Floodwater mosquitoes are here 2007.09.12

Written by David Green.

By DAVID GREEN

Not all mosquitoes are created equal.

Just step outside and meet up with a species that hasn’t shown its face—nor its sting—much recently, and certainly not in the abundance we’re seeing now.

Of the 60 species of mosquitoes that live in Michigan, a few belong to the class known as floodwater mosquitoes. Remember the heavy rains of August? The fruit of that flooding has arrived.

Dr. Edward Walker, a Michigan State University professor who studies mosquito biology and control, said the most common floodwater species in this area are Aedes vexan and Aedes trivittatus.

“Both of those are on the wing now in very large numbers,” Walker said. “It’s all a consequence of the rainfall patterns we’ve had in August.”

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Trivittatus is the smaller of the two and Walker guesses that’s the one irritating

people in this area. Many species appear a little wishy-washy about their dinner; not so with the aggressive trivittatus.

“They just hone right in and let you have it,” he said. “There’s no reticence.”

Trivittatus are different from other common species in the area in that they seem to have no hesitation in working during the sunlight, even in a breeze.

They attack in a swarm-like manner and their bite is more painful and irritating.

There’s no question about it; this is no ordinary mosquito.

Life cycle

Floodwater mosquitoes don’t actually lay their eggs in water. The females seek out areas expected to be flooded and lay eggs in the soil.

Scientists suspect that several environmental cues are at play, the most important one probably being the moisture of the subsoil.

“Females are very good at finding suitable habitat,” Walker said, noting that the process isn’t yet understood.

Perhaps odors or microbial activity is at play, he suggested.

Eggs can remain dormant in the soil for years until the right combination of flooding and temperature arrives. Then look out.

“They don’t take long to hatch in the heat that we’ve been having,” Walker said. “It can happen within a week’s time. They’re development rate is very, very fast. They seem to appear suddenly.”

That’s what happened in this area right around Labor Day.

Outlook

The floodwater mosquitoes don’t arrive until the rains arrive, but that doesn’t mean they’ll be gone after their two-week life is over. More rain can lead to another hatch.

According to the Michigan Mosquito Control Association, several generations may hatch each summer, depending on rainfall.

The inch and a quarter of rain that fell this weekend could ensure that trivittatus season is far from over.

Floodplains, ditches, tire tracks, the hoof print of a cow—habitat for floodwater mosquitoes can be widespread.

“Even small spots that look innocuous can produce thousands of them,” Walker said.

The other common floodwater species, vexans, migrate long distances from their breeding habitat.

“They actually move toward lighted horizons and settled areas,” he said. “It’s the kind of year that they’re going to be a nuisance at high school football games.”

Late-season hatches are still possible, Walker said, and the tail end of the season could persist into October. He recalls a rare season when his children were vexed by mosquitoes on Halloween.

Floodwater mosquitoes can be a carrier of the West Nile Virus, Walker said, but they aren’t good transmitters. Although the virus may be present in a mosquito’s body, it doesn’t readily spit it back out.

There are much more important transmitters of West Nile, he said, but that isn’t the case for dog heartworm. Trivittatus is the chief carrier of the heartworm larva.

After a summer almost free of the pests, humans and mosquitoes are finally becoming re-acquainted. And it’s no tame mosquito at play. These guys go right for the face.

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