The mysterious multi-modal repellency of DEET

Arthropod repellent interactions with olfactory receptors and ionotropic receptors analyzed by molecular modeling

The main insect chemoreceptors are olfactory receptors (ORs), gustatory receptors (GRs) and ionotropic receptors (IRs). The odorant binding sites of many insect ORs appear to be occluded and inaccessible from the surface of the receptor protein, based on the three-dimensional structure of OR5 from the jumping bristletail Machilis hrabei (MhraOR5) and a survey of a sample of vinegar fly (Drosophila melanogaster) OR structures obtained from artificial intellegence (A.I.) modeling. Molecular dynamics simulations revealed that the occluded site can become accessible through tunnels that transiently open and close. The present study extends this analysis to examine seventeen ORs and one GR docking with ligands that have known valence: nine that signal attraction and nine that signal aversion. All but one of the receptors displayed occluded ligand binding sites analogous to MhraOR5, and docking software predicted the known attractant and repellent ligands will bind to the occluded sites. Docking of the repellent DEET was examined, and more than half of the OR ligand sites were predicted to bind DEET, including receptors that signal aversion as well as those that signal attraction. However, DEET may not actually have access to all the attractant binding sites. The larger size and lower flexibility of repellent molecules may restrict their passage through the tunnel bottlenecks, which could act as filters to select access to the ligand binding sites. In contrast to ORs and GRs, the IR ligand binding site is in an extracellular domain known to undergo a large conformational change from an open to a closed state. A.I. models of two D. melanogaster IRs of known valence and two blacklegged tick (Ixodes scapularis) IRs having unknown ligands were computationally tested for attractant and repellent binding. The ligand-binding sites in the closed state appear inaccessible to the protein surface, so attractants and repellents must bind initially at an accessible site in the open state before triggering the conformational change. In some IRs, repellent binding sites were identified at exterior sites adjacent to the ligand-binding site. These may be allosteric sites that, when occupied by repellents, can stabilize the open state of an attractant IR, or stabilize the closed state of an IR in the absence of its activating ligand. The model of D. melanogaster IR64a suggests a possible molecular mechanism for the activation of this IR by H+. The amino acids involved in this proposed mechanism are conserved in IR64a from several Dipteran pest species and disease vectors, potentially offering a route to discovery of new repellents that act via the allosteric site.

Mosquitoes do not Like Bitter

Chemical repellents play a crucial role in personal protection, serving as essential elements in reducing the transmission of vector-borne diseases. A biorational perspective that extends beyond the olfactory system as the classical target may be a promising direction to move. The taste system provides reliable information regarding food quality, helping animals to discriminate between nutritious and potentially harmful food sources, often associated with a bitter taste. Understanding how bitter compounds affect feeding in blood-sucking insects could unveil novel molecules with the potential to reduce biting and feeding. Here, we investigated the impact of two naturally occurring bitter compounds, caffeine and quinine, on the feeding decisions in female Aedes aegypti mosquitoes at two distinctive phases: (1) when the mosquito explores the biting substrate using external taste sensors and (2) when the mosquito takes a sip of food and tastes it using internal taste receptors. We assessed the aversiveness of bitter compounds through both an artificial feeding condition (artificial feeder test) and a real host (arm-in-cage test). Our findings revealed different sensitivities in the external and internal sensory pathways responsible for detecting bitter taste in Ae. aegypti. Internal detectors exhibited responsiveness to lower doses compared to the external sensors. Quinine exerted a more pronounced negative impact on biting and feeding activity than caffeine. The implications of our findings are discussed in the context of mosquito food recognition and the potential practical implications for personal protection.

A critical review of current laboratory methods used to evaluate mosquito repellents

Pathogens transmitted by mosquitoes threaten human health around the globe. The use of effective mosquito repellents can protect individuals from contracting mosquito-borne diseases. Collecting evidence to confirm and quantify the effectiveness of a mosquito repellent is crucial and requires thorough standardized testing. There are multitudes of methods to test repellents that each have their own strengths and weaknesses. Determining which type of test to conduct can be challenging and the collection of currently used and standardized methods has changed over time. Some of these methods can be powerful to rapidly screen numerous putative repellent treatments. Other methods can test mosquito responses to specific treatments and measure either spatial or contact repellency. A subset of these methods uses live animals or human volunteers to test the repellency of treatments. Assays can greatly vary in their affordability and accessibility for researchers and/or may require additional methods to confirm results. Here I present a critical review that covers some of the most frequently used laboratory assays from the last two decades. I discuss the experimental designs and highlight some of the strengths and weaknesses of each type of method covered.

Avoidance of thiazoline compound depends on multiple sensory pathways mediated by TrpA1 and ORs in Drosophila

Transient receptor potential (TRP) channels are primary sensory molecules in animals and are involved in detecting a diverse range of physical and chemical cues in the environments. Considering the crucial role of TRPA1 channels in nocifensive behaviors and aversive responses across various insect species, activators of TRPA1 are promising candidates for insect pest control. In this study, we demonstrate that 2-methylthiazoline (2MT), an artificial volatile thiazoline compound originally identified as a stimulant for mouse TRPA1, can be utilized as a novel repellent for fruit flies, Drosophila melanogaster. We observed that 2MT induced strong, dose-dependent avoidance behaviors in adult males, regardless of their feeding states, as well as egg laying behavior in females. These aversive responses were mediated by contact chemosensation via TrpA1 and olfaction via odorant receptors. Knocking down TrpA1 revealed the essential roles of bitter taste neurons and nociceptive neurons in the legs and labellum. Furthermore, among five isoforms, TrpA1-C and TrpA1-D exclusively contributed to the aversiveness of 2MT. We also discovered that these isoforms were directly activated by 2MT through covalent modification of evolutionarily conserved cysteine residues. In conclusion, we have identified 2MT as a stimulant for multiple sensory pathways, triggering aversive behaviors in fruit flies. We propose that 2MT and related chemicals may serve as potential resources for developing novel insect repellents.

