How can we kill, repel or avoid them? Let us count the ways.

mozzie1.jpg Researcher in Ulrich Bernier’s lab runs a control trial seeing how mosquitoes are attracted to chemicals coming off human skin.  Courtesy of Uli Bernier

The female mosquito is a deadly blood-seeking machine, armed with finely attuned antennae and a proboscis serrated for easy entry. In some species, she’ll fly as far as 50 miles to find her blood meal, which she needs to lay her eggs. Her offspring later emerge to breed, feed and continue their life cycle—as well as those of the parasites and viruses that they transmit. One bite is all it takes to contract disease.

No wonder the battle against them has reached the level of high-tech devices and chemical warfare. Legions of scientists are hard at work to foil their blood-letting by figuring out how to best repel, trap or simply disable their ability to hunt us down.

Each year mosquitoes infect some 700 million people worldwide with disease. Malaria alone killed more than a million people in 2005. The first human case of West Nile virus (which is also spread by mosquitoes) hit New Mexico in 2004, and it’s since popped up in all 48 continental states.

“We used to have mosquito-born diseases quite prevalent here in the past—as late as 1935 we had 128,000 cases [per year] of malaria,” said Joseph Conlon, technical advisor to the American Mosquito Control Association, who notes that the nation’s first mosquito control programs were motivated by tourism, not disease control. “Florida would be uninhabitable without mosquito control.”

Last year mosquito repellant sales amounted to $550 million in the US, a figure that will likely only grow. Only 40% of Americans use insect repellent, according to a recent CDC survey. “It does come down to: What are the ways to keep the bugs off of you,” said Emily Zielinski-Gutierrez, a behavioral scientist at the CDC’s Division of Vector Borne Diseases. “If we could get 75% of people to say that they were usually using repellent when they were going out, I’d feel better.”

The industry standard for repellant applied to skin is N,N-diethyl-M-toluamide, or DEET.

DEET was discovered in 1946 and was first used by the US Army. Since 1957, when it hit consumer shelves, it’s been used several billion times with few reported problems. Today, roughly one third of Americans use DEET-based bug juice. It repeatedly tops the competition in efficacy studies and bears the EPA’s seal of approval, but alternatives are sorely needed.

“Florida would be uninhabitable without mosquito control,” said Joseph Conlon, technical advisor to the American Mosquito Control Association.

The fact that it melts plastic does the same to consumer confidence in it.

Jerome Klun, a research entomologist at the USDA’s Chemicals Affecting Insect Behavior lab in Beltsville, Md., believes he’s found DEET’s successor: SS220 or (1S, 2’S)-2-methylpiperidinyl-3-cyclohexene-1-carboxamide. Klun awaits funding to begin pricey EPA chronic toxicity studies. “Cosmetically it’s pleasant—it’s not sticky,” said Klun, who adds it’s more efficient that DEET. “A 20% SS220 [solution] is as good as 33% DEET.”

Other bids for more user-friendly repellents are in the pipeline. Dupont just filed a patent to use a derivative of catnip to keep us from being nipped, and scientists have found natural repellants in things ranging from tomatoes and beautyberry plants to crested auklets.

But repellents are only half the battle.

There are machines like the Mosquito Magnet, described as a “cross between a barbecue and motorboat engine,” which emits CO2 and other mosquito attractants to draw them near and kill them. Its manufacturer, American Biophysics Corporation, will soon offer networks of these traps that communicate wirelessly. This will allow them to coordinate efforts by tracking wind direction, temperature and bite rate.

“We are trying to develop what we call push/pull systems,” said Daniel Kline, a research entomologist at the Mosquito and Fly Research Unit in Gainesville, Fla. “The push would be a repellent and the pull system would be the trap.”

mozzie2.jpg Researchers in Jerome Klun’s lab test how mosquitoes respond to a person’s breath.  Courtesy of J.A. Klun

About 300 different chemicals are emitted by human skin, and figuring out which ones mosquitoes are most drawn to is tedious work. The biggest boost to narrowing down the long list came when Kline decided to stick his dirty socks in the olfactometer, a Y-shaped tube that offers mosquitoes a choice of two scents. “We put it in there and it is the fastest response—it was like every mosquito was woken up and was trying to get to that sock,” he said. The smelly sock test helped Kline find some ace attractants like acetone and dimethyl disulfide.

Nowadays Kline is branching out to studying bird, cow and flower odors that draw mosquitoes. He carefully blends them like perfume to figure out what works best as bait. “We feel that you can use attractants in a trapping strategy to divert mosquitoes away from biting people,” he said. “If you can keep them from biting people you can stop the disease transmission.”

It’s one thing to repel a mosquito, another to bait and kill it, and a miracle to disappear off their radar altogether.

Ulrich Bernier, a research chemist at the USDA’s Mosquito and Fly Research Unit, noticed that some of the 300 compounds emanating from human skin counteracted the attractiveness of others. He tweaked and fiddled with these compounds until he wound up with 17 synthetic chemicals that act as attraction inhibitors. “I can turn off mosquitoes from finding us,” Bernier said.  “It’s like the difference between smelling something really bad or not smelling anything at all.”

Bernier has filed patents for all of his cloaking compounds and is currently testing the best three at the Aswan dam in Egypt, where sandflies are a huge issue. His prototype device is about the size of a silver dollar with a little hole punched in the top, through which the chemical is slowly released. In the lab, one of his attraction inhibitors managed to reduce the attractiveness of one scent from 92% to 5%.

Bernier is hopeful that the chemicals will retain their magical properties in the field. If so, he reckons we could be out of scent, out of mind in as little as two years.

Originally published September 5, 2006


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