Insecticide-Treated Nets May Create Super Mosquitoes

Two species of mosquitos have interbred, giving rise to hybrids that can resist the most potent weapons used against them

mosquito net bed
Charles Gullung/cultura/Corbis

Evolution’s mechanisms keep life on Earth mutable, adaptable and alive. But it also presents a stumbling block when we humans attempt to control nature. When confronted with penicillin, bacteria develop resistance to the formerly miraculous drug and its successors. When challenged repeatedly with the same potent herbicides, weeds become dreaded superweeds. Now, our efforts to drive back malaria-carrying mosquitoes have created bloodsuckers unaffected by insecticides. 

Since 2000, deaths from malaria worldwide have fallen by 47 percent, according to the World Health Organization’s World Malaria Report. Much of that success in sub-Saharan Africa, where the brunt of that toll is exacted, can be attributed to the use of insecticide-treated nets. The Guardian reports that access to such nets in the region rose from 3 percent in 2002 to 49 percent in 2013. The article, written in December, states that the WHO report "estimates that 214m long-lasting insecticidal nets will have been delivered to the area by the end of this year, bringing the total number distributed in the area over the past two years to 427m."

That kind of firepower gets met by the inevitable mosquito march for survival. The malaria-carrying mosquito species Anopheles coluzzii has apparently interbred with another species Anopheles gambiae. The hybrids carry genes that give them resistance to the most commonly used insecticides, reports Arielle Duhaime-Ross for The Verge

Alarmingly, the rise of insecticide-treated nets in Mali coincides neatly with the development of this resistance, researchers found. They published their work in Proceedings of the National Academy of Sciences U.S.A.

It could be that the hybrid mosquitos who happened to have specific mutations were better able to survive and proliferate despite the nets. "[A] man-made change to the environment — the use of nets — has actually driven hybridization between two species, ultimately leading to an 'improved' mosquito," one of the study’s authors, Gregory Lanzaro of the University of California, Davis, told The Verge. Duhaime-Ross writes:

The findings shouldn’t cause anyone to stop using nets in areas with high rates of malaria; nets are still widely considered the frontline tool of malaria control. But Lanzaro believes that there's an urgent need for new methods for malaria mosquito vector control. Scientists are exploring the use of bacteria to kill mosquito larvae, Lanzaro says. And "work is underway to use genetic methods to kill or alter mosquitoes."

Epidemiologists had predicted this development years ago and urged the WHO to come up with strategies to address and even preempt resistance. Declan Butler reported for Nature in 2011 that those recommendations include alternating what types of insecticides are sprayed, using combinations of sprays and developing new classes of insecticides. 

The news—along with reports that Ebola’s presence may complicate malaria eradication efforts in West Africa—only serves to underscore the difficulty of combating diseases. Just when we think we’ve found an exploitable weakness, evolution changes the game.

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