Insecticide resistance poses a serious threat to current malaria control efforts. The Anopheles gambiae genome will enable identification of new resistance genes and will provide new molecular targets for the design of more effective insecticides.

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.O ne of the most widely used and effective insect repellents available is the synthetic compound N,N-Diethyl-mtoluamide (DEET) (1). DEET was identified more than 50 years ago by a structure-activity study of synthetic compounds and, although a number of compounds with similar activity have been identified, their efficacy is often judged by comparison with DEET (2). The mode of action of DEET has not been elucidated fully. It was originally thought to act by affecting the olfactory receptor neurons (ORNs) sensitive to lactic acid (3) and thus inhibit the mosquito's response to this, normally attractive, compound (4-6), but this view was challenged by the finding that DEET can function as a repellent even when other attractants are present (4, 7). A recent investigation on Anopheles gambiae has suggested that the ORN for 1-octen-3-ol, a component of human sweat (8) that in combination with CO 2 acts as an attractant for this species (9, 10), is blocked by DEET, and thus, in the presence of DEET, a higher concentration of the 1-octen-3-ol is required for the mosquitoes to be able to detect it (11). 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.Despite the proven efficacy of DEET as a repellent, during laboratory and field trials it is common to find that a small proportion of mosquitoes are not repelled by the compound (13-15). Boeckh et al. (4) demonstrated that DEET reduced, but did not entirely eliminate, the approach of A. aegypti to host odors, and experiments with Dros...

cDNA clones for the esterase (E4) responsible for broad insecticide resistance in peach-potato aphids (Myzus persicae Sulz.) were isolated and used to study the molecular basis of resistance. Increased esterase synthesis by resistant aphids was found to be associated with amplification of the structural gene for the esterase (E4 or its closely related variant, FE4), the degree of amplification being correlated with the activity of the esterase and the level of resistance. Hybridization of the cDNA clones to genomic Southern blots showed that only some of the esterase-related restriction fragments are amplified. Qualitative differences between restriction patterns in different clones of resistant aphids correlated with the presence or absence of a specific chromosome translocation and with production of E4 or FE4.

Overproduction of the insecticide-degrading esterases, E4 and FE4, in peach-potato aphids, Myzus persicae (Sulzer), depends on both gene amplification and transcriptional control, the latter being associated with changes in DNA methylation. The structure and function of the aphid esterase genes have been studied but the determination of their copy number has proved difficult, a common problem with gene amplification. We have now used a combination of pulsed-field gel electrophoresis and quantitative competitive PCR to determine relative esterase gene copy numbers in aphid clones with different levels of insecticide resistance (R1, R2 and R3). There are approx. 4-fold increases between susceptible, R1, R2 and R3 aphids, reaching a maximum of approx. 80 times more genes in R3; this gives proportionate increases in esterase protein relative to susceptible aphids. Thus there is no overexpression of the amplified genes, in contrast with what was thought previously. For E4 genes, the loss of 5-methylcytosine is correlated with a loss of expression, greatly decreasing the amount of enzyme relative to the copy number.

DEET (N,N-Diethyl-m-toluamide) is one of the most widely used mosquito repellents. Although DEET has been shown to be extremely effective, recent studies have revealed that certain individual insects are unaffected by its presence. A genetic basis for this has been shown in Aedes aegypti mosquitoes and the fruit fly Drosophila melanogaster, but, for the triatomine bug, Rhodnius prolixus, a decrease in response to DEET occurred shortly after previous exposure, indicating that non-genetic factors may also be involved in DEET “insensitivity”. In this study, we examined host-seeking behaviour and electrophysiological responses of A. aegypti after pre-exposure to DEET. We found that three hours after pre-exposure the mosquitoes showed behavioural insensitivity, and electroantennography revealed this correlated with the olfactory receptor neurons responding less to DEET. The change in behaviour as a result of pre-exposure to DEET has implications for the use of repellents and the ability of mosquitoes to overcome them.