript of several ORs. The WRB component with strong feeding inhibition altered mRNA transcript differently as compared to an attractant odor, showing these two odors of opposing valence already segregate at the cellular and molecular levels. Furthermore, molecular dynamics simulations demonstrated that the predicted ligand-OR binding pockets consisted mostly of hydrophobic residues with a few hydrogen bonds but a stable interaction. Finally, our electrophysiological response showed the olfactory sensory neurons of G. f. fuscipes tuned to the tsetse repellent components in different sensitivity and selectivity.

Orthosteric muscarinic receptor activation by the insect repellent IR3535 opens new prospects in insecticide-based vector control

The insect repellent IR3535 is one of the important alternative in the fight against mosquito-borne disease such as malaria, dengue, chikungunya, yellow fever and Zika. Using a multidisciplinary approach, we propose the development of an innovative insecticide-based vector control strategy using an unexplored property of IR3535. We have demonstrated that in insect neurosecretory cells, very low concentration of IR3535 induces intracellular calcium rise through cellular mechanisms involving orthosteric/allosteric sites of the M1-muscarinic receptor subtype, G protein βγ subunits, background potassium channel inhibition generating depolarization, which induces voltage-gated calcium channel activation. The resulting internal calcium concentration elevation increases nicotinic receptor sensitivity to the neonicotinoid insecticide thiacloprid. The synergistic interaction between IR3535 and thiacloprid contributes to significantly increase the efficacy of the treatment while reducing concentrations. In this context, IR3535, used as a synergistic agent, seems to promise a new approach in the optimization of the integrated vector management for vector control.

Insect repellents mediate species-specific olfactory behaviours in mosquitoes

Background

The species-specific mode of action for DEET and many other mosquito repellents is often unclear. Confusion may arise for many reasons. First, the response of a single mosquito species is often used to represent all mosquito species. Second, behavioural studies usually test the effect of repellents on mosquito attraction towards human odorants, rather than their direct repulsive effect on mosquitoes. Third, the mosquito sensory neuron responses towards repellents are often not directly examined.

Methods

A close proximity response assay was used to test the direct repulsive effect of six mosquito repellents on Anopheles coluzzii, Aedes aegypti and Culex quinquefasciatus mosquitoes. Additionally, the behavioural assay and calcium imaging recordings of antennae were used to test the response of An. coluzzii mosquitoes towards two human odorants (1-octen-3-ol and benzaldehyde) at different concentrations, and mixtures of the repellents lemongrass oil and p-menthane-3,8-diol (PMD) with DEET.

Results

Anopheles coluzzii mosquitoes were repelled by lemongrass oil and PMD, while Ae. aegypti and Cx. quinquefasciatus mosquitoes were repelled by lemongrass oil, PMD, eugenol, and DEET. In addition, high concentrations of 1-octen-3-ol and benzaldehyde were repellent, and activated more olfactory receptor neurons on the An. coluzzii antennae than lower concentrations. Finally, changes in olfactory responses to repellent mixtures reflected changes in repulsive behaviours.

Conclusions

The findings described here suggest that different species of mosquitoes have different behavioural responses to repellents. The data further suggest that high-odour concentrations may recruit repellent-sensing neurons, or generally excite many olfactory neurons, yielding repellent behavioural responses. Finally, DEET can decrease the neuronal and behavioural response of An. coluzzii mosquitoes towards PMD but not towards lemongrass oil. Overall, these studies can help inform mosquito repellent choice by species, guide decisions on effective repellent blends, and could ultimately identify the olfactory neurons and receptors in mosquitoes that mediate repellency.