Poly(oxyethylene)/Poly(oxypropylene) butyl ether prolongs the repellent effect of N,N-diethyl-3-toluamide on the skin

N,N-diethyl-meta-toluamide (DEET) is a widely used insect repellent, with minimal skin permeation and sustained repellent activity in the superficial layers of the skin. In this study, we prepared a 10% DEET formulation consisting of 40% ethanol with or without 2% poly(oxyethylene)/poly(oxypropylene) butyl ether (POE-POP), an amphiphilic random copolymer. Further, we demonstrated the effects of POE-POP on tensile stress (stickiness), hydrophobicity, skin retention, permeation, and repellent activity of DEET. Stickiness was measured in male ICR mice (7-week old), and skin retention and permeation were evaluated in male Wistar rats (7-week old). In addition, female Aedes albopictus were used to measure the repellent action of DEET. The addition of POE-POP did not affect stickiness, volatility, and degradability but decreased logP and increased viscosity of DEET. Next, we demonstrated the behavior of DEET formulations in the rat skin. POE-POP prolonged the retention of DEET in the superficial layers of the rat skin (skin surface and stratum corneum) and decreased the penetration of DEET into rat skin tissues (epithelium and dermis). The repellent effect of DEET was also enhanced by the addition of POE-POP. However, severe skin damage was not observed after repetitive treatment with DEET formulations containing POE-POP for one month (twice a day). In conclusion, we demonstrated that a 10% DEET formulation consisting of 40% ethanol and 2% POE-POP attenuated the skin penetration and prolonged the repellent action of DEET without causing stickiness and skin damage. We conclude that the combination of ethanol and POE-POP is useful as a safe and effective delivery system for the development of insect repellent formulations containing DEET.

Computational insights of the molecular recognition between volatile molecules and odorant binding proteins from the red palm weevil Rhynchophorus ferrugineus

The red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) is one of the most harmful pests for palm trees, causing serious economic damage worldwide. The present work aims to model and study the 3D structures of highly expressed odorant binding proteins from R. ferrugineus (RferOBPs) and identify possible binding modes and ligand release mechanism by docking and molecular dynamics. Highly confident 3D structures of a total of 11 odorant binding proteins (OBPs) were obtained with AlphaFold2. All 3D RferOBPs modeled structures displayed six characteristic α-helices, except for RfeOBP7 and RfeOBP10, which had an extra terminal α-helix. Among the eleven modeled RferOBPs, RferOBP4 was highly expressed in the antennae and subsequently selected for further analyses. Molecular docking analyses demonstrated that ferruginol, α-pinene, DEET, and picaridin can favorably bind the RferOBP4 cavity with low affinity energies. Molecular dynamic simulations of RferOBP4 bound to ferruginol at different pH values showed that low pH environments dictate a structural change into an apo-state that modifies the number of tunnels where the ligand can coexist, further triggering ligand release by a pH-dependent mechanism. This is the first report concerning the modelling and study of ligand binding modes and release mechanism of R. ferrugineus OBPs.Communicated by Ramaswamy H. Sarma.

Combinatorial encoding of odors in the mosquito antennal lobe

Among the cues that a mosquito uses to find a host for blood-feeding, the smell of the host plays an important role. Previous studies have shown that host odors contain hundreds of chemical odorants, which are detected by different receptors on the peripheral sensory organs of mosquitoes. But how individual odorants are encoded by downstream neurons in the mosquito brain is not known. We developed an in vivo preparation for patch-clamp electrophysiology to record from projection neurons and local neurons in the antennal lobe of Aedes aegypti. Combining intracellular recordings with dye-fills, morphological reconstructions, and immunohistochemistry, we identify different sub-classes of antennal lobe neurons and their putative interactions. Our recordings show that an odorant can activate multiple neurons innervating different glomeruli, and that the stimulus identity and its behavioral preference are represented in the population activity of the projection neurons. Our results provide a detailed description of the second-order olfactory neurons in the central nervous system of mosquitoes and lay a foundation for understanding the neural basis of their olfactory behaviors.

Dual assessment of transcriptional and metabolomic responses in the American dog tick following exposure to different pesticides and repellents

The American dog tick, Dermacentor variabilis, is a major pest to humans and animals, serving as a vector to Rickettsia rickettsii, a bacterium responsible for Rocky Mountain spotted fever, and Francisella tularensis, which is responsible for tularemia. Although several tactics for management have been deployed, very little is known about the molecular response following pesticidal treatments in ticks. In this study, we used a combined approach utilizing transcriptomics and metabolomics to understand the response of the American dog tick to five common pesticides (amitraz, chlorpyrifos, fipronil, permethrin, and propoxur), and analyzed previous experimental data utilizing DEET repellent. Exposure to different chemicals led to significant differential expression of a varying number of transcripts, where 42 were downregulated and only one was upregulated across all treatments. A metabolomic analysis identified significant changes in acetate and aspartate levels following exposure to chlorpyrifos and propoxur, which was attributed to reduced cholinesterase activity. Integrating the metabolomics study with RNA-seq analysis, we found the physiological manifestations of the combined metabolic and transcriptional differences, revealing several novel biomolecular pathways. In particular, we discovered the downregulation of amino sugar metabolism and methylhistidine metabolism after permethrin exposure, as well as an upregulation of glutamate metabolism in amitraz treated samples. Understanding these altered biochemical pathways following pesticide and repellent exposure can help us formulate more effective chemical treatments to reduce the burden of ticks.

TRPA1-mediated repellency behavior in the red flour beetle Tribolium castaneum

The sensory perception of irritant chemicals results in escape and repellency behavioral patterns in insects. Transient receptor potential channels are cation channels that function as sensor proteins for several types of signals, such as light, sound, temperature, taste, as well as chemical and physical stimuli; among these, the TRPA channel is widely conserved and activated by irritant chemicals. Certain plant-derived essential oils (EOs), produced by secondary metabolism, are mixtures of volatile compounds, which are used as repellents because they contain environmentally sustainable ingredients. Citronellal, which is present in citronella EO from Cymbopogon species, is a potentially viable insect repellent; however, the repellency capability against coleopteran beetles remains limited. We investigated the citronellal-derived repellency behavior for the red flour beetle Tribolium castaneum, in which TcTRPA1 and odorant receptor co-receptor (Orco) expressions were mediated by RNA interference. Area-preference tests showed dose-dependent repellency behavior for citronellal; additionally, both TcTRPA1 and TcOrco double-strand RNA (dsRNA) micro-injection showed clear TcTRPA1 and TcOrco transcript reductions, and only TcTRPA1 dsRNA treatment significantly impaired repellency behavior. The relative expression level of the TcTRPA1 transcripts, evaluated by quantitative reverse-transcription PCR (qRT-PCR), revealed dominant expression in the antennae, indicating the antennae-expressed TcTRPA1-mediated repellency behavior.

