Smelling the Way to Better Mosquito Management

(ISNS) -- If there's one thing you can count on while enjoying a summer evening outside, it's the after-dusk descent of the mosquitoes. Some people may seem less appetizing than others for the creatures' blood meal, but most will quickly rush inside to avoid the insects' onslaught.

Why do some people become a mosquito's main course, while others go unnoticed?

This anomaly is due to the chemicals given off by both humans and plants, according to biologist Zainulabeuddin Syed from the University of Notre Dame.   

Syed and his team have studied which people and plants are or aren't attractive to mosquitoes, and they're hoping to use that information against the petite pests by recreating plant odors to use in traps or repellents.

This method is feasible because mosquitoes depend on their sense of smell to find a host.

"Insects evolutionarily have simple, centralized brains. Almost half of the brain is dedicated to smell," Syed said. "So if we know what [mosquitoes] smell, then we can probably synthesize it, attract them and kill them."

Syed's research has been focusing on Culex pipiens, more commonly known as southern house mosquitos. These widespread pests are mainly active around dusk and are the dominant carriers of diseases like West Nile virus in the U.S. Other diseases carried, like malaria, pose a global threat, claiming the lives of nearly 3,000 children daily in Africa alone, according to the United Nations Children’s Fund.

After studying other insects for years, Syed became interested in how a floral- and fruity-smelling chemical known as nonanal, short for nonanaldehyde, seemed to dictate the path of Culex mosquitoes. Originally the insect fed on birds, but at some point developed a taste for people.

Syed's team collected odors coming from birds and humans, attached electrodes to the mosquitoes' antennae, and found that nonanal was the dominant chemical that attracted the mosquito to both species, in combination with carbon dioxide and other compounds. When nonanal was combined with carbon dioxide during Syed's previous experiments, he caught 2,000 mosquitoes per night, compared to only 20 mosquitoes with nonanal alone.

"So far I haven't found that some people have a certain 'chemical X' that makes them more attractive," Syed said. "It seems like we give off the same or very similar compounds but in different ratios."

The researchers are now investigating why certain plants attract or repel mosquitoes, because the only food they actually need to survive is plant sugars. Male mosquitoes feed only on sugars, and while females need blood meals to lay eggs, the rest of their life is also spent feeding on foliage.  

"There's no way you can attract a male by just using chemicals from [animal hosts]," Syed said. "If you have something that is based on plants, you can essentially attract and trap both of the sexes. This would significantly reduce the population density."

In order to identify which plants and odors the mosquitoes favor, Syed provides them with an "all-you-can-eat buffet" of plant species in his lab. After the feasting has subsided, his team inspects the mosquitoes' stomachs to identify which vegetation they were feeding on. New trials have just begun now that the plants are blooming, but so far two favorites seem to be goldenrod and milkweed.

Alternatively, the odors from non-attractive plants can also be tested by attaching electrodes to the mosquitoes' antennae to see which compounds they dislike. These chemicals could then be used as repellents.

Like Syed, biologist Remco Suer shares a fascination for the olfaction of mosquitoes. Last March, Suer cofounded In2Care, in order to develop sustainable, cost-effective insect control products.

Using odors to target mosquitoes is a very promising venue, said Suer. He added that what’s rare about Syed’s work is his focus on plant odors rather than human odors. Suer's company has also made use of this approach by adding plant odors to traps.

The ideal repellent in the U.S. has to be chemically engineered to meet a long list of requirements, according to Joseph Conlon, technical advisor at the American Mosquito Control Association. Some of the criteria include being: long-lasting, non-toxic, non-allergenic, non-irritating, odorless, inexpensive, and effective over a broad range of species -- a tall order to fill for a natural repellent.

"The best repellent is the one you will actually use. DEET is the best repellent in the world, but if people aren't using it, it's not doing them any good," Conlon said. "We've got to make repellents so that people actually use them, otherwise we're barking up the wrong tree."

Syed and his researchers are currently analyzing mosquito behavioral data, and will soon begin investigating the chemistry and physiology of attractive plants. He's excited about the possibility of plant-based mosquito traps and repellents, because they would be natural and much safer to use than controversial repellents like DEET, which is a plasticizer, meaning they have softening properties that can damage certain synthetic fabrics and some rubber, plastic, vinyl, or elastic materials such as contact lenses and eyeglasses, according to the National Pesticide Information Center.

For those who don't want to use DEET, Syed and Conlon recommend essential oils, like lemon eucalyptus, that have been diluted with water to achieve a 20-40 percent concentration. But even here, there are caveats. The EPA recommends that commercial repellents containing lemon eucalyptus oil should not be used on children under 3 years old. And unlike DEET, which lasts for up to six hours, these diluted oils are only effective for 30 minutes, according to Syed. Picaridin, a synthetic derivative of the active compound in pepper plants, is also a popular alternative because it is less irritating than DEET. Conlon urges people to eliminate any standing water around the house in order to discourage mosquito breeding.

"Repellency is such a complex phenomenon," Conlon said. "Certain people have a genetic predisposition to not be attractive to mosquitoes, so just because it works for you doesn't mean it's going to work for someone else."

Allison Jarrell is a contributing writer to Inside Science News Service

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