Stop blaming the rain

a rainy year means more mosquitoes? Not quite, suggests a study. Researchers have found that because dry weather knocks out key mosquito predators and competitors, last year's drought -- not this year's annual rainfall -- may best predict mosquito outbreaks.

The idea came to Jonathan M Chase of Washington University in St Louis, Missouri, and Tiffany Knight, a researcher at the University of Florida in Gainesville. A pond they were studying in Pennsylvania dried during a drought. The next year when it filled again, the number of mosquito larvae skyrocketed. To explore what was happening, the two ecologists surveyed about 30 ponds of three types: permanent, semi-permanent and temporary (as per their water level throughout the year).

In permanent ponds, they found few mosquito larvae but plenty of fish, water beetles and other critters that feed on mosquito larvae. Mosquitoes were also scarce in ponds that dried up every year, as the ponds were rife with competition: zooplankton, snails, and tadpoles that compete with the mosquito larvae for algae and other food. But in ponds that were usually full but dried out after a drought in 1999, mosquito larvae burgeoned the next year. Reason: drought killed predators and competitors, as they were unable to adapt to the dry spells.

Chase and Knight studied only two species of mosquitoes (i>Anopheles quadrimaculatus and Culex pipiens) that breed in wetlands. Experts suggest that drought also plays a role in the outbreak of mosquitoes that breed in tree holes.