Pyrethroid-Derived Acids and Alcohols: Bioactivity and Synergistic Effects on Mosquito Repellency and Toxicity

Pyrethroids are one of the most commonly used classes of insecticides, and their acid and alcohol components are esterase degradation products, usually considered to be biologically inactive. In this study, it was found that several pyrethroid acids had a spatial repellent activity that was greater than DEET, often more active than the parent pyrethroids, and showed little cross resistance in a pyrethroid-resistant Puerto Rico strain of Aedes aegypti mosquitoes. Further investigation revealed that the acids can synergize not only contact repellent standards but also other pyrethroid components as well as the parent pyrethroids themselves. Synergism by the pyrethroid acids is expressed as both increased spatial repellency and vapor toxicity as well as human bite protection. Electrophysiological studies confirmed that pyrethroid acids (100 μM) had no effect on neuronal discharge in larval Drosophila melanogaster CNS and were detected by electroantennography, and there was little resistance to olfactory sensing of these acids in antennae from Puerto Rico strain mosquitoes carrying kdr mutations. Thus, the data suggest that the pyrethroid acids have a different mode of action than the parent pyrethroids, unrelated to the voltage-sensitive sodium channel. The results highlight the potential of pyrethroid acids to be useful in future repellent formulations.

Multiple channels of DEET repellency in Drosophila

BACKGROUND

N,N-Diethyl-meta-toluamide (DEET) is the prophylactic insect repellent used most widely to inhibit insect bites. Despite its use since 1944, the mechanism of DEET repellency remains controversial. Here, we revisited the role of smell and taste in DEET repellence using Drosophila as a model.

RESULTS

Analysis of the responses of individual olfactory receptor neuron (ORN) classes to DEET reveals that 11 ORNs are activated and two are inhibited by this compound. Blocking individual ORN classes in the antenna does not block DEET repellence. This argues against the existence of a single ORN mediating DEET repellence in Drosophila. Activation of all ORCO-expressing neurons using channelrhodopsin favors attraction, not repellence, in behavioral valence. We also demonstrate that gustatory neurons are highly sensitive to DEET. We used RNA interference to screen candidate receptors encoded by gene families involved in the detection of bitter compounds, including 34 gustatory receptors (Grs), 14 ionotropic receptors (Irs), five pick-pocket subunits (PPKs), three transient receptor potential ion channels (TrpA, TrpL, Painless) and one metabotropic glutamate receptors gene (DmXR). We saw striking defects in DEET-mediated oviposition behavior when expression of either Gr32a or Gr33a was inhibited.

CONCLUSION

Our findings support a multimodal mechanism for DEET detection in fruit flies and indicate a prominent role for taste detection mediating DEET repellence. © 2019 Society of Chemical Industry.

Reduced effectiveness of repellents in a pyrethroid-resistant strain of Aedes aegypti (Diptera: culicidae) and its correlation with olfactory sensitivity

BACKGROUND

The mosquito, Aedes aegypti (Diptera: Culicidae), is a vector of dengue fever, zika, chikungunya, and yellow fever viruses, and in many areas possesses significant levels of resistance to pyrethroids. Behavioral performance was assessed in 15, 30, and 60 min exposures in a high throughput vapor phase spatial repellency assay to three contact repellent standards: N,N-diethyl-3-methylbenzamide (DEET), ethyl 3-[acetyl(butyl)amino] propanoate (IR3535), and 2-undecanone, as well as pyrethrum extract, transfluthrin, and metofluthrin in susceptible (Orlando) and a pyrethroid-resistant Puerto Rico strain of Aedes aegypti. Additionally, electroantennographic studies were used to investigate the antennal sensitivities to these compounds in both strains.

RESULTS

Resistance was found to all tested insect repellents in the Puerto Rico strain of Ae. aegypti. Resistance ratios at the different time points were about 2 for DEET, 3 for 2-undecanone, and 12 for IR3535. Resistance was also observed to pyrethrum extract (∼9-fold), transfluthrin (∼5-fold), and metofluthrin (∼48-fold) in repellent behavioral response. Electrophysiological analysis found decreased antennal sensitivity to all repellents tested, consistent with their behavioral effects.

CONCLUSION

The reduced sensitivity to these repellents may represent a fitness cost arising from the kdr mutation present in Puerto Rico Aedes aegypti. This work highlights the need for understanding collateral effects from the evolution of pesticide resistance in mosquitoes, and the importance of finding alternative strategies to control resistance development. © 2019 Society of Chemical Industry.

