Repellent/insecticide combination proposed to fight malarial mosquito

Researchers in their efforts to fight malaria have proposed using insect repellents along with insecticides to extend the lifetime of the insecticides we have today and, paradoxically, evolve mosquitoes with greater aversion to the repellent.

"If you combine these two, you should get better repellents and you should get a slower resistance to the insecticide, so it will last longer or forever," said population biologist Michael Boots, a University of California, Berkeley, professor of integrative biology, who worked with his colleagues at Exeter University in the United Kingdom on the project and described the technique in an article in the open-access journal eLife.

Insecticides don't kill 100 percent of mosquitoes, those with some resistance survive and come to dominate the population, rendering the insecticide ineffective.

Repellents, which can be as simple as a scent that insects avoid, can help protect humans by keeping homes free of malarial mosquitoes. Many of the mosquitoes that carry the malaria parasite have evolved to bite humans indoors at night.

First author Penny Lynch, a postdoctoral fellow with joint appointments at the two universities, said the proposed technique doesn't require an initially very effective repellent to work.

A mediocre repellent will drive mosquitoes to evolve so that the repellent becomes more effective. "That is the premise," she said. "Evolution should be selecting for the ones that are repelled, because they don't get killed by the insecticide."

As fewer mosquitoes encounter the insecticide, there is less pressure for the mosquitoes to develop resistance to it.

"The idea is, if you combine a spatial repellent that is not particularly good initially, with an effective insecticide in the house, then a certain proportion of the mosquitoes will be repelled and some proportion will go inside the house and get killed," Boots was quoted as saying in a UC Berkeley news release.

"So populations that aren't going into the house are under selection for increased repellence or deflection, and as that happens you get less selection for insecticide resistance."

Thanks to work over the past 15 years, much of it funded by the Gates Foundation, residents of many countries in Africa and Asia now have access to bed nets impregnated with insecticides such as pyrethroid compounds.

This has helped drive malaria cases down almost to levels achieved decades ago with the insecticide DDT, Boots said. Now that DDT has been phased out almost everywhere, however, there are fewer fallback options when current insecticides become ineffective because of resistance.

The idea of repellent/insecticide combination came to Lynch as she was working on her Ph.D. thesis on insecticides that kill older mosquitoes, the ones that actually spread malaria.

A financial modeler by training, she began thinking about the genetic fitness of mosquitoes, and how to manipulate that. She teamed up with Boots to use mathematical models of evolutionary competition between vectors and hosts - the malaria parasite and the mosquito, for example - to test whether the strategy would work.

One advantage of the strategy is that it can be implemented without interrupting current mosquito control programs, such as spraying insecticides around homes and using bed nets. A repellent could easily be applied in a home at the same time as insecticides are sprayed.

Boots and Lynch plan to test the hypothesis that mosquitoes can actually evolve to be more repelled by mediocre repellents in Africa or Asia, and determine what levels of effectiveness are necessary for both repellent and insecticide. Endit