Does stormwater breed mosquitoes?

Only stagnant water is a problem

Last summer, I saw a notice posted over the biggest pool at the east end of the greenway, announcing that the city had treated the pool for mosquitoes.  So, I wondered if our plan for a chain of rain gardens and pools in the ravine would create a mosquito problem.

As you read on, you'll discover that some kinds of mosquitoes do love stormwater, but only if it's stagnant for several weeks.  If the pool dries up in a week or two, no problem.  If the stream flows again, it flushes out the larvae, so again, no problem.  And the mosquito larvas can be killed by a larvacide, or by stocking the pools with minnows. 

Neighbors can watch to see if pools linger, and easily take action if they do.    In conclusion: the stream in our ravine is unlikely to present a mosquito problem.

Tracking mosquitoes for the sake of public health

The following is condensed version from an article on CALS News.  Culex is one species of mosquito found in Madison.

"...Unlike most mosquitoes, Culex prefers to deposit its eggs in the most stinking, fetid water imaginable. In fact, when they want to attract Culex females that are ready to lay, the scientists make a mix of chicken poop, brewer’s yeast and grass clippings, and then “brew it up on the back deck until it smells really awful,” laughs Paskewitz.

And where in Madison do such rank conditions exist? Mostly within stormwater ditches, say the researchers, although not all the time. When ditches are streaming with runoff from city streets, there’s no problem. But as soon as flows stop and the water becomes stagnant, they can become “superproducers” of Culex.

To control the mosquito’s numbers, Hausbeck’s department began applying a mosquito larvacide to the 12 most productive sites shortly after Irwin identified them. Similar to Bt – the natural, bacterial insecticide the city uses to combat gypsy moth – the larvacide is environmentally benign and highly specific to mosquitoes. Still, its use has raised concerns, the main one being the development of resistance, says Hausbeck.

“The more we use these larvacides, the more we have to worry about mosquitoes becoming tolerant and them becoming less effective,” he says.

That’s why the trio has launched an experiment this summer to see if supplementing the larvacide with an inexpensive native fish, the fathead minnow, can reduce Culex numbers even further. The idea is that if the minnows eat large numbers of larvae, the city could add larvacides less often or perhaps stop using them altogether.

In the meantime, Paskewitz and Irwin continue to monitor Madison’s overall community of adult mosquitoes, as well as those that target people. So far, the news is good. In human landing catch studies they’ve conducted during the prime mosquito-feeding hours of 5 and 10 p.m., the pair has bagged thousands of mosquitoes, but only a handful of Culex: just 13 last year, and about six the year before, says Paskewitz."

Written by Madeline Fisher on 9/3/2008 for CALS News