A study by researchers from the London School of Hygiene and Tropical Medicine shows that one of the most commonly used insect repellents worldwide,commonly known as DEET, seems to be losing its potency against mosquitoes.
Although DEET has been shown to be extremely effective, recent studies have revealed that certain individual insects are unaffected by its presence.
The study published in Journal of Plos One, a US publication, recently made available to the public, said a genetic basis for this has been shown in Aedes aegypti mosquitoes.
Researchers examined host-seeking behaviour and electrophysiological responses of A. aegypti after pre-exposure to DEET.
The study said 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.
However, despite its common use over the last 60 years, and evidence that it could repel 100 per cent of mosquitoes in the laboratory, semi-field and field tests, there are several studies suggesting that certain individual insects are not repelled by DEET.
The study said repeated exposure of mosquitoes to a repellent was likely to occur in situations where more than one host may be treated with a repellent, and mosquitoes feed multiple times during their lifespan.
In order to investigate the effect of pre-exposure to DEET on a. aegypti, the study repeatedly exposed female mosquitoes to DEET and determined their subsequent behavioural and/or the electrophysiological responses.
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).
The research examined whether female a. aegypti mosquitoes would change their behaviour when tested twice with a DEET treatment on a human arm.
It said mosquito responses were determined using an arm-on-cage repellency assay, during which mosquitoes, which attempted to probe despite the presence of DEET were considered insensitive.
The research said mosquitoes probing in response to DEET on an arm when first exposed were removed from the experiment, thus, mosquitoes probing on second exposure to DEET were all initially sensitive to DEET and had altered their behaviour.
It found that previously, DEET-sensitive females, which were exposed again to an arm treated with DEET, landed and probed significantly more on the second DEET exposure than mosquitoes tested for the first time with DEET, or tested with DEET following exposure to a control arm.
The study noted, however, that the proportion of mosquitoes probing on second exposure to DEET was still lower than the response to the untreated arm.
It said mosquitoes did not change their behaviour to the untreated control arm if pre-exposed to it.
The researchers sought to understand how repellents work and how mosquitoes detect them, hence they could better work out ways to get around the problem when they do become resistant to repellents.