Carbon dioxide and blood-feeding shift visual cue tracking during navigation in Aedes aegypti mosquitoes

Haematophagous mosquitoes need a blood meal to complete their reproductive cycle. To accomplish this, female mosquitoes seek vertebrate hosts, land on them and bite. As their eggs mature, they shift attention away from hosts and towards finding sites to lay eggs. We asked whether females were more tuned to visual cues when a host-related signal, carbon dioxide, was present, and further examined the effect of a blood meal, which shifts behaviour to ovipositing. Using a custom, tethered-flight arena that records wing stroke changes while displaying visual cues, we found the presence of carbon dioxide enhances visual attention towards discrete stimuli and improves contrast sensitivity for host-seeking Aedes aegypti mosquitoes. Conversely, intake of a blood meal reverses vertical bar tracking, a stimulus that non-fed females readily follow. This switch in behaviour suggests that having a blood meal modulates visual attention in mosquitoes, a phenomenon that has been described before in olfaction but not in visually driven behaviours.

Mosquito Repellents: Efficacy Tests of Commercial Skin-Applied Products in China

As a prevention tool for mosquito-borne diseases, mosquito repellents have received substantial attention. To make a convincing recommendation for repellent products to Chinese consumers, we compared the protection time (landing time and probing time) of the 26 best-selling commercial repellents in the Chinese market in a controlled laboratory environment. The data were analyzed by one-way ANOVA. Meanwhile, prices and favorable rates of repellents are also taken into consideration. In our study, N, N-diethyl-m-toluamide (DEET)-based products provided the longest protection time (0.5–3.88 h landing time and/or 1–5.63 h probing time) and lower prices (13.9–21.9 yuan) than other components (ethyl butylacetylaminopropionate (IR3535), picaridin, and botanical. Among the 26 selected products, only 17 repellents showed repellency, and the best repellent was Green Jungle (15% DEET), with a mean (±SD) landing and/or probing time of 3.88 ± 1.65 h and/or 5.63 ± 0.36 h. For botanicals, only ICE King, OMNIbaby, and Ren He showed a little repellency. Autan (20% picaridin) performed best in the picaridin group. Run Ben (7% IR3535) stood out from the IR3535 group. In conclusion, DEET repellent is highly recommended to consumers. The combination of botanicals and synthesized chemicals is a new prospect for eco-friendly repellents.

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.

In Search of Synergistic Insect Repellents: Modeling of Muscarinic GPCR Interactions with Classical and Bitopic Photoactive Ligands

Insect vector-borne diseases pose serious health problems, so there is a high demand for efficient molecules that could reduce transmission. Using molecular docking and molecular dynamics (MD) simulation, we studied a series of compounds acting on human and insect muscarinic acetylcholine receptors (mAChRs), a novel target of synergistic agents in pest control. We characterized early conformational changes of human M1 and fruit fly type-A mAChR G protein-coupled receptors (GPCRs) in response to DEET, IR3535, and muscarine binding based on the MD analysis of the activation microswitches known to form the signal transduction pathway in class A GPCRs. We indicated groups of microswitches that are the most affected by the presence of a ligand. Moreover, to increase selectivity towards insects, we proposed a new, bitopic, photoswitchable mAChR ligand—BQCA-azo-IR353 and studied its interactions with both receptors. Modeling data showed that using a bitopic ligand may be a promising strategy in the search for better insect control.

N,N-Diethyl-3-toluamide Formulation Based on Ethanol Containing 0.1% 2-Hydroxypropyl-β-cyclodextrin Attenuates the Drug's Skin Penetration and Prolongs the Repellent Effect without Stickiness

N,N-diethyl-3-toluamide (DEET) is one of the most widely used insect repellents in the world. It was reported that a solution containing 6-30% cyclodextrin (CD) as a solvent instead of ethanol (EtOH) provided an enhancement of the repellent action time duration of the DEET formulation, although the high-dose CD caused stickiness. In order to overcome this shortcoming, we attempted to prepare a 10% DEET formulation using EtOH containing low-dose CDs (β-CD, 2-hydroxypropyl-β-CD (HPβCD), methyl-β-CD, and sulfobutylether-β-CD) as solvents (DEET/EtOH/CD formulations). We determined the CD concentration to be 0.1% in the DEET/EtOH/CD formulations, since the stickiness of 0.1% CDs was not felt (approximately 8 × 10^-3 N). The DEET residue on the skin superficial layers was prolonged, and the drug penetration into the skin tissue was decreased by the addition of 0.1% CD. In particular, the retention time and attenuated penetration of DEET on the rat skin treated with the DEET/EtOH/HPβCD formulation was significantly higher in comparison with that of the DEET/EtOH formulation without CD. Moreover, the repellent effect of DEET was more sustained by the addition of 0.1% HPβCD in the study using Aedes albopictus. In conclusion, we found that the DEET/EtOH/HPβCD formulations reduced the skin penetration of DEET and prolonged the repellent action without stickiness.

In silico prediction of mosquito repellents for clothing application

Use of protective clothing is a simple and efficient way to reduce the contacts with mosquitoes and consequently the probability of transmission of diseases spread by them. This mechanical barrier can be enhanced by the application of repellents. Unfortunately the number of available repellents is limited. As a result, there is a crucial need to find new active and safer molecules repelling mosquitoes. In this context, a structure-activity relationship (SAR) model was proposed for the design of repellents active on clothing. It was computed from a dataset of 2027 chemicals for which repellent activity on clothing was measured against Aedes aegypti. Molecules were described by means of 20 molecular descriptors encoding physicochemical properties, topological information and structural features. A three-layer perceptron was used as statistical tool. An accuracy of 87% was obtained for both the training and test sets. Most of the wrong predictions can be explained. Avenues for increasing the performances of the model have been proposed.

Cellular and molecular basis of IR3535 perception in Drosophila

BACKGROUND

IR3535 is among the most widely used synthetic insect repellents, particularly for the mitigation of mosquito-borne diseases such as malaria, yellow fever, dengue and Zika, as well as to control flies, ticks, fleas, lice and mites. These insects are well-known vectors of deadly diseases that affect humans, livestock and crops. Moreover, global warming could increase the populations of these vectors.

RESULTS

Here, we performed IR3535 dose-response analyses on Drosophila melanogaster, a well-known insect model organism, using electrophysiology and binary food choice assays. Our findings indicated that bitter-sensing gustatory receptor neurons (GRNs) are indispensable to detect IR3535. Further, potential candidate gustatory receptors were screened, among which GR47a was identified as a key molecular sensor. IR3535 concentrations in the range 0.1-0.4% affected larval development and mortality. In addition, N,N-diethyl-m-toluamide (DEET, another commonly used insecticide) was found to exert synergistic effects when co-administered with IR3535.

