Penn.'s retention ponds: Breeding ground for West Nile mosquitoes?

Tyler Miller

Are township-mandated water retention and detention basins in new developments and commercial areas contributing to the local mosquito population and, in turn, the risk of a West Nile outbreak?

Last month, the Pennsylvania Department of Environmental Protection announced that mosquitoes caught in Washington Township had tested positive for West Nile virus.

West Nile is a disease carried by certain species of mosquitoes and passed on through bites to animals and humans, in some cases causing illness and even encephalitis, an inflammation in or around the brain.

In 2006, Franklin County's 65 detected cases of West Nile were more than any other Pennsylvania county and 21 percent of the state's total cases. This year, the county has seen three positive tests: two in Washington Township and one in Lurgan Township.

Because of the spate of new construction in Washington Township, retention and detention basins designed to accommodate storm water runoff have popped up all over the municipality.

And with mosquitos known to breed in stagnant water, some local residents may wonder whether these basins intended to protect local waterways are actually spawning greater numbers of the nocturnal, blood-feeding insects.

Breeding Grounds?

Several years ago, Quincy and Washington townships and the boroughs of Waynesboro and Mont Alto agreed to have a state-funded study conducted of the Antietam Creek watershed.

As a result, each municipality adopted a storm water ordinance.

In Washington Township, the Antietam Creek Watershed Storm Water Management Ordinance requires professional engineers and surveyors to complete a storm water management plan, which then dictates what basins are constructed.

“The basins are all designed individually to meet the quantity of water,” said Washington Township Manager Mike Christopher. “They're designed to meet the criteria of the best engineering available when they were designed.”

Retention basins slow the push of rainwater from deluging a creek, stream or other waterway, mitigating storm water runoff, while detention basins hold water indefinitely, according to the Franklin County Conservation District.

The basins allow the water to cool after having been heated running across blacktop. Wetlands detention basins offer cattails and other vegetation that help eat away at automobile oil carried by the water runoff.

Standing Water

“Mosquitoes will breed in any water that is stagnant for at least four days,” said Sandra Roderick, a community relations supervisor with DEP's south-central region office. “That (length of time) enables a mosquito to lay eggs and actually hatch.”

Roderick was skeptical that storm water basins exacerbate problems with mosquito populations.

“Basins could possibly (create problems) - as could a bird bath in somebody's yard. I wouldn't point at any one thing as being a hazard or particularly susceptible area,” she said. “Most residential storm water retention basins are designed to drain in less than four days, which prevents mosquito larvae from completing their development.”

Roderick said if basins are functioning properly and not clogged with trash, the water will drain and be absorbed into the ground. Since most basins have a backup pipe that channels overflow to an emergency spillway, even a clog in the main outlet pipe shouldn't allow a basin to fill.

“A well designed retention basin should not breed mosquitoes,” agreed Ray Eckhart, Franklin County's West Nile Coordinator with Penn State Cooperative Extension.

“Mosquitoes take advantage of intermittent water,” he said. “If it's permanent water (like a wetlands detention basin), mosquitoes are less likely to be a problem ... In general, a permanent body of water will have enough natural predators to control the mosquito population.”

Those predators include dragonflies and frogs, he said, making mosquito proliferation less likely in a detention or retention basin than in buckets, tires and other objects left outside that inadvertently collect rainwater.

Testing Positive

Eckhart noted, however, that in the course of building new developments, there is a break-in period for water basins when they may not be draining as designed and may be more susceptible to mosquito reproduction.

Those locations are among the areas tested for West Nile in the county, assured Eckhart.

The testing is conducted by Penn State Cooperative Extension and DEP officials and includes larvae sightings from March onward and the setting of mosquito traps.

In Washington Township, Penn State Cooperative Extension and DEP have been using ATVs with mounted spray equipment for localized spraying of areas that have tested positive for West Nile.

An agricultural insecticide commonly known as Biomist 3+15 ULV, or ultra low volume, is used to kill mosquitos.

The odorless, light-brown liquid is sprayed as a cold fog into the air. Mosquitoes come into contact with the vapor and die, though the Biomist is only effective for the hour or so it hangs above the ground.

“You're releasing a cloud of fog, basically, that mosquitoes fly into,” said Eckhart. “Any decision to do an adult control event is based on surveillance or information that (mosquitoes are active in a given area).”

Eckhart called adult control a “last resort” and said mosquitoes will remain active until the first frost.

Not Usually Dangerous

After previously having been confined to Africa and parts of the Eurasian continent, West Nile first appeared in the United States - in New York - in 1999 and in Pennsylvania in 2000.

There have been 352 human cases of West Nile virus in Pennsylvania since 2001. Twenty-four deaths have resulted, with most of the fatalities being the elderly.

West Nile itself usually isn't dangerous; according to the Centers for Disease Control and Prevention in Atlanta, Ga., only 20 percent of infected people will experience any symptoms - fever, headache, body ache, rash and so forth.

But West Nile does have the potential to cause encephalitis, an inflammation of the brain, and CDC data indicates about one infected person in 150 will experience severe symptoms: high fever, disorientation, coma, convulsions and/or paralysis.

Doctors have yet to devise a specific treatment or vaccine for West Nile, and 983 people nationwide have died from the effects of the virus since 1999.

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