How to Bug Bugs

By Laura Sanders, Science News

Bug dope is loathsome to insects in all sorts of ways, new research finds.

Insects are known to intensely dislike the smell of the common repellent DEET, but they also hate the taste of it, a study appearing online August 25 in Neuron shows. And the lemony-fresh molecule citronellal stinks in two very different ways, a study appearing the same day in Current Biology finds.

A thorough understanding of exactly what makes these repellents nasty could lead to a new generation of bug dopes in the future, which would more effectively reduce the spread of diseases such as malaria and dengue fever.

The research “hopefully points more and more toward a new horizon for replacements for things like DEET that will have higher effectiveness, lower toxicity and more environmental friendliness,” says molecular neuroscientist Laurence Zwiebel of Vanderbilt University in Nashville, Tenn., who was not involved in the current studies.

DEET, the main component in popular insect repellents, has been warding off mosquitoes for about 50 years now. It works presumably through its smell, but exactly how it repels insects has remained unclear, says Craig Montell of Johns Hopkins University School of Medicine in Baltimore, who is a coauthor of both studies.

Using fruit flies as stand-ins for mosquitoes, Montell and his colleagues wanted to know whether insects might be able to taste the repellent as well as smell it. Flies avoided drinking sugar water laced with DEET, the team found. To rule out the sense of smell, the researchers genetically engineered fruit flies that lacked a functional version of an important smell receptor. Those nonsmelling flies still avoided the DEET-laced sugar water, suggesting that some other sense was also at work.

Further experiments showed that three specific proteins — all of which respond to bitter tastes — responded to DEET. When the researchers disrupted these taste receptors, the flies no longer stayed away from the DEET, suggesting that normally, the flies were tasting the nasty chemical. The results, published in Neuron, show that DEET is “working though two senses, not just one sense,” Montell says.

When a female mosquito lands on skin, she uses special taste neurons on her body to determine whether there’s good eating to be had. Since the mosquito tastes before she chomps down, she might be persuaded to leave without eating.

“The first line of defense is you want to keep the bugs away from you,” Montell says. “But even if they do land on you, because DEET is also detected through the sense of taste, it also provides some protection by preventing them from biting.”

Although the DEET study was done on fruit flies, which are easier to genetically engineer than mosquitoes, the researchers think that mosquitoes and other biting insects, such as ticks, fleas or chiggers, may also taste and smell repellants in a variety of ways.

Montell’s Current Biology study focused on a different bug deterrent: citronellal, the lemon-scented botanical chemical that is common in tiki torches and outdoor candles. Vapor containing citronellal repulses fruit flies in two very different ways, the researchers found. One way is through a smell receptor, and the other is through a receptor called TRPA1. TRPA1 is also found in mosquitoes and humans, and it senses other noxious stimuli, including painful cold sensations, sharp needle pokes and the spicy compound in chili peppers. When the researchers genetically interfered with either TRPA1 or the smell receptor, the flies no longer avoided citronellal, suggesting that both were required for the aversion.

Knowing exactly how DEET and citronellal are repulsive to bugs could allow researchers to uncover better repellents that trigger the same reaction, Montell says. “This opens the possibility of screening through literally hundreds of thousands of compounds to find more effective repellents.”

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