CONCLUSION

Our findings confirmed that IR3535 directly activates bitter-sensing GRNs, which are mediated by GR47a. This relatively safe and highly potent insecticide can be largely used in combination with DEET to increase its efficiency to protect livestock and crops. Collectively, our findings suggest that the molecular sensors elucidated herein could be used as targets for the development of alternative insecticides. © 2021 Society of Chemical Industry.

Drosophila melanogaster Chemosensory Pathways as Potential Targets to Curb the Insect Menace

Simple Summary

The perception and processing of chemosensory stimuli are indispensable to the survival of living organisms. In insects, olfaction and gustation play a critical role in seeking food, finding mates and avoiding signs of danger. This review aims to present updated information about olfactory and gustatory signaling in the fruit fly Drosophila melanogaster. We have described the mechanisms involved in olfactory and gustatory perceptions at the molecular level, the receptors along with the allied molecules involved, and their signaling pathways in the fruit fly. Due to the magnifying problems of disease-causing insect vectors and crop pests, the applications of chemosensory signaling in controlling pests and insect vectors are also discussed.

Abstract

From a unicellular bacterium to a more complex human, smell and taste form an integral part of the basic sensory system. In fruit flies Drosophila melanogaster, the behavioral responses to odorants and tastants are simple, though quite sensitive, and robust. They explain the organization and elementary functioning of the chemosensory system. Molecular and functional analyses of the receptors and other critical molecules involved in olfaction and gustation are not yet completely understood. Hence, a better understanding of chemosensory cue-dependent fruit flies, playing a major role in deciphering the host-seeking behavior of pathogen transmitting insect vectors (mosquitoes, sandflies, ticks) and crop pests (Drosophila suzukii, Queensland fruit fly), is needed. Using D. melanogaster as a model organism, the knowledge gained may be implemented to design new means of controlling insects as well as in analyzing current batches of insect and pest repellents. In this review, the complete mechanisms of olfactory and gustatory perception, along with their implementation in controlling the global threat of disease-transmitting insect vectors and crop-damaging pests, are explained in fruit flies.

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.

Effect of interventions to reduce malaria incidence among military personnel on active duty: study protocol for a cluster randomised controlled trial of the impact of etofenprox-treated uniforms, permethrin-treated uniforms and DEET insect repellent

BACKGROUND

While there is strong evidence that bite protection methods such as permethrin-treated clothing and topical insect repellents are protective against insect bites, there are few studies assessing the impact on malaria infection. This study will estimate the protective efficacy of treated uniforms and DEET insect repellent on the incidence of malaria infection among military personnel in an operational setting. Permethrin-treated uniforms used with DEET lotion will be compared to etofenprox-treated uniforms with DEET lotion. The effect of DEET lotion will be estimated by comparing permethrin-treated uniforms with DEET or placebo lotion.

METHOD

A cluster randomised double-blind placebo-controlled trial is planned to evaluate the effectiveness of the interventions on preventing malaria infections in soldiers on active duty at Mgambo National Service Camp in Tanga, Tanzania. The arms are (1) permethrin-treated uniform with 30% DEET liposome formula; (2) permethrin-treated uniform with placebo lotion; (3) candidate insect repellent system, i.e. etofenprox-treated uniform with 30% DEET liposome formula; and (4) placebo, i.e. untreated uniforms with placebo lotion. The primary outcome is the incidence of Plasmodium falciparum malaria infection detected by polymerase chain reaction (PCR) by active case detection using surveys every 2 weeks for 12 months. Rapid diagnostic tests will be used for the diagnosis of participants with symptoms. The unit of randomisation will be combania: companies formed by recruits aged 18 to 25 years; combania do activities together and sleep in the same dormitory. Unequal randomisation will be used to optimise statistical power for the primary comparison between permethrin-treated uniforms with DEET and etofenprox-treated uniforms with DEET.

DISCUSSION

This trial will provide the estimate of the effects of permethrin with DEET compared to those of the new fabric treatment etofenprox with DEET and any additional effect of using DEET. The results will inform strategies to protect military personnel and civilians who have more outdoor or occupational malaria exposure than the general public.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02938975 .

Transcriptional profiling of Dermacentor variabilis (Acari: Ixodidae) provides insights into the role of the Haller's organ in spatial DEET recognition

DEET is the most common active ingredient in commercial repellents, providing effective protection against blood-sucking insects and ticks. However, its mode of action is not fully understood, with several theories put forward to explain its repellency effect. Unique to ticks, the Haller's organ recognizes a variety of external stimuli through non-contact mechanisms, yet the extent to which the organ plays a role in tick chemoreception is not fully known. We previously found that DEET inhibited the expression/activity of cytochrome P450s and cholinesterases in Dermacentor variabilis, however, our experimental design could not distinguish between sexes (males/females), method of exposure (volatile/tactile) or the roles of chemosensory tissues (Haller's organ). In this study, we used RNA sequencing to assess changes in transcript expression induced by volatile DEET in D. variabilis males/females with/without intact Haller's organs. Male ticks showed much greater transcriptional responses to DEET than females, which may be at least partially attributed to the sexual dimorphism of the Haller's organ. Female transcript expression profiles were most influenced by condition (i.e., intact/excised Haller's organs) with minimal changes due to repellent exposure. On the other hand, removal of the Haller's organs caused DEET treated male ticks to exhibit similar expression profiles as control (ethanol) ticks with intact Haller's organs. Consequently, the transcript-level responses to spatial DEET exposure appears largely based on males possessing their Haller's organs. The molecular signature of this response included the suppression of a large number of transcripts involved in detoxification, lipid metabolism and immunity. Taken collectively, this study furthers our understanding of the Haller's organ role in volatile DEET recognition.

Skin bacterial volatiles: propelling the future of vector control

The skin microbiota plays an essential role in the protection against pathogens. It is our skin microbiota that makes us smell different from each other, rendering us more or less attractive to mosquitoes. Mosquitoes exploit skin bacterial odours to locate their hosts and are vectors of pathogens that can cause severe diseases such as malaria and dengue fever. A novel solution for long-lasting protection against insect vectors of disease could be attained by manipulating the bacterial commensals on human skin. The current options for protection against biting insects usually require topical application of repellents that evaporate within hours. We discuss possible routes for the use of commensal bacteria to create a microbial-based repellent.

