A new study shows that fertilizer present in water where mosquitoes such as Anopheles gambiae breed can boost growth of bacteria, algae, and fungi, which mosquito larvae feed on, resulting in accelerated larval development and greater survival rates to adulthood. (Photo credit: James Gathany, CDC Public Health Image Library)

As mosquitoes seek standing water to lay their eggs, many are attracted to water rife with plant matter, which serves as a food source for larvae as they develop. Examples of such locations range from backyard flowerpots to flooded rice paddies. But what happens when humans apply a fertilizer that runs off into the mosquitoes’ breeding sites?

A new study published in December in the Journal of Medical Entomology shows the mosquitoes of at least two species develop both faster and in greater numbers when plant matter and fertilizer are combined in the water where the mosquito larvae grow.

Frédéric Darriet, a researcher at France’s Institute of Research for Development (MIVEGEC), tested the larval growth rate and adult emergence of Aedes aegypti and Anopheles gambiae mosquitoes in laboratory breeding settings containing varying levels of nitrogen, phosphorous, and potassium fertilizer and plant matter. Across the tests, 1.7 to 3 times as many mosquito larvae developed to adulthood in settings with both plant matter and fertilizer compared to those in water with plant matter alone, and the mosquitoes’ development rate was two to four times as fast on the combo, as well.

“Fertilizer is not directly assimilated by the mosquito larvae,” says Frédéric Darriet. “However, the three minerals [nitrogen, phosphorous, and potassium] enhance the development of bacteria, algae, and fungi, increasing the food biomass of the breeding sites. Larvae of mosquitoes exploit this additional biomass to proliferate.”

The mosquitoes seem to sense this advantage, too. In previous research, Frédéric Darriet found female mosquitoes were more attracted to breeding sites with chemical fertilizers in them. The problem, though, is not just one of simple proliferation of potentially disease-carrying mosquitoes. Because pesticides are often used alongside fertilizers in agricultural settings—again, think of those rice paddies—the conditions are ripe for accelerated insecticidal resistance developing in mosquitoes as well.

“The selection pressure induced is huge, and the resistance mechanisms present in the mosquitoes are selected all the more rapidly and efficiently as the mineral fertilizations and the insecticide treatments are frequent within a time span,” says Frédéric Darriet. “All my research on the subject shows us how the not-so-well-thought-out use of pesticides and fertilizers in agriculture not only impacts the mosquito environment but even generates new ecological systems beneficial to the proliferation of mosquitoes.”

These hotspots for mosquitoes fall into something of a no-man’s land between agriculture and mosquito management, both in practice and research.

“Insofar as the interface of agriculture and public health doesn’t come under the farmer’s competences nor the ones of the vector-control services, there still is a huge research area even nowadays that has only partly been explored,” Frédéric Darriet says. “The synergy of such a partnership between the scientists, the rice-growers, and the vector-control services would initiate pluridisciplinary research programs whose goal would be to protect the crops while reducing the mosquito populations as much as possible.”