A female mosquito (Aedes aegypti) fills up on human blood. In the process, she can also spread viruses, including Zika, dengue and chikungunya. A new review of existing research shows spatial repellents using volatile pyrethroids can be a useful tool in protecting humans from mosquitoes that may carry disease. (Photo credit: James Gathany, CDC Public Health Image Library)

By Leslie Mertz

Step 1: Complain about how bad the mosquitoes are this year.
Step 2: Purchase a can of bug spray to keep mosquitoes away.
Step 3: Use it once and be disgusted by the greasy feel and weird smell.
Step 4: Put the can in a closet, never to be used again.
Step 5: Complain about how bad the mosquitoes are this year.

Florida entomologist Christopher Bibbs has a better idea. He recommends using spatial repellents—candles and other devices—that emit highly effective compounds, such as volatile pyrethroids, to keep mosquitoes away. He reviews this group of repellents in a new paper published this week in the open-access Journal of Integrated Pest Management (JIPM).

Leslie Mertz

Pyrethroids are synthetic organic compounds based on a substance (pyrethrum) discovered in chrysanthemums, and they offer good alternatives to botanical options, such as citronella and clove oil, according to Bibbs. “While many botanicals have been shown to repel mosquitoes, they are not easy to work with and are often very unstable, breaking down almost immediately in sun or warm temperatures,” he says. “To make more-effective green repellents available, there have to be chemists involved, because botanicals are chemicals, and at present, that’s a bottleneck. We just don’t have people who know how to do it.” Volatile pyrethroids, on the other hand, work very well and are available today to reduce exposure to disease-carrying mosquitoes.

Bibbs is all too familiar with mosquitoes and the pathogens they transmit, including Zika, chikungunya and dengue viruses. Not only does he live in a state where the mosquito is jokingly called the “state bird,” but he is also a biologist at the Anastasia Mosquito Control District of St. Johns County as well as a doctoral student in the University of Florida’s Entomology and Nematology Department. “One of the fundamental things that makes spatial repellents different from topical repellents is that one person can use the spatial repellent and it can actually create a zone of protection to safeguard multiple people,” he says. “On top of that, they are simple to use. You set them down, you turn them on—or, in some cases, you light them—and then you just hang out in the area.”

As a biologist at the Anastasia Mosquito Control District of St. Johns County in Florida, Christopher Bibbs (right) spends a good amount of his time spreading the word about importance of mosquito control. Here, he and former intern Jesse Crosier provide mosquito education and summer safety reminders for a local radio station. (Photo credit: Molly Clark, Anastasia Mosquito Control District of St. Johns County)

Another advantage of spatial repellents is that they don’t suffer as much from what he calls the push/pull effect that occurs with topical repellents. “Imagine that you have six people in a group, all of them breathing out carbon dioxide and other compounds that are like whiffs of really nice-smelling food to mosquitoes,” Bibbs says. The repellents are pushing the mosquitoes away from the five protected people, and the one defenseless individual is inadvertently pulling them in. “That unfortunate person is probably going to get all of them,” he says. “But spatial repellents can protect even non-users, in addition to harming mosquitoes that are successfully repelled so they don’t get the chance to bite someone else.”

Perhaps the biggest drawback with spatial repellents is that people don’t realize just how well they work.  “We are finding that volatile pyrethroids actually affect mosquitoes in a very broad range of ways. They can bring about acute symptoms, such as ‘knockdown’ that causes mosquitoes to become paralyzed in mid-flight, and they can flat-out kill the mosquitoes,” Bibbs says. “This provides an excellent multi-pronged attack, and it’s done using a very simple deployment method.”

Christopher Bibbs applies volatile pyrethroids to perfumery strips that are then introduced into cages of mosquitoes to observe their toxic effects on them (right). (Photo credit: Molly Clark, Anastasia Mosquito Control District of St. Johns County)

He explains that volatile pyrethroids, such as allethrin, metofluthrin, and transfluthrin, work by interfering with the mosquito nervous system. The mosquito only needs to inhale an extremely small amount of the spatial repellent to be debilitated, and the effects are almost immediate. “It happens in an instant, as soon as they fly into the gas,” he says.

Part of the reason the public remains in the dark about spatial repellents is that topical repellents have historically drawn the lion’s share of marketing efforts, as well as much more research attention. Bibbs hopes to influence at least the latter. “While we know that volatile pyrethroids can repel, damage, and kill mosquitoes, we don’t know the degree to which all of these things happen, especially in comparison to well-known and established products, such as DEET. This JIPM review paper is aimed at giving the cast-net of what we do know at the moment about spatial repellents and how these different attributes can come together to make a tool that has a lot of different functions for vector management, to reduce the spread of mosquito-borne diseases,” he says. “At the same time, it’s a call to arms that we need more research to make spatial repellents, including volatile pyrethroids, a more significant tool in our arsenal.”

Until then, Bibbs says consumers should do everything they can to reduce their exposure to mosquitoes and consequently their vulnerability to mosquito-borne diseases. That includes using EPA-approved spatial repellents or topical repellents. “I don’t care which repellent you carry, but carry something and use it,” he says. “You don’t have the right to complain about being bit if you’re not taking a measure to protect yourself.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.