The structural basis of odorant recognition in insect olfactory receptors

Olfactory systems must detect and discriminate amongst an enormous variety of odorants1. To contend with this challenge, diverse species have converged on a common strategy in which odorant identity is encoded through the combinatorial activation of large families of olfactory receptors1-3, thus allowing a finite number of receptors to detect a vast chemical world. Here we offer structural and mechanistic insight into how an individual olfactory receptor can flexibly recognize diverse odorants. We show that the olfactory receptor MhOR5 from the jumping bristletail4 Machilis hrabei assembles as a homotetrameric odorant-gated ion channel with broad chemical tuning. Using cryo-electron microscopy, we elucidated the structure of MhOR5 in multiple gating states, alone and in complex with two of its agonists-the odorant eugenol and the insect repellent DEET. Both ligands are recognized through distributed hydrophobic interactions within the same geometrically simple binding pocket located in the transmembrane region of each subunit, suggesting a structural logic for the promiscuous chemical sensitivity of this receptor. Mutation of individual residues lining the binding pocket predictably altered the sensitivity of MhOR5 to eugenol and DEET and broadly reconfigured the receptor's tuning. Together, our data support a model in which diverse odorants share the same structural determinants for binding, shedding light on the molecular recognition mechanisms that ultimately endow the olfactory system with its immense discriminatory capacity.

Drosophila sensory receptors-a set of molecular Swiss Army Knives

Genetic approaches in the fruit fly, Drosophila melanogaster, have led to a major triumph in the field of sensory biology-the discovery of multiple large families of sensory receptors and channels. Some of these families, such as transient receptor potential channels, are conserved from animals ranging from worms to humans, while others, such as "gustatory receptors," "olfactory receptors," and "ionotropic receptors," are restricted to invertebrates. Prior to the identification of sensory receptors in flies, it was widely assumed that these proteins function in just one modality such as vision, smell, taste, hearing, and somatosensation, which includes thermosensation, light, and noxious mechanical touch. By employing a vast combination of genetic, behavioral, electrophysiological, and other approaches in flies, a major concept to emerge is that many sensory receptors are multitaskers. The earliest example of this idea was the discovery that individual transient receptor potential channels function in multiple senses. It is now clear that multitasking is exhibited by other large receptor families including gustatory receptors, ionotropic receptors, epithelial Na+ channels (also referred to as Pickpockets), and even opsins, which were formerly thought to function exclusively as light sensors. Genetic characterizations of these Drosophila receptors and the neurons that express them also reveal the mechanisms through which flies can accurately differentiate between different stimuli even when they activate the same receptor, as well as mechanisms of adaptation, amplification, and sensory integration. The insights gleaned from studies in flies have been highly influential in directing investigations in many other animal models.

Development and Validation of a Simple HPLC-UV Method to Assay DEET Repellents and its Application to Different Commercial Forms

Background:

N’,N’-diethyl-m-toluamide (DEET) is the most widely used repellent substance worldwide. It is formulated as aerosol, solution, lotion, gel and patches. However, the official compendia report monographs to analyze only DEET drug substance and solution.

Objective:

In this study an isocratic HPLC method was validated to assay DEET in lotion, gel and solution, under the same analytical conditions.

Methods:

The method was validated according to ICH requirements and DEET detection was achieved at around 11 min, using a C-18 column, a mobile phase composed by methanol, acetonitrile and water pH 4.5 (45:10:45), flow rate at 1 mL min-1and detection at 270 nm.

Results:

A linear relationship was observed in the range of 2.5 to 100 μg mL-1, the method was precise (relative standard deviation<2%) and accuracy was demonstrated by DEET recovery values ranging from 99.5 to 100.2%. The specificity was studied by a forced degradation test, where degradation products were observed after alkaline degradation and ultraviolet radiation. Appropriate resolution between DEET, degradation products and excipient peaks indicated the method specificity. Robustness was evaluated by a full factorial design, and no effect on DEET assay was observed under simultaneous variation in analytical parameters. The method was applied to assay nine marketed formulations, demonstrating its good applicability.

Conclusion:

The validated HPLC method was successfully applied to the quantitative analysis of DEET in lotion, gel and solution, contributing to improve the quality control and the efficacy of these formulations.

Mechanisms of DEET gustation in Drosophila

DEET is the most widely used active ingredient in insect repellents and offers protection against insect bites. We previously reported that DEET suppresses the feeding behavior of Drosophila, which is guided by gustatory receptors (GRs) in bitter-sensing gustatory receptor neurons. Here, we sought to identify new candidates using egg-laying assays. Upon screening all GR mutants, GR89a was identified as a potential DEET receptor. Gr89a mutants exhibited reduced oviposition avoidance, feeding avoidance, and electrophysiological responses compared to Gr32a, Gr33a, and Gr66a mutants. However, GR89a was found to modulate DEET avoidance, as demonstrated by genetic and RNA interference assays. Furthermore, we found that DEET ingestion severely affected larval and pupal development and survival, and therefore may act as an effective larvicide.

2-Pentylfuran: a novel repellent of Drosophila suzukii

BACKGROUND

Drosophila suzukii (Matsumura), spotted-wing drosophila (SWD), is a major invasive pest of soft-skinned fruits in North America and Europe. Although insecticides are currently the primary method of SWD control, it is imperative to develop alternative management approaches, such as behavioral control through the use of repellents and attractants. This study explores the repellent properties of 2-pentylfuran as an oviposition deterrent on raspberries.

RESULTS

2-Pentylfuran was found to be aversive to SWD in laboratory multiple-choice tests. When co-released from a vial (loaded as neat compound) with a synthetic SWD lure, 2-pentylfuran reduced SWD attraction to the SWD lure by 98% and the effect appeared 17% stronger compared to 1-octen-3-ol, a known SWD repellent. Releasing 50% 2-pentylfuran mixed with mineral oil from a vial located near ripe raspberries resulted in 30% reduction in SWD oviposition in the field. In laboratory no-choice assays, 2-pentylfuran reduced SWD oviposition on raspberries above 2.5 mg h^-1 with greater repellency achieved at higher release rates. A release rate of 10 mg h^-1 from a polyethylene sachet reduced egg-laying on raspberries by 60% in a semifield cage choice experiment. In a field experiment using fruiting raspberry clusters, 14 mg h^-1 release rate of 2-pentylfuran was effective at reducing SWD infestations by 56% compared to untreated plots.

CONCLUSION

2-Pentylfuran acts as a repellent for SWD and can significantly reduce fruit infestations under field conditions and high SWD pressure. Given that 2-pentylfuran is a registered food additive and generally regarded as safe, 2-pentylfuran has a potential use in behavioral control strategies against SWD. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

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.

