Bite Me: Weather's Impact on Mosquitoes

July 20, 2010; 12:41 PM ET
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A mosquito lands on a man's hand Friday, June 26, 2009, in Olmsted Falls, Ohio. (AP Photo/Mark Duncan)

Flooding rain in certain parts of the country has created ideal breeding conditions for summer's greatest pest: the mosquito.

Mosquito eggs are laid in water, which is why the insects are often associated with marshes. However, puddles and standing water can also foster the larva.

Urban entomologist Steve Jacobs of Penn State, said that even "pools, birdbaths and boat tarps" where water collects can become breeding grounds for mosquitoes.

The key thing is that the water be still. Frequent rain can actually wash out the eggs, diminishing the chance for mosquitoes to become a problem.

"Rarely do you have a problem in moving water," Jacobs said.

While most mosquitos in the U.S. can be found in the southern parts of the country, Jacobs said they can be found just about anywhere. Jacobs said that some types of mosquitoes can be found in pools coming off melting glaciers - indicating that certain varieties of the insect can withstand cold temperatures.

This year, parts of the country that have experienced significant rainfall include much of western and southern Texas, much of the central Plains, including Iowa and Nebraska, and central Indiana. All of these regions were at above-average rainfall according to the July 10, 2010, Short Term Drought Index. Some of those areas had received as much as 4 inches more rain than they typically do for this time of year.

Western and southern Texas were doused when Hurricane Alex moved through at the end of June, beginning of July.

An increase in the mosquito population "is not uncommon following big storms," Jacobs said.

What really matters, though, is how long the water on the ground remains after the storm. If the water stagnates, there is a greater chance for mosquitoes. Jacobs said the topography of a region can have a significant influence in whether the water stays around.

Map shows the distribution of non-human activity (shaded in light green) and human infections including PVDs (dark green) occurring during 2010 by state. If West Nile virus infection is reported from any area of a state, that entire state is shaded. Map courtesy of the Center for Disease Control.

However, another weather factor may inhibit the activity of mosquitoes: the heat.

The hottest June on record may actually be downplaying some of the effects of mosquitoes.

Bill Meredith, the program administrator for the Delaware Mosquito Control Section, said, "You can actually get too hot and dry for mosquitoes to be active."

However, Meredith also noted that mosquitoes like humid conditions.

An additional concern comes in, however, because research indicates that with mosquitoes carrying a disease, such as West Nile virus, the virus can be amplified within the mosquito in warmer weather.

According to Meredith, this means there is "more possibility of the virus being passed on [to a human or animal], when the mosquito takes a blood meal."

"In extreme hot weather, in some areas, mosquitoes would be less active, but the level of virus could be higher. It's a trade-off. If they're less active, they're going to do less biting..." Meredith said, but as soon as there is a cool spell, mosquitoes with a lot of virus in their bodies will become more active.

The "big five" of viruses carried by mosquitoes are West Nile, Eastern Equine Encephalitis, LaCross virus, and both Western and St. Louis Encephalitis.

Meredith also said there is currently an outbreak of Dengue Fever in Key West.

"Within the past few weeks, it appears that the virus activity, as monitored in New Jersey, Pennsylvania and Massachusetts is picking up." Meredith said.

In the longer term,'s Expert Senior Meteorologist Paul Pastelok said the weather near the Gulf Coast has created favorable conditions for mosquitoes.

"Right along the Gulf Coast, it's been pretty dry across the interior southeast and that may be the case for a while still there," he said.

In general, Pastolek said overnight temperatures will influence the mosquito population.

"The nights have been incredibly high and that's the driver actually. Daytime highs have been high; don't get me wrong," he said, "but the nighttime lows have been up, and that's when you get your most infested."

Meredith said that a peak time period for mosquito-born viruses is between mid-August and October.

Pastolek said the warm nighttime temperatures will allow mosquitoes to thrive. "That's been the case for a good portion of the nation."

"I think the Gulf Coast is an area to watch and here in the Northeast, we're going to stay warm, probably all the way though October," Pastolek said.

Related to the story:

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