Exploring the Potentiality of Natural Products from Essential Oils as Inhibitors of Odorant-Binding Proteins: A Structure- and Ligand-Based Virtual Screening Approach To Find Novel Mosquito Repellents

Odorant-binding proteins (OBPs) are the main olfactory proteins of mosquitoes, and their structures have been widely explored to develop new repellents. In the present study, we combined ligand- and structure-based virtual screening approaches using as a starting point 1633 compounds from 71 botanical families obtained from the Essential Oil Database (EssOilDB). Using as reference the crystallographic structure of N,N-diethyl-meta-toluamide interacting with the OBP1 homodimer of Anopheles gambiae (AgamOBP1), we performed a structural and pharmacophoric similarity search to select potential natural products from the library. Thymol acetate, 4-(4-methyl phenyl)-pentanal, thymyl isovalerate, and p-cymen-8-yl demonstrated a favorable chemical correlation with DEET and also had high-affinity interactions with the OBP binding pocket that molecular dynamics simulations showed to be stable. To the best of our knowledge, this is the first study to evaluate on a large scale the potentiality of NPs from essential oils as inhibitors of the mosquito OBP1 using in silico approaches. Our results could facilitate the design of novel repellents with improved selectivity and affinity to the protein binding pocket and can shed light on the mechanism of action of these compounds against insect olfactory recognition.

Commonly Used Insect Repellents Hide Human Odors from Anopheles Mosquitoes

The mode of action for most mosquito repellents is unknown. This is primarily due to the difficulty in monitoring how the mosquito olfactory system responds to repellent odors. Here, we used the Q-system of binary expression to enable activity-dependent Ca²⁺ imaging in olfactory neurons of the African malaria mosquito Anopheles coluzzii. This system allows neuronal responses to common insect repellents to be directly visualized in living mosquitoes from all olfactory organs, including the antenna. The synthetic repellents N,N-diethyl-meta-toluamide (DEET) and IR3535 did not activate Anopheles odorant receptor co-receptor (Orco)-expressing olfactory receptor neurons (ORNs) at any concentration, and picaridin weakly activated ORNs only at high concentrations. In contrast, natural repellents (i.e. lemongrass oil and eugenol) strongly activated small numbers of ORNs in the Anopheles mosquito antennae at low concentrations. We determined that DEET, IR3535, and picaridin decrease the response of Orco-expressing ORNs when these repellents are physically mixed with activating human-derived odorants. We present evidence that synthetic repellents may primarily exert their olfactory mode of action by decreasing the amount of volatile odorants reaching ORNs. These results suggest that synthetic repellents disruptively change the chemical profile of host scent signatures on the skin surface, rendering humans invisible to Anopheles mosquitoes.

DEET and other repellents are inhibitors of mosquito odorant receptors for oviposition attractants

In addition to its primary function as an insect repellent, DEET has many "off-label" properties, including a deterrent effect on the attraction of gravid female mosquitoes. DEET negatively affects oviposition sites. While deorphanizing odorant receptors (ORs) using the Xenopus oocyte recording system, we have previously observed that DEET generated outward (inhibitory) currents on ORs sensitive to oviposition attractants. Here, we systematically investigated these inhibitory currents. We recorded dose-dependent outward currents elicited by DEET and other repellents on ORs from Culex quinquefasciatus, Aedes aegypti, and Anopheles gambiae. Similar responses were observed with other plant-derived and plant-inspired compounds, including methyl jasmonate and methyl dihydrojasmolate. Inward (regular) currents elicited by skatole upon activation of CquiOR21 were modulated when this oviposition attractant was coapplied with a repellent. Compounds that generate outward currents in ORs sensitive to oviposition attractants elicited inward currents in a DEET-sensitive receptor, CquiOR136. The best ligand for this receptor, methyl dihydrojasmolate, showed repellency activity but was not as strong as DEET in our test protocol.

Insecticide resistance modifies mosquito response to DEET and natural repellents

Background

Pyrethroid and organophosphate resistance in the malaria vector Anopheles gambiae has led to the search for not only alternative insecticides, but also repellent chemical compounds. However, little is known about the potential actions of repellents and the cross-resistance risk between insecticide and repellent compounds.