Interactions of DEET and Novel Repellents With Mosquito Odorant Receptors

The carboxamide N,N-di-ethyl-meta-toluamide (DEET) is the most effective and widely used insect repellent today. However, drawbacks concerning the efficacy and the safety of the repellent have led to efforts to design new classes of insect repellents. Through quantitative structure-activity relationships, chemists have discovered two chemical groups of novel repellents: the acylpiperidines and the carboxamides, with the acylpiperidines generally more potent in biological assays. Although the exact mechanism of action of DEET and other repellents has not yet been thoroughly elucidated, previous research shows that the activity of insect odorant receptors are inhibited in the presence of repellents. The present electrophysiological study employs two-electrode voltage clamp with Xenopus laevis oocytes expressing AgOR2/AgOrco and AgOR8/AgOrco receptors to assess the effects of the novel repellents on Anopheles gambiae Giles (Insecta: Diptera: Culicidae) mosquito odorant receptors. The novel acylpiperidines and carboxamides reversibly inhibited (12-91%) odorant-evoked currents from both AgOR2/AgOrco and AgOR8/AgOrco receptors in a dose-dependent manner at all tested concentrations (30 μM to 1 mM). Furthermore, all the novel agents were more potent inhibitors of the receptors than DEET, with the acylpiperidines producing on average greater inhibition than the carboxamides. Interestingly, there was a correlation (r2 = 0.72) between the percentage inhibition of AgOR2/AgOrco receptor currents and protection times of the acylpiperidines. Our results add to existing evidence that the repellency of a compound is linked to its ability to disrupt the insect olfactory system and that the acylpiperidines could represent a class of more effective alternatives to the current gold standard, DEET.

Olfaction: Repellents that Congest the Mosquito Nose

How does the common insect repellent DEET modify a mosquito's ability to detect humans? New research using GCaMP-expressing mosquitoes suggests that DEET works differently for different mosquito species. For An. coluzzii, DEET and other non-volatile repellents mask the mosquitoes' ability to detect odors.

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.

Novel repellents for the blood-sucking insects Rhodnius prolixus and Triatoma infestans, vectors of Chagas disease

BACKGROUND

Studying the behavioral response of blood-sucking disease-vector insects to potentially repellent volatile compounds could shed light on the development of new control strategies. Volatiles released by human facial skin microbiota play different roles in the host-seeking behavior of triatomines. We assessed the repellency effect of such compounds of bacterial origin on Triatoma infestans and Rhodnius prolixus, two important vectors of Chagas disease in Latin America.

METHODS

Using an exposure device, insects were presented to human odor alone (control) and in the presence of three individual test compounds (2-mercaptoethanol, dimethyl sulfide and 2-phenylethanol, the latter only tested in R. prolixus) and the gold-standard repellent NN-diethyl-3-methylbenzamide (DEET). We quantified the time the insects spent in the proximity of the host and determined if any of the compounds evaluated affected the behavior of the insects.

RESULTS

We found volatiles that significantly reduced the time spent in the proximity of the host. These were 2-phenylethanol and 2-mercaptoethanol for R. prolixus, and dimethyl sulfide and 2-mercaptoethanol for T. infestans. Such an effect was also observed in both species when DEET was presented, although only at the higher doses tested.

CONCLUSIONS

The new repellents modulated the behavior of two Chagas disease vectors belonging to two different triatomine tribes, and this was achieved using a dose up to three orders of magnitude lower than that needed to evoke the same effect with DEET. Future efforts in understanding the mechanism of action of repellent compounds such as 2-mercaptoethanol, as well as an assessment of their temporal and spatial repellent properties, could lead to the development of novel control strategies for these insect vectors, refractory to DEET.

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.

Dynamic modelling of personal protection control strategies for vector-borne disease limits the role of diversity amplification

Personal protection measures, such as bed nets and repellents, are important tools for the suppression of vector-borne diseases like malaria and Zika, and the ability of health agencies to distribute protection and encourage its use plays an important role in the efficacy of community-wide disease management strategies. Recent modelling studies have shown that a counterintuitive diversity-driven amplification in community-wide disease levels can result from a population's partial adoption of personal protection measures, potentially to the detriment of disease management efforts. This finding, however, may overestimate the negative impact of partial personal protection as a result of implicit restrictive model assumptions regarding host compliance, access to and longevity of protection measures. We establish a new modelling methodology for incorporating community-wide personal protection distribution programmes in vector-borne disease systems which flexibly accounts for compliance, access, longevity and control strategies by way of a flow between protected and unprotected populations. Our methodology yields large reductions in the severity and occurrence of amplification effects as compared to existing models.

Aedes aegypti Mosquitoes Use Their Legs to Sense DEET on Contact

DEET (N, N-diethyl-meta-toluamide) is the most effective and widely used insect repellent, but its mechanism of action is both complex and controversial [1]. DEET acts on insect smell [2-6] and taste [7-11], and its olfactory mode of action requires the odorant co-receptor orco [2, 3, 6]. We previously observed that orco mutant female Aedes aegypti mosquitoes are strongly attracted to humans even in the presence of DEET, but they are rapidly repelled after contacting DEET-treated skin [6]. DEET inhibits food ingestion by Drosophila melanogaster flies, and this repellency is mediated by bitter taste neurons in the proboscis [9]. Similar neurons were identified in the mosquito proboscis, leading to the hypothesis that DEET repels on contact by activating an aversive bitter taste pathway [10]. To understand the basis of DEET contact chemorepellency, we carried out behavioral experiments and discovered that DEET acts by three distinct mechanisms: smell, ingestion, and contact. Like bitter tastants, DEET is a feeding deterrent when ingested, but its bitterness per se does not fully explain DEET contact chemorepellency. Mosquitoes blood fed on human arms treated with high concentrations of bitters, but rapidly avoided DEET-treated skin and did not blood feed. Insects detect tastants both through their proboscis and legs. We show that DEET contact chemorepellency is mediated exclusively by the tarsal segments of the legs and not the proboscis. This work establishes mosquito legs as the behaviorally relevant contact sensors of DEET. These results will inform the search for molecular mechanisms mediating DEET contact chemorepellency and novel contact-based insect repellents.

