If you are diligently following the experts’ advice on mosquito control—getting rid of standing water in old tires, pots, gouges in your patio and other places where water pools—scientists have made a discovery that can reduce your labors: concentrate on the puddles where leaves are floating. That might be especially welcome news for Midwesterners who, after suffering the floods of June, are now dealing with plagues of mosquitoes that are in some cases 20 times the usual number.

Entomologists have long known that female mosquitoes—the ones that bite—are drawn to water to lay their eggs, but exactly what the draw was has been a mystery. Scientists at North Carolina State University therefore gave Aedes aegypti, the species that carries yellow fever, dengue fever and other diseases, a choice: lay eggs in plain water or in those where leaves have fallen. As they report this evening in Proceedings of the National Academy of Sciences, the females definitely prefer the latter, by something like 16-to-1.

What seems to happen is that bacteria find the leaves (the scientists tried both bamboo and white oak) and start decomposing them. Chemicals released by the bacteria are sensed by female mosquitoes, who then decide that the water is an acceptable nursery for junior, conclude NCSU’s Charles Apperson and colleagues. Specifically, carboxylic acids and methyl esters released by the bacteria scream “lay your eggs!” to mama mosquito.

How much do mosquitoes prefer leaf-infused water? When the female lands on water in a container, she senses the presence of various bacteria and the chemicals they release, using chemoreceptors on her antennae, mouthparts or ovipositor. Given a choice between pure water and the leaf-infused variety, Ae.aegypti laid 94 percent of their eggs in cups containing bacteria from bamboo infusions and 6.5 percent in plain water; in the next experiment, the insects laid 91 percent of their eggs in cups containing bacteria from white-oak leaf infusions and 9.8 percent in plain water.

“Some water-filled containers are rejected by the female mosquito,” Apperson says. “If we filter the bacteria out, the mosquitoes want no part of the water container. But put the filtered bacteria back in the water container, and the mosquitoes will be stimulated to lay eggs.” Once they hatch, the larvae will chow down on the microbes.

Knowing what stimulates disease-carrying mosquitoes to lay their eggs is getting more important now that once-tropical diseases are invading temperate latitudes. (The World Health Organization estimates that 51 million people are infected with dengue fever every year, that the disease occurs in 100 countries, that there has been a sharp rise in the number of cases in Asia and that it has made its way to Central and South America, on America’s doorstep.) Lesson: be extra vigilant about getting rid of standing water where leaves have fallen. Or have a large supply of calamine lotion on hand this summer.

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