Sewage overflows are a fact of life in urban areas, and in many cities, excess sewage water enters streams and lakes with rain runoff. Although this pollution is harmful to most organisms, there is one group of insects that thrives on it: mosquitoes.

Luis Fernando Chaves, a post-doctoral researcher at Emory University, and his team discovered mosquitoes in abundance in a sewage-contaminated stream in Atlanta, but rarely in a nearby clean stream. They also found that mosquitoes were largest in streams with high levels of organic minerals – in this case, nitrogen and phosphorous – that originated from the sewage treatment plants. Likewise, in laboratory experiments, mosquitoes reared in sewage overflow were larger than those reared in clean water. "In this food web, mosquitoes feed on microorganisms that require nitrogen and phosphorous to grow," Chaves says. "This translates into an input of food for the mosquitoes." When there's more organic matter, the microorganisms flourish, he says, and larval mosquitoes can eat like kings.

To make matter worse, it's possible that other aquatic insects that would feed on mosquito larvae are poisoned in sewage overflow water. Many aquatic insects breathe underwater through gill-like structures, so excess nitrogen and phosphorous could be toxic to them. Mosquito larvae, however, breathe air at the water's surface through a specialized siphon. Chaves says that this combination of an increase in food and a potential decline in predators could be the key to these mosquitoes' success.

Larger mosquitoes tend to result from a longer lifespan, which is dangerous because it gives mosquitoes a better chance of harboring pathogens that cause human diseases. Chaves makes the case that cities should separate their sewage overflow from their rain runoff to avoid creating ideal habitat for mosquitoes. This research was presented at the Ecological Society of America's Annual Meeting on August 3, 2009.

(Science Daily, 03/08/2009)