Female mosquitoes of some species are generalists and will blood-feed on a variety of vertebrate hosts, whereas others display marked host preference. Anopheles gambiae and Aedes aegypti have evolved a strong preference for humans, making them dangerously efficient vectors of malaria and Dengue haemorrhagic fever1. Specific host odours likely drive this strong preference since other attractive cues, including body heat and exhaled carbon dioxide (CO2) are common to all warm-blooded hosts2, 3. Insects sense odours via several chemosensory receptor families, including the odorant receptors (ORs). ORs are membrane proteins that form heteromeric odour-gated ion channels4, 5 comprised of a variable ligand-selective subunit and an obligate co-receptor called Orco6. Here we use zinc-finger nucleases to generate targeted mutations in the Ae. aegypti orco gene to examine the contribution of Orco and the OR pathway to mosquito host selection and sensitivity to the insect repellent DEET. orco mutant olfactory sensory neurons have greatly reduced spontaneous activity and lack odour-evoked responses. Behaviourally, orco mutant mosquitoes have severely reduced attraction to honey, an odour cue related to floral nectar, and do not respond to human scent in the absence of CO2. However, in the presence of CO2, female orco mutant mosquitoes retain strong attraction to both human and animal hosts, but no longer strongly prefer humans. orco mutant females are attracted to human hosts even in the presence of DEET, but are repelled upon contact, indicating that olfactory- and contact-mediated effects of DEET are mechanistically distinct. We conclude that the OR pathway is crucial for an anthropophilic vector mosquito to discriminate human from non-human hosts and to be effectively repelled by volatile DEET.

Highlights d DEET and bitters inhibit Aedes aegypti female mosquito sugar ingestion d Only DEET completely prevents blood feeding on contact d Repellency of DEET on skin contact is mediated by the tarsal segments of the legs d Any pair of legs can sense DEET to prevent mosquito biting

Animals evolved in complex environments, producing a wide range of behaviors, including navigation, foraging, prey capture, and conspecific interactions, which vary over timescales ranging from milliseconds to days. Historically, these behaviors have been the focus of study for ecology and ethology, while systems neuroscience has largely focused on short timescale behaviors that can be repeated thousands of times and occur in highly artificial environments. Thanks to recent advances in machine learning, miniaturization, and computation, it is newly possible to study freely moving animals in more natural conditions while applying systems techniques: performing temporally specific perturbations, modeling behavioral strategies, and recording from large numbers of neurons while animals are freely moving. The authors of this review are a group of scientists with deep appreciation for the common aims of systems neuroscience, ecology, and ethology. We believe it is an extremely exciting time to be a neuroscientist, as we have an opportunity to grow as a field, to embrace interdisciplinary, open, collaborative research to provide new insights and allow researchers to link knowledge across disciplines, species, and scales. Here we discuss the origins of ethology, ecology, and systems neuroscience in the context of our own work and highlight how combining approaches across these fields has provided fresh insights into our research. We hope this review facilitates some of these interactions and alliances and helps us all do even better science, together.

DEET (N,N-diethyl-meta-toluamide) is a synthetic chemical identified by the United States Department of Agriculture in 1946 in a screen for repellents to protect soldiers from mosquito-borne diseases1,2. 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 system4,6–12 and requires the olfactory receptor co-receptor orco7,9–11, 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 its mechanism of action. We found that DEET is not a volatile repellent, but interferes selectively with chemotaxis to a variety of attractant and repellent molecules. In a forward genetic screen for DEET-resistant animals, we identified a single G protein-coupled receptor, str-217, expressed in a single pair of DEET-responsive chemosensory neurons, ADL. Misexpression of str-217 in another chemosensory neuron conferred responses to DEET. Both engineered str-217 mutants and a wild isolate of C. elegans carrying a str-217 deletion are DEET-resistant. 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 increased average pause length. This is consistent with the “confusant” hypothesis, in which DEET is not a simple repellent but modulates multiple olfactory pathways to scramble behavioural responses10,11. Our results suggest a consistent motif for the effectiveness of DEET across widely divergent taxa: an effect on multiple chemosensory neurons to disrupt the pairing between odorant stimulus and behavioural response.