Drought Cause Of
Higher Mosquito Populations
Old Mosquito Theory Holds No Water

Washington University In St. Louis

So, you think you know mosquitoes? Consider the venerable law that rainy weather is the cause of increased mosquito populations.
An ecologist at Washington University in St. Louis says if you believe that, you're all wet.
Jon Chase, Ph.D., assistant professor of biology in Arts & Sciences at Washington University, has found that the previous year's drought is the cause of high mosquito populations in wetlands the following year because drought drastically reduces mosquito predators from fish to water beetles and other competitor species.
This conclusion stands the ecological world on its head and, with more extensive scrutiny, eventually could have implications in prediction and control of diseases like West Nile virus, St. Louis encephalitis and malaria. Chase and his group, including his wife Tiffany Knight, a postdoctoral researcher at the University of Florida in Gainesville, are analyzing data from locales across North America to see if their results found in wetlands translate to other landscapes, such as urban and suburban areas.
The study appears in the October issue of Ecology Letters.
"We're dealing with the pure ecology of mosquito larvae and wetlands and trying to see if this translates to the landscape level," said Chase. "This is important because mosquitoes live in all sorts of habitats besides ponds and lakes Ð consider tire swings and puddles. With the data we have we're trying to run correlations between mosquito density over 20-year and 30-year spans to see how well it predicts mosquito abundance. We've found that the current year explains almost nothing but the past year explains a lot. We're also looking into disease implications. This is much harder to prove because there is so much epidemiology to take into consideration, for instance the role of intermediate hosts and other complexities. Very few people are looking at these mosquito-borne diseases in a modern ecological context. Most of the work comes from the epidemiological, genomic and/or molecular biology perspectives."
Chase, a theoretical ecologist who is changing notions of "niche" ecology, and Knight came to their conclusion by accident. A pond they had been studying in Pennsylvania dried up during a drought and the next year, after the pond was replenished, mosquito larvae came back like gangbusters. They then surveyed approximately 30 ponds, some permanent but others semipermananet and temporary. They found few mosquito larvae in permanent ponds but lots of mosquito predators; in ponds that dried up annually mosquito larvae were scarce because of lots of competitor species, zooplankton, snails and tadpoles. Those ponds that were usually full but dried out after a 1999 drought, however, had lots of mosquito larvae and very few predators and competitors because, Chase reasons, those species aren't adapted to dry spells in these ponds.
The team was able to recreate this natural study by filling tanks with soil and water and stocking them with mosquito larvae and other species found in natural ponds. They gave these artificial ponds three years to allow the ecosystem to form and stabilize, and then slowly drained some of the tanks to simulate a drought. Next year, the mosquito larvae numbers were very large compared with tanks that were drained each year or remained full.
Analyzing data provided by the city of Winnipeg, Canada, Chase and Knight found that drought the previous year is a much better predictor of high mosquito populations than the current year's rainfall.




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