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

Abstract

N,N-Diethyl-m-toluamide (DEET) is one of the most effective and commonly used mosquito repellents. However, during laboratory trials a small proportion of mosquitoes are still attracted by human odors despite the presence of DEET. In this study behavioral assays identified Aedes aegypti females that were insensitive to DEET, and the selection of either sensitive or insensitive groups of females with males of unknown sensitivity over several generations resulted in two populations with different proportions of insensitive females. Crossing experiments showed the "insensitivity" trait to be dominant. Electroantennography showed a reduced response to DEET in the selected insensitive line compared with the selected sensitive line, and single sensillum recordings identified DEET-sensitive sensilla that were nonresponders in the insensitive line. This study suggests that behavioral insensitivity to DEET in A. aegypti is a genetically determined dominant trait and resides in changes in sensillum function.

Fig. 1.

Percentage of female A. aegypti insensitive to DEET in unselected and selected populations in two experiments, A and B. Unselected (dashed bar) = female mosquitoes from standard culture. s (white bar) = line bred from female mosquitoes sensitive to DEET at each generation. i (shaded bar) = line bred from female mosquitoes insensitive to DEET at each generation (F₄ in experiment B was not tested because of low numbers). F₇ and F₈ of experiment A and F₆ and F₇ of experiment B were reared without selection. N(F₁) = 600 experiment A, 480 experiment B. N(F₂–F₉) = 100–400 per generation. Means are given ± SEM.

Fig. 2.

EAG responses (mV/average control value, data log¹⁰ transformed) of A. aegypti from the F₉ generation of the s (white bar) and i (shaded bar) lines of experiments A and B. N = 20. Same cartridge and separate cartridges refer to DEET and 1-octen-3-ol tested simultaneously by different delivery methods. Concentrations of compounds used: methyl salicylate = 1 × 10^-4 g; 1-octen-3-ol = 1 × 10^-4 g; DEET = 1 × 10^-3 g. * indicates a significant difference (p < 0.001). Means are given ± standard error of the difference.

Fig. 3.

A. aegypti sensilla morphological types: (1) long sharp sensilla; (2) short sharp sensilla; (3) sbtI sensilla; (4) sbtII sensilla; (5) grooved peg sensilla.

Fig. 4.

Response spectra of olfactory receptor neurons housed in sbtII sensilla trichodea of A. aegypti. The neuronal responses of the two neurons, A (white bar) and B (shaded bar), housed in five functional classes are shown as an average over (N) replicates. SbtII 1, N = 5 s, 0 i. SbtII 2, N = 2 s, 5 i. SbtII 3, N = 2 s, 1 i. SbtII 4, N = 7 s, 4 i. SbtII 5, N = 7 s, 1 i. Means are ± SEM.

Fig. 5.

Responses of sbtII type 4 in behaviorally selected females from the s and i lines. (A) Microelectrode recordings from sbtII 4 sensilla. (a) s line control. (b) s line tested with 1 × 10^-3 g DEET. (B) Dose–response curve of sbtII 4 to DEET in the s and i lines (N = 7 s, 4 i). Here only the responses of the A neuron are shown, because the B neuron showed no response difference to the control. The s line showed an excitatory response to DEET in comparison to the control at 1 × 10^-4 g (p < 0.05). * indicates a significant difference in response by the s and i lines (p < 0.05), ** (p < 0.01). (C) Neuronal responses to DEET and (±)-linalool by the A and B neurons of sbtII 4 sensilla in female A. aegypti from the s (white bar) and i (shaded bar) lines. N = 7 s, 4 i. Means are ± SEM. * indicates a significant difference in response by the s and i lines (p < 0.05), ** (p < 0.01).