Stinging news: mosquitoes learning to avoid insect repellent

Robin Wulffson MD's picture
mosquitoes, mosquito repellent, DEET, learned behavior
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Mosquitoes are extremely annoying little critters. They buzz you, bite you, create tender welts, and some can infect you with malaria. When threatened by an attack from the buzzing menaces, many of us rely on an application of mosquito repellent. However, when mosquito season arrives this year, you might find that a squadron will attack you despite a generous application of repellant. Researchers from the London School of Hygiene and Tropical Medicine published their stinging report online on February 20 in the journal PLoS ONE.

The study authors note that 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 and in the field, some studies have surfaced that certain individual insects are not repelled by DEET. They explain that a genetic basis for this has been shown in Aedes aegypti mosquitoes and the fruit fly Drosophila melanogaster; however, for another insect, 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 view of the foregoing, the researchers examined host-seeking (yes, us humans are an unwilling host) behavior and electrophysiological responses of A. aegypti after pre-exposure to DEET. They found that mosquitoes were initially repelled by their initial encounter with the substance; however, they then get used to the smell in about three hours and then ignore it. The British researchers have thrown down the gauntlet and have determined that by increasing their understanding about how repellents work and how mosquitoes detect them, the better they can work out ways to get around the problem when they do become resistant to repellents,

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DEET was created by the U.S. military in 1946 and has been determined not to present any health concerns to the general population. The oil generally works by creating an unpleasant smell for insects, repelling them rather than killing them. To determine DEET’s effectiveness, the investigators tempted some mosquitoes in the lab with an arm covered in DEET. At first the mosquitoes were repelled; however, a few hours later, the mosquitoes were tested again and the researchers found the DEET was less effective in preventing the insects from biting.

The investigators placed electrodes on the mosquitoes’ antennae to measure what was going on in the miniscule heads of the insects. They found that the mosquitoes were growing less sensitive to the chemical. The researchers theorized that first-time exposure to DEET results in changes to their olfactory system (changes their sense of smell) and their ability to smell the chemical, which makes it less effective. The authors concluded that change in behavior as a result of pre-exposure to DEET has implications for the use of repellents and the ability of mosquitoes to overcome them.

Take home message:
If you find that your mosquito repellent with DEET is becoming ineffective, you might try one with another ingredient. Other repellents recommended by the CDC include picaridin, Eucalyptus oil (PMD) and IR3535. Others you might try are indalone, dimethyl pthalate, dimethyl carbate, and ethyl hexanediol. Some plants can repel mosquitoes, such as citronella, catnip, marigolds, ageratum, and horsemint.

Reference: PLoS ONE

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