The mosquito repellent Deet, which is widely used by holidaymakers and residents in warm climates, is losing its effectiveness, scientists say.
People living or travelling in areas plagued by mosquitoes are more at risk of bites after researchers found the insects are first deterred by Deet, but then later ignore it.
Deet, which was developed by the US military following operations in the jungle during World War II, is one of the most widely used ingredients in insect repellents.
But scientists said they needed to find alternatives to the substance, which works because mosquitoes do not like the smell, after tests showed the insects are becoming increasingly resistant to Deet.
Researchers from the London School of Hygiene and Tropical Medicine took a species of mosquito that spreads dengue and yellow fever and put it in a room with a human arm covered in Deet.
The first time the mosquitoes were tempted with the arm, they were putt off by the smell. However, the second time, researchers found the Deet was less effective.
Dr James Logan from the London School of Hygiene and Tropical Medicine, said that mosquitoes are "very good" at evolving "very, very quickly".
He said: "We were able to record the response of the receptors on the antenna to Deet, and what we found was the mosquitoes were no longer as sensitive to the chemical, so they weren't picking it up as well.
"There is something about being exposed to the chemical that first time that changes their olfactory system – changes their sense of smell – and their ability to smell Deet, which makes it less effective.
"The more we can understand about how repellents work and how mosquitoes detect them, the better we can work out ways to get around the problem when they do become resistant to repellents."
He stressed that people should not stop using Deet but said scientists were now trying to find out how long the effect of the chemical works after mosquitoes are initially exposed.
Alice Philipson Telegraph.co.uk