Pyrethroid Resistance Associated With a Decreased DEET Repellency in the Common Bed Bug (Hemiptera: Cimicidae)

The global resurgence of the bed bug (Cimex lectularius L.) and the widespread resistance of this insect to pyrethroid insecticides have created the need to find alternatives to chemical control. Thus, synthetic or natural repellents have been considered as a suitable choice to control bed bug infestations. Repellents not only keep insects away from their hosts, but also allow a decrease in insecticide selection pressure. The aim of this study was to evaluate the repellence effectivity of DEET against two bed bug colonies from Argentina (a susceptible one and a field-collected pyrethroid-resistant one), under two starving periods. We found different repellent effects of DEET on the two C. lectularius colonies and no differences between the two starving periods. In fact, DEET had a lower effect on the insecticide-resistant colony. The methodology used in this study proved to be useful to test the variation of the effect of DEET between pyrethroid-susceptible and -resistant bed bugs.

Use of machine learning to identify novel, behaviorally active antagonists of the insect odorant receptor co-receptor (Orco) subunit

Olfaction is a key component of the multimodal approach used by mosquitoes to target and feed on humans, spreading various diseases. Current repellents have drawbacks, necessitating development of more effective agents. In addition to variable odorant specificity subunits, all insect odorant receptors (ORs) contain a conserved odorant receptor co-receptor (Orco) subunit which is an attractive target for repellent development. Orco directed antagonists allosterically inhibit odorant activation of ORs and we previously showed that an airborne Orco antagonist could inhibit insect olfactory behavior. Here, we identify novel, volatile Orco antagonists. We functionally screened 83 structurally diverse compounds against Orco from Anopheles gambiae. Results were used for training machine learning models to rank probable activity of a library of 1280 odorant molecules. Functional testing of a representative subset of predicted active compounds revealed enrichment for Orco antagonists, many structurally distinct from previously known Orco antagonists. Novel Orco antagonist 2-tert-butyl-6-methylphenol (BMP) inhibited odorant responses in electroantennogram and single sensillum recordings in adult Drosophila melanogaster and inhibited OR-mediated olfactory behavior in D. melanogaster larvae. Structure-activity analysis of BMP analogs identified compounds with improved potency. Our results provide a new approach to the discovery of behaviorally active Orco antagonists for eventual use as insect repellents/confusants.

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.

Use of repellents by travellers: A randomised, quantitative analysis of applied dosage and an evaluation of knowledge, Attitudes and Practices (KAP)

BACKGROUND

Prevention of arthropod-borne infections hinges on bite prevention. We aimed to investigate travellers' use of repellents.

METHODS

We measured the amount of applied repellent with a spray containing 30% DEET and 20% Icaridin versus a lotion with 20% Icaridin alone. We calculated the concentration of active ingredient reached on the skin and evaluated formulation acceptability. The travellers completed a questionnaire evaluating Knowledge, Attitudes and Practice (KAP) to anti-vectorial protective measures (AVPM).

RESULTS

Some 200 volunteers travelling to mosquito borne infection endemic areas were recruited. The mean concentration of active substance achieved on the skin of the left arm was 0.52 mg/cm² of DEET/Icaridin spray versus 0.21 mg/cm² of Icaridin lotion. These levels are below the recommended protective dose (1 mg/cm²) for each formulation. Women were significantly more likely to apply a higher, protective dose of repellent. Travellers to Africa, women and older participants showed higher projected adherence to AVPM.

CONCLUSIONS

Only 2.5% of recruited travellers applied the recommended protective dose of repellent. Women and older travellers are the most adherent users of repellents. The pre-travel health consultation should provide more information on the application quantity and correct use of repellents.

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.

Membrane Proteins Mediating Reception and Transduction in Chemosensory Neurons in Mosquitoes

Mosquitoes use chemical cues to modulate important behaviors such as feeding, mating, and egg laying. The primary chemosensory organs comprising the paired antennae, maxillary palps and labial palps are adorned with porous sensilla that house primary sensory neurons. Dendrites of these neurons provide an interface between the chemical environment and higher order neuronal processing. Diverse proteins located on outer membranes interact with chemicals, ions, and soluble proteins outside the cell and within the lumen of sensilla. Here, we review the repertoire of chemosensory receptors and other membrane proteins involved in transduction and discuss the outlook for their functional characterization. We also provide a brief overview of select ion channels, their role in mammalian taste, and potential involvement in mosquito taste. These chemosensory proteins represent targets for the disruption of harmful biting behavior and disease transmission by mosquito vectors.

2D and 3D structure-activity modelling of mosquito repellents: a review $

Repellents disrupt the behaviour of blood-seeking mosquitoes protecting humans against their bites which can transmit serious diseases. Since the mid-1950s, N,N-diethyl-m-toluamide (DEET) is considered as the standard mosquito repellent worldwide. However, DEET presents numerous shortcomings. Faced with the heightening risk of mosquito expansion caused by global climate changes and increasing international exchanges, there is an urgent need for a better repellent than DEET and the very few other commercialised repelling molecules such as picaridin and IR3535. In silico approaches have been used to find new repellents and to provide better insights into their mechanism of action. In this context, the goal of our study was to retrieve from the literature all the papers dealing with qualitative and quantitative structure-activity relationships on mosquito repellents. A critical analysis of the SAR and QSAR models was made focusing on the quality of the biological data, the significance of the molecular descriptors and the validity of the statistical tools used for deriving the models. The predictive power and domain of application of these models were also discussed. The hypotheses to compute homology and pharmacophore models, their interest to find new repellents and to provide insights into the mechanisms of repellency in mosquitoes were analysed. The interest to consider the mosquito olfactory system as the target to compute new repellents was discussed. The potential environmental impact of these chemicals as well as new ways of research were addressed.

Commercially Available Natural Benzyl Esters and Their Synthetic Analogs Exhibit Different Toxicities against Insect Pests

Benzyl methyl ester, also known as methyl benzoate (MB), is a volatile organic compound that exists naturally as a floral fragrance in many plants. Our behavioral bioassays show that MB and some of its naturally occurring and synthetic analogs kill insects at different life stages. Compared to commercial pesticides containing pyriproxyfen and acetamiprid, MB and some analogs are 1.3 to 3.4 times more toxic to gypsy moth larvae and brown marmorated stinkbug nymphs. The arthropod repellent DEET is also a benzyl ester, and shares the same chemical skeleton with MB. They differ by the diethylamide ester and a methyl group on the benzene ring in DEET. However, unlike MB, DEET does not kill insects; instead, it deters or repels them. Exactly how DEET causes the repellent effect in target organisms is still a mystery. Due to the MB's structural similarity to DEET, exploring the structure - activity relationship (SAR) of the MB analogs will provide useful information for the discovery of the mode and mechanistic actions of DEET as an insect repellent. In addition, the SAR will allow researchers to modify the chemical structure of the MB molecule, leading to the development of more efficient, safe, and environmentally - friendly green pesticides.