Methods

Here we show the action of permethrin, DEET, geraniol, carvacrol, culminaldehyde and cinnamaldehyde against three A. gambiae strains: ‘Kis’ (Kisumu susceptible strain), ‘KdrKis’ (pyrethroid resistant strain) and ‘AcerKis’ (organophosphate resistant strain), the last two differing from the first by a mutation on the kdr and ace1 genes, respectively.

Conclusions

Results from the DEET assays show it induced repellency for the resistant KdrKis and AcerKis strains but maintained irritancy for the susceptible strain. More generally, we show resistance genes modify the behavior of An. gambiae, increasing or decreasing the effectiveness of DEET and natural compounds, depending on the mutation. These findings offer a new avenue for research on the target and mechanism of repellent compounds. We discuss these findings in the context of vector control strategies.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3343-9) contains supplementary material, which is available to authorized users.

Molecular evidence for the inhibition of cytochrome p450s and cholinesterases in ticks by the repellent DEET

For more than 50 years DEET (N,N-Diethyl-m-toluamide) has been considered the gold standard of repellents. It is applied to the skin or clothing to deter mosquitoes and other blood-sucking invertebrate pests from approaching and/or settling, and ultimately it provides temporary protection from bites. Despite rampant global use, surprisingly little is understood about DEET's mode of action and the molecular targets of the active ingredient. Furthermore, the theories into its mechanism for repellency are largely based off fruit fly and mosquito research. Since ticks possess a unique sensory structure, the Haller's organ, the specific genes and pathways associated with DEET avoidance may differ from insects. In these studies, we collected American dog ticks (Dermacentor variabilis) from four natural populations within Manitoba, Canada. We first carried out behavior assays, which showed DEET effectively repelled the ticks. RNA sequencing revealed that DEET caused a rapid and substantial reduction in the abundance of transcripts encoding cytochrome P450 and acetylcholinesterase genes, which gradually recovered over the 24 h time course. Finally, enzymatic kinetics provided functional support for DEET's role as an effective inhibitor of P450 s. While many facets of its mode of action remain to be worked out, our study provides valuable insights into the molecular underpinnings of DEET's repellence in ticks.

A natural variant and engineered mutation in a GPCR promote DEET resistance in C. elegans

DEET (N,N-diethyl-meta-toluamide) is a synthetic chemical identified by the US Department of Agriculture in 1946 in a screen for repellents to protect soldiers from mosquito-borne diseases^1,2. Since its discovery, DEET has become the world's most widely used arthropod repellent and is effective against invertebrates separated by millions of years of evolution-including biting flies³, honeybees⁴, ticks⁵, and land leeches³. In insects, DEET acts on the olfactory system^5-12 and requires the olfactory receptor co-receptor Orco^7,9-12, but exactly how it works remains controversial¹³. Here we show that the nematode Caenorhabditis elegans is sensitive to DEET and use this genetically tractable animal to study the mechanism of action of this chemical. We found that DEET is not a volatile repellent, but instead interferes selectively with chemotaxis to a variety of attractant and repellent molecules. In a forward genetic screen for DEET-resistant worms, we identified a gene that encodes a single G protein-coupled receptor, str-217, which is expressed in a single pair of chemosensory neurons that are responsive to DEET, called ADL neurons. Mis-expression of str-217 in another chemosensory neuron conferred responses to DEET. Engineered str-217 mutants, and a wild isolate of C. elegans that carries a str-217 deletion, are resistant to DEET. We found that DEET can interfere with behaviour by inducing an increase in average pause length during locomotion, and show that this increase in pausing requires both str-217 and ADL neurons. Finally, we demonstrated that ADL neurons are activated by DEET and that optogenetic activation of ADL neurons increased average pause length. This is consistent with the 'confusant' hypothesis, which proposes that DEET is not a simple repellent but that it instead modulates multiple olfactory pathways to scramble behavioural responses^10,11. Our results suggest a consistent motif in the effectiveness of DEET across widely divergent taxa: an effect on multiple chemosensory neurons that disrupts the pairing between odorant stimulus and behavioural response.

Effect of DEET-multiple exposures on behavior and life history traits in the malaria mosquito Anopheles gambiae (s.s.)

