Memorial Day weekend is not only the kickoff for the summer season; it is also the start of an intensive regionwide offensive against mosquitoes.

In Virginia, Prince William County has declared unmaintained swimming pools an official "health menace'' because they can serve as giant breeding grounds for the kinds of mosquitoes that carry West Nile virus -- a disease considered endemic to the area.

The "menace" designation allows county officials to go onto private property and remove aboveground pools, place covers on in-ground pools or put mosquito larvicide into the water.

"We're not just going to march in and throw our weight around. We will work with homeowners," said Earl Tester, a supervisor at the Prince William Health District, who said neglected pools are a significant problem in the county. "But they can't ignore the problem."

Across the region, localities are gearing up for another summer of fighting West Nile, a potentially deadly virus that has spread quickly since it was noticed in 1999.

"Standing water is a concern with every jurisdiction, and it goes beyond pools," said Sean O'Donnell, an official with the Metropolitan Washington Council of Governments. "Swimming pool covers, drain spouts, anything that can hold standing water for a week or more is a vital concern."

O'Donnell said the region's major jurisdictions have taken steps to control stagnant water where mosquitoes love to breed.

Fairfax County has hired a contractor to target storm-drain catch basins, which are being treated with the larvicide Bacillus sphaericus, which dissolves in water. The substance contains proteins that, when consumed, cause mosquito larvae to starve, said Jorge Arias, the county's West Nile virus program supervisor.

Fairfax also is distributing education materials in English, Chinese, Korean, Spanish and Vietnamese that give residents tips about reducing the mosquito menace, from gutter cleaning to swimming pool maintenance.

"Most people are very aware of the problem and want to resolve it,'' Arias said. "I don't think anyone wants a slimy swimming pool in their yard.''

Montgomery County doesn't have a crew out looking for standing water, but it will respond to complaints, said Mary Anderson, a county spokeswoman.

"If someone doesn't consistently cut their knee-high grass and leaves garbage all over the place, we will [clean it up] and bill them," she said. "It is the same theory."

The District recently passed legislation to crack down on standing water, a spokeswoman with the city Health Department said.

Prince William, however, is the jurisdiction most aggressively targeting backyard pools, and officials hope the extra enforcement will capture the attention of thousands of pool owners in the county.

"We used to look at mosquitoes as a nuisance, like ants,'' Tester said. "But since 1999, we've had West Nile. We are now faced with a mosquito-borne disease that brings illness and death.''

Prince William has 30 to 40 active investigations of "problem" pools that the county is trying to bring into compliance. Last year, the health district handled about 150 complaints about unmaintained pools and other sites with standing water. A pool is considered to be in violation if the water has turned black or green because of the presence of organic matter, Tester said.

"Certainly, our first approach is persuasion,'' Tester said. "But there is an ordinance on the books that allows us to enforce the rule, instead of just trying to convince people to do this on a voluntary basis."

Prince William has a reputation for being proactive when it comes to fighting mosquitoes. It has a mosquito control program that includes one of the few adult-mosquito spraying programs in the region.

The target of the regionwide hunt is called Culex pipiens, or the northern house mosquito. Of the 25 breeds of mosquitoes in Virginia, Culex pipiens is the most common "night biter'' and is an effective carrier of West Nile. That species is known as a container breeder, and Tester said it would be possible for that mosquito to breed in a tablespoon of water left undisturbed in warm weather for three days.

Lakes and ponds are not as great a risk for mosquitoes because those bodies of water are likely to have mosquito predators, such as fish and frogs.

Tester said the source of a mosquito problem is often quite nearby, perhaps an old tire, a tarp covering firewood, old toys -- or unmaintained pools.

"People tend to grow their own mosquito problems,'' Tester said.

Swimming pools can be mosquito breeding grounds. "Standing water is a concern with every jurisdiction," one official says.