Abstract
BackgroundThe 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.MethodsA 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.ResultsAnopheles 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.ConclusionsThe 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.
Highlights
The species-specific mode of action for DEET and many other mosquito repellents is often unclear
Species‐specific differences in mosquito behavioural response to repellents DEET does not directly activate odorant receptors (ORs) in An. coluzzii mosquitoes, and does not directly repel them [13]. This is in contrast to what has been reported for Ae. aegypti [6,7,8] and Cx. quinquefasciatus [4, 9] mosquitoes
The direct effect of DEET was tested in the close proximity response assay on these two mosquito species (Fig. 2)
Summary
The species-specific mode of action for DEET and many other mosquito repellents is often unclear. The mosquito sensory neuron responses towards repellents are often not directly examined. Female mosquitoes can carry a number of deadly infectious agents transmittable to humans via a bite. In 2017, approximately 700,000 deaths occurred as a result of mosquito bites from three divergent species of mosquitoes (Anopheles, Aedes and Culex) [1]. Mosquitoes use their sense of smell to seek out and distinguish a vertebrate host for a blood meal. Disturbing a mosquito’s sense of smell can reduce host-seeking behaviours. The use of insect repellents, which can alter olfactory responses in mosquitoes, is one strategy of personal protection from host-seeking mosquitoes. There are two broad categories of insect repellents available on the market: products
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