Background

Vector-borne diseases are major public health concerns, and their control is threatened by the spread of insecticide resistance in vector populations. In this context, the use of repellents is an alternative approach to limit vector-host interactions. However, prior exposure to repellents is suspected to affect mosquito behavior at the subsequent exposure, possibly reducing the efficacy of the compound. Despite this, the effect of mosquito experience on repellent efficacy remains poorly documented. In the present study, we tested whether a first blood meal successfully obtained upon a DEET-treated net would affect the success at taking a second blood meal in spite of DEET in the malaria mosquito Anopheles gambiae (s.s.). The impact of DEET on mosquito life history traits after the first and the second exposure was also measured, in order to assess the long-term consequences of multiple exposures to DEET in vector insects.

Results

A first blood meal obtained upon a DEET-treated net did not influence the success of An. gambiae females to take a second blood meal in spite of DEET. However, data showed that a prior exposure to DEET negatively affected all life history traits tested in this study related to fecundity and fertility. DEET pre-exposed females displayed a reduction in blood engorgement at the second exposure, as well as a reduction in the number of eggs laid and in the proportion of offspring that reach adult stage. Also, an increase of mosquito activity was observed during the second blood meal in DEET-pre-exposed females. Taken together, these data suggest an overall impact of DEET exposure on mosquito fitness.

Conclusions

Our results did not evidence any effect of a prior exposure to DEET on its efficacy during the second exposure. However, data show a negative impact of DEET exposure on mosquito fitness. These results give insights to understand the long-term efficacy of the most used mosquito repellent, and highlight that DEET induces deleterious effects on mosquito fitness in addition to repellency, potentially increasing its efficacy for controlling vector-borne diseases.

Identification of novel bioinspired synthetic mosquito repellents by combined ligand-based screening and OBP-structure-based molecular docking

In this work we report a fast and efficient virtual screening protocol for discovery of novel bioinspired synthetic mosquito repellents with lower volatility and, in all likelihood, increased protection time as compared with their plant-derived parental compounds. Our screening protocol comprises two filtering steps. The first filter is based on the shape and chemical similarity to known plant-derived repellents, whereas the second filter is based on the predicted similarity of the ligand's binding mode to the Anopheles gambiae odorant binding protein (AgamOBP1) relative to that of DEET and Icaridin to the same OBP. Using this protocol, a chemical library containing 42,755 synthetic molecules was screened in silico and sixteen selected compounds were tested for their affinity to AgamOBP1 in vitro and repellence against A. gambiae female mosquitoes using a warm-body repellent assay. One of them showed DEET-like repellence (91%) but with significantly lower volatility (2.84 × 10^-6 mmHg) than either DEET (1.35 × 10^-3 mmHg) or its parental cuminic acid (3.08 × 10^-3 mmHg), and four other compounds were found to exhibit repellent indices between 69 and 79%. Overall, a correlation was not evident between repellence and OBP-binding strength. In contrast, a correlation between binding mode and repellence was found.

Effect of the Topical Repellent para-Menthane-3,8-diol on Blood Feeding Behavior and Fecundity of the Dengue Virus Vector Aedes aegypti

Dengue fever is an acute disease caused by the dengue virus and transmitted primarily by the mosquito Aedes aegypti. The current strategy for dengue prevention is vector control including the use of topical repellents to reduce mosquito biting. Although N,N-diethyl-m-methylbenzamide (DEET) is the most common active ingredient in topical repellent products, para-menthane-3,8-diol (PMD) is also used commercially. Studies have indicated PMD reduced biting by 90–95% for up to 6–8 h, similar to the efficacy of DEET, depending on the testing environment. The purpose of this study was to evaluate the behavioral effects of PMD on Ae. aegypti blood feeding and fecundity to explore the potential impact of PMD on downstream mosquito life-history traits. Two experiments were performed. In both experiments, cohorts of female Ae. aegypti (Belize strain) were exposed to 20% PMD or ethanol for 10 min in a closed system and introduced to an artificial membrane feeding system. Following a 30min feed time, mosquitoes of Experiment 1 were killed and weighed as a proxy measure of blood meal, whereas mosquitoes of Experiment 2 were monitored for oviposition, a measure of fecundity. Results showed a statistically significant reduction (p < 0.001) in the percentage of Ae. aegypti that blood-fed when exposed to PMD (38%) compared to those non-exposed (49%). No significant difference in fecundity between test populations was indicated. These findings suggest that exposure of Ae. aegypti to 20% PMD may influence the probability of subsequent blood feeding but of those mosquitoes that do blood feed, egg-lay density is not affected. Further studies are warranted to investigate the full range of effects of PMD exposure on other Ae. aegypti life-history traits such as mating, to continue characterizing the potential effects of PMD to impact overall vector population dynamics.

