February 20, 2013
If you’re someone that’s naturally irresistible to mosquitoes, a new finding published today in PLOS ONE could make for a rude awakening. A group of researchers from the London School of Hygiene and Tropical Medicine discovered that three hours after an exposure to DEET, many Aedes aegypti mosquitoes were immune to the chemical, ignoring its typically noxious smell and attempting to land on irresistible human skin.
Normally, DEET—short for N,N-Diethyl-meta-toluamide, which is the active ingredient in most insect repellents on the market—works because mosquitoes seem to find the chemical’s smell unpleasant and actively avoid landing on surfaces where it has been applied. But in this study, led by Nina Stanczyk, the researchers found mosquito behavior that runs contrary to scientists’ previous understanding of how the insects interact with the chemical.
Initially, the researchers split a number of Aedes aegypti mosquitoes (a common species found on all continents, including North America) into two groups, each in a metal mesh cage. Then they had volunteers hold their arms about an inch over each cage, with one treated with a 20-percent DEET solution and another that had no repellent (a control arm).
Three hours later, they repeated the experiment, and counted exactly how many mosquitoes overcame the DEET and attempted to get through the metal mesh to reach the arms. They found that about half of the mosquitoes who’d been initially exposed to DEET on their first go-round seemed immune to the chemical during the second trial and tried to reach the DEET-covered arm, compared to the 10-20 percent that had attempted to do so during their first trial. This number was still less than the proportion of mosquitoes trying to reach the plain arm (70-80 percent).
Further proof the development of DEET immunity, though, lies in a third group of mosquitoes, who were exposed to a control arm first and a DEET arm second. Because they hadn’t had the chance to become habituated to the chemical, a much lower amount of them (less than 10 percent) tried to reach the DEET-covered arm.
To ensure that some sort of interaction between chemicals in human skin and DEET wasn’t responsible, the researchers also replicated the experiment with a heating device—to which mosquitoes are naturally attracted—that was also covered in DEET. The results were similar, indicating that the insects were somehow becoming habituated to DEET itself, regardless of the surface it was covering.
So why did the mosquitoes, as a whole, overcome their dislike of DEET? Previous studies by this group and others have found particular mosquitoes with a genetic mutation that made them innately immune to DEET, but they say that this case is different, because they didn’t demonstrate this ability from the start.
They suspect, instead, that the insects’ antennae became less chemically sensitive to DEET over time, as evidenced by electroantennography on the mosquitoes’ odor receptors after each of the tests—a phenomenon not unlike a person getting used to the smell of, say, the ocean or a manufacturing plant near his or her house.
Of course, this sort of aromatic habituation is significantly less convenient, because DEET-based repellents are relied upon not just to help us avoid irritating bites but also to stop the spread of mosquito-borne diseases like malaria and dengue. But the researchers don’t recommend dropping DEET entirely, for a few reasons.
For one, mosquitoes live as adults for only a few days at most, and the habituation likely isn’t passed along to offspring, so the odds that a particular mosquito you come across has already been exposed to DEET is pretty low. Additionally, even if it has, not all of the individual mosquitoes in the trial became used to the DEET, so it should still be somewhat effective as a repellent.
Most important, though, is the fact that we still haven’t developed any other repellent that is as consistently potent as DEET—so for now, they say, people living in areas with high risks of mosquito-borne illnesses have little other choice than to keep using it.
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