Date Published: February 20, 2013
Publisher: Public Library of Science
Author(s): Nina M. Stanczyk, John F. Y. Brookfield, Linda M. Field, James G. Logan, John Vontas. http://doi.org/10.1371/journal.pone.0054438
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.
The insect repellent N,N-diethyl-m-toluamide (DEET) is one of the most commonly used repellents worldwide . However, despite its common use over the last 60 years, and evidence that it can repel 100% of mosquitoes in the laboratory, semi-field and field tests –, there are several studies suggesting that certain individual insects are not repelled by DEET. For example, a small proportion of individuals in populations of Aedes aegypti mosquitoes and Drosophila melanogaster fruit flies will move towards an attractant despite the presence of DEET, a genetic “insensitivity” which can be selected for in the population –, and which corresponds to changes in the function of the peripheral olfactory system . However, in a recent study, the triatomine bug, Rhodnius prolixus, showed a decrease in behavioural repellency after continuous stimulation with DEET , indicating that other, non-genetic, factors may play a role in preventing insects from responding to DEET.
Four separate experiments were performed to determine whether pre-exposure to DEET affected the behavioural and/or olfactory responses of mosquitoes to DEET when applied to a human arm, or when applied to an artificial heat source (to remove the effects of human volatiles). A summary of the experiments is given in Table 1.
The genetic insensitivity to DEET found in previous studies – cannot be the cause of the change in behaviour of A. aegypti which occurred over a short, three hour, period in the experiments reported here. Our observed increase in insensitivity to DEET on a second exposure, by previously DEET-sensitive mosquitoes, initially suggested they may have adapted to DEET, possibly by associating it with the presence of a host arm, and were able to ‘overcome’ the natural repellent effect. This would be consistent with other studies showing that mosquitoes can learn to respond differently to odours to maximise feeding success , . Both Culex quinquefasciatus mosquitoes , the parasitic wasp M. croceipes, and the triatomine bug R. Prolixus– can learn to associate a neutral odour with a food source through Pavlovian conditioning, and adapt their host-seeking preferences accordingly. In C. quinquefasciatus this conditioning could last for up to 24 hours in colony mosquitoes, though fewer mosquitoes responded over time . However, in our study, altered behaviour towards DEET did not result in a reward (i.e. the mosquitoes were not given a blood meal) other than the ability to move towards a human arm/heat source, and this behaviour occurred even when there was no host-related stimulus present. Interestingly, mosquitoes showed increased repellency by DEET on the artificial heat source when pre-exposed to the heat, which was not seen towards DEET on an arm after pre-exposure to the control arm (Fig. 1A, B). The presence of human volatiles with the DEET stimulus may have been a greater incentive for the mosquitoes to persist in host seeking when re-exposed, compared to the weaker attraction of heat alone. Overall, the increased response to a second treatment with DEET on an attractive stimulus, after pre-exposure to DEET with no attractant present, indicates that the learned behaviour is not by association with an attractant as was found in other studies with host-seeking insects , , . It is instead a direct response to a single exposure to the DEET, Such habituation to DEET has been shown in R. prolixus, where continuous stimulation led to 10–20 minutes of reduced repellency . Thus it seems likely that in our experiments increased DEET-insensitivity results from sensory adaptation or habituation, whereby there is an decrease in response to a stimulus (in this case, DEET) after repeated exposure , .