Pesticides Aren’t the Best Way to Fight Zika-Carrying Mosquitoes

Chemical spraying is less sustainable than other mosquito-fighting methods, and may give people a false sense of security.

A health worker fumigates a neighborhood on the outskirts of Panama City
A health worker fumigates a neighborhood as part of the preventive measures against the Zika virus on the outskirts of Panama City. (Carlos Jasso / Reuters)

When Zika came to Florida this summer, the government launched an aerial attack. Planes flew over Miami, spraying a mist of insecticide over the neighborhoods where the virus was spreading. Attacks from above were supplemented with boots (and trucks) on the ground, where people sprayed certain chemicals to target the adult Aedes aegypti mosquitoes that carry Zika, and others to target their larvae.

All that spraying is an impressive show of force, but a recent study calls into question just how well it works. In a meta-review published in PLOS Neglected Tropical Diseases, researchers at the University of East Anglia (UEA), in the United Kingdom, studied the effectiveness of different methods control Aedes aegypti.

In addition to chemicals, the researchers looked at education efforts, biological methods (like the use of animals that eat mosquito larvae), and combinations of the three. They found that, overall, there isn’t a lot of good evidence out there for any of these methods—which isn’t to say that they can’t work, just that they haven’t been well-studied.

A lot of the research they reviewed was in the form of a before-and-after study, looking at the changes in one place after using one of these methods. It would be better to compare different communities over the same time period—some that got the intervention and some that didn’t, says Paul Hunter, a professor of health protection at UEA, and an author on the study.

But, based on the evidence they have, Hunter says the most effective way to fight the mosquitoes is to remove their breeding sites. Aedes aegypti are hardy little bugs, who can breed in even the smallest amounts of standing water. That means that any standing water near people’s homes puts them at risk. So education campaigns that teach people how to make Zika-free zones around their houses are the best bet. For example, earlier this year in Brazil, soldiers went door-to-door, handing out leaflets.

Of course, for worried citizens, that isn’t as visually impressive of a strategy as people in fumigation suits tromping out to fight the mosquitoes, or planes blanketing neighborhoods with chemical mist. The World Health Organization also comes down on the side of removing standing water, writing somewhat sassily that “although fogging to kill adult mosquitoes provides the most visible evidence that a government is taking action, WHO stresses that the elimination of mosquito breeding sites is the most effective intervention for protecting populations.”

Insecticides, both those sprayed and those added to water to fight larvae, are effective, but not particularly sustainable, Hunter says. “For a few days afterward, they’re brilliant, and then the mosquitoes come back.” So the chemicals need to be applied regularly to make a long-term difference. Biological methods, the researchers found, seem to work a little better.

Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine, favors an integrative strategy. “Chemical methods can be highly effective with a proven track record,” he wrote in an email. “But they are also highly labor intensive and need improvement.” Some biological methods, like genetically modified mosquitoes or Wolbachia, the bacterium that prevents Aedes aegypti from spreading Zika and other diseases, “may represent the future,” Hotez writes, “but it does not mean we need to abandon the old, at least just yet. I think the point is, it can’t be either-or.”

There’s also a more worrying complication of chemical methods that some of the research Hunter and his colleagues reviewed pointed to: Chemical spraying may make people complacent. “People think, ‘I don’t need to worry about removing breeding sites, because, hey, it’s been sprayed so everything’s going to be fine,’” Hunter says.