Mosquitos can grow immune to repellent in as quickly as three hours, according to new research.
The study, which tested mosquitoes' response to the insect repellent DEET, showed that the insects were able to ignore the smell of the repellent within a few hours after first being exposed to it.
Published in science journal PLOS ONE, the research was done by the London School of Hygiene & Tropical Medicine in the UK.
Researchers James Logan and Nina Stanczyk tested the responses to DEET in Aedes aegypti mosquitoes, which are known for biting during the day and can transmit dengue fever.
They found that a brief exposure to DEET was enough to make some mosquitoes less sensitive to it, and three hours later the insects proved to be undeterred in their quest for heat and human skin, despite the presence of DEET.
The researchers attributed the insensitivity to a decrease in the sensitivity of odour receptors on the mosquito's antennae following a previous exposure.
"We think that the mosquitoes are habituating to the repellent, similar to a phenomenon seen with the human sense of smell also. However, the human olfactory system is very different from a mosquito's, so the mechanism involved in this case is likely to be very different," said Mr Logan.
Logan's previous research found that some flies and mosquitoes carry a genetic change in their odour receptors which make them insensitive to the smell of DEET.
But this study now suggests that the mosquitoes' response is based on short-term changes rather than genetic ones.
"Our study shows that the effects of this exposure last up to three hours. We will be doing further research to determine how long the effect lasts," said Mr Logan.
"This doesn't mean that we should stop using repellents - on the contrary, DEET is a very good repellent, and is still recommended for use in high risk areas. However, we are keeping a close eye on how mosquitoes can overcome the repellent and ways in which we can combat this."