The folks from the Mosquito and Forest Pest Management Branch of the Prince William County Public Works Department regularly test for mosquitos carrying the West Nile Virus. When they find a group, or pool, of infected mosquitos, they get to work and spray to kill the little blood suckers.
Rachel Kempf, a Mosquito and Forest Pest Management specialist with the county, said fogging trucks generally spray after dark when most people are inside and mosquitos are most active. "We spray in the evenings after sunset and will sometimes go until 12 or 1 a.m. depending on how many spray blocks are scheduled."
The spray is a synthetic pyrethroid, which is a common pesticide that mimics the natural pyrethrum derived from the chrysanthemum plant. The spray has very little residual activity, due to rapid degradation in sunlight.
While the spray is considered a low toxicity pesticide, Kempf says that it is always prudent for citizens and their pets to stay indoors while the trucks are conducting spray operations. "The risk is low with proper application, but contact with the spray can lead to minor eye and skin irritation. As a precaution, bring pets and their food and water dishes, as well as children's toys and clothing inside, and avoid contact with wet surfaces until they have completely dried, around one hour post spray. If the spray ends up in your eyes or on your skin, flush immediately with water."
To determine where to spray, specialists from the pest management branch trap adult mosquitos and send them to George Mason University for a DNA amplification test, which determines if West Nile Virus was present in that pool of mosquitos.
The sprays occur Monday through Friday between June and the end of September and only occur as necessary. An area will typically be treated once during the season, unless the traps indicate more spraying is needed, said Kempf. "We spray only in response to West Nile Virus positive mosquito pools. Some areas of the county will not be sprayed, and some areas will be sprayed more than once over the course of the summer, depending on what we find in our adult traps."
It is important to note that while Mosquito and Forest Pest Management continually manages mosquito populations from April through October by treating for mosquito larvae, populations of adult mosquitoes can still rise and become infected with diseases in certain areas. Therefore, Kempf said people should take precautions to knock out the places mosquitos breed. "Mosquitos tend to breed in stagnant streambeds, ditches, that kind of thing, but they also breed in containers. So, it's important for homeowners to go out and check their property for any amount of standing water. Mosquitos can breed in as little as a quarter-inch of water."
In addition, the Mosquito and Forest Pest Management Branch conducts free mosquito site inspections for Prince William County residents to help locate, eliminate and treat sources of mosquito breeding on their property. Residents can call the branch's office at 703-792-6279 to find out more about this service.
For more information about the county's mosquito abatement program and what areas will be sprayed, visit the branch's spray information page.