Standing water following flooding could mean more mosquitoes

Mosquito activity across Iowa has been quiet so far this year, with average populations measuring about half as big as last year at this time, according to Iowa State University entomologists.

But that could change in the coming weeks as floodwaters are expected to recede and leave behind plenty of standing water. Puddles and pools are 'prime real estate” for mosquitoes to lay eggs, according to Brendan Dunphy, a research associate in ISU's Department of Entomology.

In fact, Dunphy said in a news release, the 'running dogma is that with all this rain, the mosquito population should explode at any time now.”

'Frankly,” he said, 'I'm a little surprised that it hasn't done so already.”

Mosquito traps managed by the ISU Medical Entomology Laboratory averaged twice as many mosquitoes last June than this June. The statewide weekly average reached about 600 mosquitoes per trap in June 2013, and the highest average this year was about 300 per trap.

ISU entomologists say the drop might be related to this spring's late frost, which could have wiped out immature mosquitoes. Or low mosquito numbers late last summer could have resulted in fewer eggs.

And this year's numbers are statewide averages, Dunphy said, meaning some mosquito populations at the local level might be more on par with last year's activity. Still, he said, mosquitoes clearly aren't as problematic as they were last summer - so far.

Wetter conditions this summer have caused rivers across northern, central and Eastern Iowa to swell, flooding many communities - including parts of Johnson and Linn counties. Recent forecasts show less moisture in the forecast and floodwaters receding, but the standing water that gets left behind historically has meant more mosquitoes.

ISU data shows a consistent rise in mosquito numbers in flood years like 2008 and 2010. In 2010, according to the lab, traps in western Iowa attracted up to 3,000 mosquitoes in the weeks following widespread flooding there.

Lyric Bartholomay, an associate professor of entomology and director of the medical entomology lab, said in a news release that those flood-year trends might provide a glimpse of what's in store for the rest of this summer.

'What we've experienced so far probably isn't a good indicator of what's going to happen in July,” Bartholomay said in a news release.

Increases in mosquito populations can take about two weeks because the insects have to lay eggs and then wait for immature mosquitoes to grow into adults.

Mosquitoes are mostly harmless insects - except those carrying West Nile virus, an infection that can cause minor symptoms like fever and headache or life-threatening illness like inflammation of the brain.

Since the virus first appeared in Iowa in 2002, it has been found in every county, either in humans, horses or birds. Last year, the Iowa Department of Public Health reported 44 human cases of West Nile virus and no deaths.

The public health department last week announced the state's first confirmed human case of West Nile virus in 2014. The infected adult male - aged 18 to 40 - is from Clay Count and is recovering, according to public health officials.

Iowa's first human cases of the virus were reported around the same time last year and in 2012, and both those years saw many more reports through October. Nationally, as of June 10, 14 counties in eight states had reporting some form of West Nile activity - although most reports were in non-human species like dead birds.

Iowa public health officials also warned that recent rains and flooding could increase mosquito activity, although they said 'flood mosquitoes” rarely carrying the West Nile virus.

'Therefore, heavy rain and flooding don't necessarily result in increased West Nile virus cases,” according to the department.

But mosquitoes carrying West Nile virus can lay eggs in stagnant water, which is why public health officials are urging Iowans to eliminate standing water, where possible.

'It is when flood water lies stagnant for several weeks that the threat of mosquitoes carrying West Nile virus increases,” according to the department.

ISU associate professor Bartholomay said homeowners should rid their property even of standing water in flower pots, gutters and bird baths to avoid increased mosquito activity.

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