Heavy rains bring high numbers of floodwater mosquitoes

Fewer mosquitoes capable of carrying West Nile

When heavy rains fell in southern Wisconsin this month, flood water mosquito numbers rose rapidly.

“Mosquitoes certainly are out in force. I mean this is Wisconsin, after all. We’ve had a lot of rain this year, so it is not surprising the mosquito numbers we’re seeing, but they seem to have really picked up in the last couple of weeks,” said P.J. Liesch, extension entomologist with UW-Madison.

The entomology lab at UW-Madison handles the count and identification of mosquitoes collected by county health departments in southern Wisconsin. The traps from those surveillance programs have at times been so full of floodwater mosquitoes that counting them wasn’t realistic. On those occasions, instead of counting the mosquitoes, they weighed them all.

“There were two traps we had to that for last week that were just thousands more than we have time to count right now,” said Tom Richards, a UW-Madison graduate who works in the entomology lab.

While the number of flood water mosquitos is high the risk they pose to the public is low.

“If there is a silver lining to it, those mosquitoes aren’t playing an important role with diseases such as West Nile virus,” said Liesch.

As of Monday there have been no confirmed cases of West Nile virus in humans in Wisconsin this year. There have been 31 West Nile virus cases found in birds, but that carries less significance because of the migratory nature of birds.

The same conditions that led to high numbers of floodwater mosquitos may be responsible for a low number of culex pipien mosquito, which can carry West Nile virus. While flood water mosquitoes thrive in rainy conditions, culex pipiens do not.

“Luckily for us, despite all of the rainfall and other mosquitoes, that particular mosquito doesn’t fair very well under rainy conditions. One of the reasons for that, it likes to use bodies of water that are more permanent, and in an urban area that includes rainwater catch basins in our storm water runoff systems. If you get enough rainfall it actually gets into those and flushes the water out and those developing larvae get washed away and die,” said Liesch.

Even in a year with fewer culex pipien mosquitoes it is still recommended that individuals protect themselves by wearing a mosquito repellent with DEET.