Efforts to control mosquitoes take on new urgency

The major mosquito spreading Zika virus has quirks that make it one of the toughest to fight

Aedes aegypti mosquito larvae

SCIENCE VS. MOSQUITO  Geniuses at growing in tiny containers of water, larvae of Zika-spreading mosquito Aedes aegypti dangle head down at the water’s surface. 

© Alex Wild

Brazil, now a poster child for mosquito-borne virus spread, was once a model for mosquito eradication.

“It was amazing,” says Dan Strickman, medical entomologist with the Bill and Melinda Gates Foundation. The Aedes aegypti mosquito, today identified by epidemiologists as one of the carriers of the Zika virus, was attacked in the 1930s with the simple tools then available. By 1965, the mosquito was certified as eradicated from Brazil and 17 other countries in the Americas (but not the United States). The feat took ferocious effort, but as the threat dwindled, so did money and the political will to stay vigilant. 

Whether eradication would even be possible now is unclear. But the question of how to cope with Ae. aegypti has taken on new urgency as that mosquito species sweeps Zika virus through South and Central America and into parts of North America. Known as the yellow fever mosquito, Ae. aegypti can also spread dengue, chikungunya and West Nile viruses (SN: 6/13/15, p. 16).

It’s “the most difficult mosquito in the Americas to control,” says Michael Doyle, director of mosquito control for the Florida Keys. The mosquito’s resistance to major pesticides and its unusual biology foil many standard control measures. Some scientists have developed high-tech control approaches. Other specialists are going back to the basics to search for biological vulnerabilities that have been overlooked. 

Casual slappers of mosquitoes tend to lump all of them into one annoying category, but there are 3,500 or so species, with a wide range of idiosyncrasies. Some species, for instance, don’t drink blood. As a group, though, mosquitoes are among the most dangerous animals on Earth, claiming more than 400,000 lives a year just from spreading malaria.

Researchers propose that several species might spread Zika, depending on location: Ae. hensilli was suspected in a previous outbreak on Yap in Micronesia and Ae. polynesiensis in French Polynesia. In the Americas, epidemiologists are watching two widespread invasives: the Asian tiger mosquito Ae. albopictus (SN: 6/29/13, p. 26) as well as the notorious Ae. aegypti.

A forest-dwelling form of Ae. aegypti, native to Africa, frequents tree holes and sucks blood from animals. The worldwide invaders, however, have become domesticated. “They bite almost exclusively humans; they live almost exclusively within feet of humans,” Doyle says.

For many mosquitoes, blood is for motherhood, usually one drink per batch of eggs. The insects meet everyday energy needs with plant sugars such as flower nectar. Ae. aegypti females, however, sip blood often, raising the chances of passing on disease. That’s because they’re unusually adept at extracting energy from blood instead of nectar, Laura Harrington, who studies mosquito biology at Cornell University, and her colleagues found. 

A common way to fight bloodsuckers is spraying pesticides from trucks or aircraft. But spraying often does little to Ae. aegypti holing up in houses, resting on clothing in closets or hiding under beds. And don’t count on nighttime protection from bed nets. Ae. aegypti readily bite during the day.

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To fight such a foe, crews start by trying to kill larvae before they reach vampire age. Mosquitoes generally go through their first life stage in water, and Ae. aegypti needs only a little containerful. So Doyle sends inspectors on house-by-house quests for stray minipools: in a bucket, a Fritos bag, old tires, a kayak and plant saucers by the dozens. And that could be in just one yard. His difficulties make a fine case study in how hard — and expensive — fighting a human-specialist mosquito can be.

The human-versus-mosquito battle isn’t all door-to-door. The company Oxitec, based in the United Kingdom, engineered male Ae. aegypti mosquitoes to carry genes that cause their offspring to die (SN: 7/14/12, p. 22). Using an old insect-control strategy, mass releases of dysfunctional males seduce wild females and, in time, shrink the problem population.

Experimental releases of Oxitec’s genetically modified Ae. aegypti males have reduced the size of mosquito populations by more than 80 percent in a test site of about 5.5 hectares in a suburb of Juazeiro, Brazil. The Brazilian government has approved these engineered mosquitoes for widespread use. Oxitec has applied for U.S. Food and Drug Administration approval to do a similar test in the Florida Keys. On March 11, the FDA released a draft statement predicting “no significant impact” to the environment from the test (SN Online: 3/11/16).

Efforts to genetically sterilize mosquitoes may improve with advances in CRISPR/Cas9 techniques to cut and paste genes (SN: 12/12/15, p. 16). And an advance in the genetics of sex determination last year opens new possibilities for refining sterile-male releases. What’s called an M factor determines maleness in certain insects, and for the first time in any mosquito, researchers determined the sequence of the genetic components of Ae. aegypti’s M factor. Manipulating it to produce entire generations of only males could have many uses, says Virginia Tech’s Zach Adelman.

microscope image of Wolbachia bacteria
Wolbachia bacteria (green, in a mosquito ovary) can pass along their virus-blocking ability to the next generation. D.A. Joubert et al/Plos One 2016
In a different approach to reengineering mosquitoes ( SN: 7/14/12, p. 22 ), researchers with the international consortium called Eliminate Dengue are testing a nongenetically modified mosquito in Brazil among other places. Instead of wiping out a population, the goal is to reduce its disease-spreading power. Infection with a strain of Wolbachia , bacteria common in insects, can render these mosquitoes less likely to transmit dengue virus. A paper due out soon will show that the Wolbachia -carrying mosquitoes are also less likely than uninfected ones to transmit chikungunya, as well as  Zika, says Wolbachia project leader Scott O’Neill at Monash University in Melbourne, Australia.

Search for an Achilles’ heel

What interests Cornell’s Harrington are the undiscovered mosquito-fighting targets. Her lab studies courtship and reproduction in mosquitoes. “I really believe that’s where their Achilles’ heel is,” she says.

Courtship among Ae. aegypti is unusual and more complex than anyone had imagined, Harrington, her student Lauren Cator and colleagues reported in 2009. The scent of a human host attracts amorous male mosquitoes, which fly nearby until a female arrives looking for a blood meal. Male mosquitoes’ wide, feathery antennae pick up harmonic overtones of the whine of female wingbeats. The mosquitoes then synchronize one of the wingbeat overtones. “They’re singing to each other,” says Ethan Degner, a Harrington graduate student. Perhaps there’s a way to disrupt this courtship.

Another of the Harrington lab’s findings might be more immediately relevant. Conventional wisdom is that Ae. aegypti mosquitoes mate only once in their lives. Degner offered lab females a second chance to mate, but with a collaborating lab’s genetically engineered males that produce fluorescent red sperm. In lab conditions, a low percentage of females showed red in their reproductive tracts, indicating they mated twice, Degner and Harrington reported online February 15 in the American Journal of Tropical Medicine and Hygiene. This result agrees with observations of what looked like occasional second matings in the wild. With millions of dollars going into mass releases of sterile competitors to local fertile males, female willingness takes on new importance. 

If biologists come up with some new way to eradicate Ae. aegypti, then humankind would have to decide whether to use it. Aside from moral questions, removing any species from an ecosystem can have unexpected risks and consequences. The weighing of arguments will differ species by species, even for mosquitoes. But the human-seeking form is a relative newcomer to the Americas. So in this era of Zika and other rampant mosquito-borne diseases, whether to blast this mosquito out of the hemisphere, should it ever be possible, might not be a difficult decision at all.

This article appears in the April 2, 2016 issue with the headline, “Science versus mosquito: Controlling one of the world’s most dangerous animals takes on new urgency.”

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