Mosquitoes become desensitised to DEET
Mosquitoes become less sensitive to the common repellent, DEET (N,N-Diethyl-meta-toluamide), after continued exposure to the chemical.
DEET is a widely used mosquito repellent. and has been shown in various studies to repel up to 100% of mosquitoes. But a new study, published this week in PLoS ONE, suggests that repeated exposure to the chemical can temporarily reduce the insects' aversion to it, potentially leaving a person vulnerable to bites and mosquito-borne diseases, like malaria.
Nina M. Stanczyk and her colleagues at Nottingham University exposed female mosquitoes to a human arm that had been treated with DEET. Any that approached on the first trial were removed.
The remaining mosquitoes were then presented with the DEET covered arm again. This time, around 50% of them were no longer repelled by the chemical.
The same was found when a heat source was used in place of the arm, and even when the first exposure was to DEET with no arm or heat source present.
This study is interesting, as the mosquitoes can't just have learned to overcome the DEET because it was associated with a reward. They weren't given a blood meal in any of the conditions, and the effect occurred even when the first exposure was to DEET alone, with no attractive stimulus along-side. This means it is most likely to be due to processes called "sensory adaptation", or a related phenomenon called habituation.
This occurs in humans too. If you walk into a room where there is a powerful smell it can seem overpowering at first; but, as time passes, you notice it progressively less as your nose "gets used to it". This is because of sensory adaptation. Habituation is similar, but is down to attention. Rather than specific sensory neurons becoming adapted, it relates to the fact that if we sense something over and over we become bored of it, and stop paying so much attention to it. Either of these mechanisms could be responsible for the reduction in the mosquitoes' sensitivity to DEET.
To find out what was causing them to become less sensitive to the DEET, the researchers made recordings of electrical signals from the insects' antennae. The mosquitoes' that had become less sensitive to DEET at the second exposure also showed a reduced signal in response to the chemical.
This study is important when it comes to testing new repellents, as if they are tested repeatedly on the same insects, their effectiveness may decline. It is also extremely important to find out if this is going to cause problems in tropical countries, where repellents are used to prevent the transmission of diseases like malaria.