Putting evolution in elimination: Winning our ongoing battle with evolving malaria mosquitoes and parasites

Since 2000, the world has made significant progress in reducing malaria morbidity and mortality, and several countries in Africa, South America and South-East Asia are working hard to eliminate the disease. These elimination efforts continue to rely heavily on antimalarial drugs and insecticide-based interventions, which remain the cornerstones of malaria treatment and prevention. However, resistance has emerged against nearly every antimalarial drug and insecticide that is available. In this review we discuss the evolutionary consequences of the way we currently implement antimalarial interventions, which is leading to resistance and may ultimately lead to control failure, but also how evolutionary principles can be applied to extend the lifespan of current and novel interventions. A greater understanding of the general evolutionary principles that are at the core of emerging resistance is urgently needed if we are to develop improved resistance management strategies with the ultimate goal to achieve a malaria-free world.

Repellency of 29 Synthetic and Natural Commercial Topical Insect Repellents Against Aedes aegypti (Diptera: Culicidae) in Central Mexico

In Mexico, the use of repellents to prevent insects from landing and biting is a common practice. However, variation in the efficiency of natural and synthetic repellents has been observed. In this study, we evaluated the repellency and protection time of 16 synthetic and 13 natural-based commercial products against Aedes aegypti (L.) from an endemic dengue area (Jojutla, Morelos) in Central Mexico. The "arm exposure" cage test was used to assess the efficacy of the repellents. Tests were conducted by three adult volunteers. Results showed that DEET (N, N-Diethyl-3-methylbenzamide) repellents provided the highest protection and duration times against Ae. aegypti. However, low repellency and short-time protection was observed (when compared with the manufacturers' protection times). Natural-based products did not repel (either landing or biting) mosquitoes for >30 min. These results show that most of the repellent products did not provide satisfactory levels of personal protection against mosquito bites. Frequent reapplication of repellents (synthetic and natural-based) may compensate for their short duration of action. Repellent efficacy data must be integrated into the decision-making process for an optimal response to the local (or specific region) situation.

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

Escape responses, knockdown (KD), and toxicity of laboratory strains of Anopheles minimus Theobald and Culex quinquefasciatus Say to three synthetic mosquito repellents, DEET (N, N-diethyl-3-methylbenzamide), IR3535, or picaridin, at 5% v/v concentrations, were evaluated using repellent-treated papers in standard WHO tube assays and an excito-repellency (ER) test chamber system. The tube assays recorded knockdown effects of each repellent immediately after 30-min exposure and the final morality following a 24-h holding period. DEET showed 100% KD at 30 min and complete toxicity at 24 h against both species. Both actions were either minimal or absent for IR3535 and picaridin, respectively. Culex quinquefasciatus showed significantly greater escape with DEET compared with the other compounds in both contact irritancy (excitation) and noncontact spatial repellency trials. Anopheles minimus showed much more pronounced irritancy and repellency flight escape to IR3535 than picaridin. DEET was the most active irritant and repellent compound against Cx. quinquefasciatus. When adjusting contact test responses based on paired noncontact repellency assays, DEET and IR3535 showed much stronger spatial repellent properties than irritancy with An. minimus. Picaridin performed poorly as an irritant or repellent against both species. We conclude that DEET, followed by IR3535, act as strong spatial repellents at 5% concentration. DEET also performs as a strong toxicant. Our findings show that different mosquitoes can respond contrastingly to repellents, thus the importance to test a wider range of species and populations to assess the full range of chemical action.