Mosquitoes smell and avoid the insect repellent DEET

Abstract

The insect repellent DEET is effective against a variety of medically important pests, but its mode of action still draws considerable debate. The widely accepted hypothesis that DEET interferes with the detection of lactic acid has been challenged by demonstrated DEETinduced repellency in the absence of lactic acid. The most recent hypothesis suggests that DEET masks or jams the olfactory system by attenuating electrophysiological responses to 1-octen-3-ol. Our research shows that mosquitoes smell DEET directly and avoid it. We performed single-unit recordings from all functional ORNs on the antenna and maxillary palps of Culex quinquefasciatus and found an ORN in a short trichoid sensillum responding to DEET in a dosedependent manner. The same ORN responded with higher sensitivity to terpenoid compounds. SPME and GC analysis showed that odorants were trapped in conventional stimulus cartridges upon addition of a DEET-impregnated filter paper strip thus leading to the observed reduced electrophysiological responses, as reported elsewhere. With a new stimulus delivery method releasing equal amounts of 1-octen-3-ol alone or in combination with DEET we found no difference in neuronal responses. When applied to human skin, DEET altered the chemical profile of emanations by a ''fixative'' effect that may also contribute to repellency. However, the main mode of action is the direct detection of DEET as indicated by the evidence that mosquitoes are endowed with DEET-detecting ORNs and corroborated by behavioral bioassays. In a sugar-feeding assay, both female and male mosquitoes avoided DEET. In addition, mosquitoes responding only to physical stimuli avoided DEET.DEET mode of action ͉ DEET-detecting neuron ͉ fixative effect of DEET ͉ mosquito olfaction ͉ surface-landing bioassay T he insect repellent DEET, N,N-diethyl-3-methylbenzamide, has been used for Ͼ50 years, with 200 million people using it worldwide to reduce their risk of vector-borne diseases (1) but its mode of action has yet to be elucidated. The report that DEET modulates the physiological response of lactic acid-sensitive olfactory receptor neurons (ORNs) in the antennae of the yellow fever mosquito, Aedes aegypti (2), led to the hypothesis that DEET may interfere with and inhibit the response of the olfactory system to a normally attractive chemical signal (3). This notion of ''jamming'' the olfactory system has been substantiated, on the one hand, by behavioral observations indicating that lactic acid per se is a mosquito attractant and suggesting that DEET inhibits attraction to lactic acid (4, 5) and by the recent report on DEET attenuation of mosquito response to 1-octen-3-ol (6). However, repellency solely by inhibition of lactic acid detection was challenged by indoor (4) and field experiments (7) demonstrating the repellent effect of DEET with carbon dioxide as the only attractant. Intrigued by this controversy and puzzled by the dichotomy between sensory physiology and behavioral observations, we undertook a multidisciplinary approach aimed at ...

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

“…Single sensillum recordings have identified an olfactory sensillum type sensitive to DEET in C. quinquefasciatus (12), in which the A neuron responds to DEET, α,β-thujone, linalool, and 1-8-cineole and the B neuron to 1-octen-3-ol. In A. aegypti, a short blunt sensilla responding to DEET in the A and B neurons has been located (4).…”

“…The two compounds were also tested simultaneously, but in separate cartridges, to test for any interaction between the two compounds before they reach the antennae, which would give the appearance of DEET interfering with the 1-octen-3-ol sensitive ORNs when this is not the case (12). In both experiments the s lines showed lower responses to DEET with the 1-octen-3-ol in the same cartridge than when the compounds were in different cartridges (experiment A, p ¼ 0.002; experiment B, p ¼ 0.001).…”

“…However, Syed and Leal (12) investigated this theory by using Culex quinquefasciatus and suggest that the reduction in response to the 1-octen-3-ol in this species is not because of a diminished response of the ORN but to interactions between the two compounds when DEET and 1-octen-3-ol are tested in the same cartridge. Additionally, single sensillum recordings have identified an ORN that responds directly to DEET in C. quinquefasciatus (12) and Aedes aegypti (4), indicating that these mosquito species are actively detecting DEET rather than DEET interfering with an ORN detecting another compound.…”

Acute olfactory response of Culex mosquitoes to a human- and bird-derived attractant

“…Surprisingly, we found chemical commonalities that may explain the well-documented host shifts in Culex (17), which lead to the transmission of WNV. We developed a simple system for collecting human odorants at the skin surface, employing SPME technology (18), and identified odorant profiles from various ethnic groups (Fig. 1).…”

“…In addition to these sensilla responding to plant and human compounds, we recorded also from two functionally distinct classes of A1 sensilla. One houses ORNs sensitive to the insect repellent DEET, whereas the other type responded to a mosquito oviposition pheromone (MOP) (10,18).…”