Trends in insect repellent formulations: A review

The use of natural and synthetic repellents, marketed in different pharmaceutical forms, is growing in the world due to the emerging vector-borne viral diseases as Dengue, Zika, Chikungunya, Yellow Fever and Malaria. The choice of the ideal formulation will depend on a series of factors to be analyzed: type of repellent active (natural or synthetic), pharmaceutical forms (spray, lotion, cream, gel), action time duration (short or long), environment of exposure and the user (adult, pregnant women, children, newborn). The most used repellents are DEET, IR3535 (Ethyl Butylacetylaminopropionate) (EB), Icaridin (Picaridin) and essential oils, each of them presenting advantages and disadvantages. DEET is the oldest and the most powerful repellent available in the market, thus being the reference standard. For this reason, there are many classic formulations available in the market containing the chemical component DEET in spray forms and lotions. However, due to its toxicity, DEET is not recommended for children up to 6 months and pregnant women. DEET has been an option along with other market-shared products as IR3535 and Icaridin (Picaridin), which present less toxicity in their composition. IR3535 is the less toxic and may be prescribed for children over 6 months of age and pregnant women so that they have been the best option because of the lower toxicity levels presented. IR3535 is the one that has the lowest toxicity level among the three options and may be prescribed for children above 6 months of age and pregnant women. Icaridin is as potent as DEET, but less toxic, and has the advantage of having the long-lasting action among the aforementioned repellents. The new formulations have been based on controlled release systems (CRS). The CRSs for repellents comprise polymer micro/nanocapsules, micro/solid lipid nanoparticles, nanoemulsions/microemulsions, liposomes/niosomes, nanostructured hydrogels and cyclodextrins. There are many formulations based on micro and nanocapsules containing DEET and essential oils to increase repellent action time duration and decrease permeation and consequently, systemic toxicity. The development of new formulations for the IR3535 and Icaridin is a research field yet to be explored. The current trend is the use of natural repellent actives such as essential oils, which present low toxicity, do not harm the environment, but present reduced repellent action time due to rapid evaporation after skin application. CRSs have been used as vehicle of natural repellents to improve long-lasting repellent action, reduce skin permeation and systemic effects.

Robust Manipulations of Pest Insect Behavior Using Repellents and Practical Application for Integrated Pest Management

In agricultural settings, examples of effective control strategies using repellent chemicals in integrated pest management (IPM) are relatively scarce compared to those using attractants. This may be partly due to a poor understanding of how repellents affect insect behavior once they are deployed. Here we attempt to identify potential hallmarks of repellent stimuli that are robust enough for practical use in the field. We explore the literature for success stories using repellents in IPM and we investigate the mechanisms of repellency for two chemical oviposition deterrents for controlling Drosophila suzukii Matsumura, a serious pest of small fruit crops. Drosophila suzukii causes injury by laying her eggs in ripening fruit and resulting larvae make fruit unmarketable. In caged choice tests, reduced oviposition was observed in red raspberry fruit treated with volatile 1-octen-3-ol and geosmin at two initial concentrations (10% and 1%) compared to untreated controls. We used video monitoring to observe fly behavior in these caged choice tests and investigate the mode of action for deterrence through the entire behavioral repertoire leading to oviposition. We observed fewer visitors and more time elapsed before flies first landed on 1-octen-3-ol-treated fruits than control fruits and concluded that this odor primarily inhibits behaviors that occur before D. suzukii comes in contact with a potential oviposition substrate (precontact). We observed some qualitative differences in precontact behavior of flies around geosmin-treated fruits; however, we concluded that this odor primarily inhibits behaviors that occur after D. suzukii comes in contact with treated fruits (postcontact). Field trials found reduced oviposition in red raspberry treated with 1-octen-3-ol and a combination of 1-octen-3-ol and geosmin, but no effect of geosmin alone. Recommendations for further study of repellents for practical use in the field are discussed.

Inhibition of insect olfactory behavior by an airborne antagonist of the insect odorant receptor co-receptor subunit

Response to volatile environmental chemosensory cues is essential for insect survival. The odorant receptor (OR) family is an important class of receptors that detects volatile molecules; guiding insects towards food, mates, and oviposition sites. ORs are odorant-gated ion channels, consisting of a variable odorant specificity subunit and a conserved odorant receptor co-receptor (Orco) subunit, in an unknown stoichiometry. The Orco subunit possesses an allosteric site to which modulators can bind and noncompetitively inhibit odorant activation of ORs. In this study, we characterized several halogen-substituted versions of a phenylthiophenecarboxamide Orco antagonist structure. Orco antagonist activity was assessed on ORs from Drosophila melanogaster flies and Culex quinquefasciatus mosquitoes, expressed in Xenopus laevis oocytes and assayed by two-electrode voltage clamp electrophysiology. One compound, OX1w, was also shown to inhibit odorant activation of a panel of Anopheles gambiae mosquito ORs activated by diverse odorants. Next, we asked whether Orco antagonist OX1w could affect insect olfactory behavior. A Drosophila melanogaster larval chemotaxis assay was utilized to address this question. Larvae were robustly attracted to highly diluted ethyl acetate in a closed experimental chamber. Attraction to ethyl acetate was Orco dependent and also required the odorant specificity subunit Or42b. The addition of the airborne Orco antagonist OX1w to the experimental chamber abolished larval chemotaxis towards ethyl acetate. The Orco antagonist was not a general inhibitor of sensory behavior, as behavioral repulsion from a light source was unaffected. This is the first demonstration that an airborne Orco antagonist can alter olfactory behavior in an insect. These results suggest a new approach to insect control and emphasize the need to develop more potent Orco antagonists.

The neurobiology of gustation in insect disease vectors: progress and potential

For insect vectors of human diseases, mealtimes are a key moment of infection. Understanding how and when such species decide on what to feed is both an interesting problem in sensory neurobiology and a source of information for intervention of these behaviors to control spread of infectious agents. Here I review the current knowledge of the molecular and cellular mechanisms of gustation in insect disease vectors, covering blood-feeders as well as scavengers that spread pathogens indirectly. I also consider how these behaviors are modulated over short and long timescales, and describe efforts to artificially modulate them. Though a relatively nascent field, gustatory neurobiology in insect vectors has much promise for future fundamental discoveries and practical applications.