Prince William County's war on mosquitoes begins at dusk on hot, sticky nights as a county truck laden with 10 gallons of insecticide pulls out of a Lake Ridge parking lot and heads into nearby neighborhoods.

Recently, workers Filipinas Caliboso and Jeffrey Gallagher spent nearly four hours spraying four subdivisions with Anvil, a common insecticide that smells mostly of mothballs. They traveled 50 miles, most of it at about 15 mph, up one street and down the next.

Some residents waved merrily from their yards like kids to an ice cream truck, while others quickly scurried inside as though in fear of an alien invasion.

Prince William is the only county in Northern Virginia that sprays regularly every summer. Other counties and towns have deemed spraying to be less effective at addressing mosquito-borne illness, such as West Nile virus and malaria, than killing larvae by putting pellets in ponds and other water sources.

"Adult spraying only gets those mosquitoes flying at the time," said David Goodfriend, director of the Health Department in Loudoun County, which has not sprayed since three cases of malaria occurred there two years ago.

But Goodfriend said Prince William's mosquito spraying is fine if residents want it. "My sense is they get fewer complaints because people are used to it," he said.

Prince William treats mosquitoes not just as a health hazard but also as a nuisance -- a threat to summer suburban rites such as sitting in the yard and barbecuing on the patio. Spraying has become an accepted tradition, said Karen Walker, chief of the county's gypsy moth and mosquito control branch. Out of 80,000 households contacted about spraying this season, only 133 opted out.

Although there is a threat of West Nile virus, a mosquito-borne disease that can lead to encephalitis or meningitis, there have been no human cases of the virus in Prince William. There were five human cases in Virginia last year, including one death, in Fairfax County, according to state health statistics. In 2003, there were 24 cases and one death.

At 6:43 p.m., Caliboso held a small, red device -- a sort of "everything" meter -- out the driver's side window of the white Silverado truck. A digital readout pegged the relative humidity at 42 percent, the temperature at 86 and the wind speed at 1.2 mph. Perfect conditions for spraying.

"Beacon light. On," Gallagher said as he hit a switch on the truck's dashboard. The flashing amber light sent flickers of orange into the dusk. "Spray. On," Gallagher said, flipping the switch on a black box labeled Phoenix Fogger.

In the bed of the truck, a sprayer that looks like a Kelly green machine gun sent a billowing plume of insecticide into the air as Caliboso drove. Two minutes later, the sprayer sputtered like an empty hairspray can.

Back to Lake Ridge for another truck.

By 7:26 p.m., they were back in business. "All right, we've got a nice cloud," Gallagher said as they sprayed from a fresh tank. "We're sitting pretty."

Gallagher, 28, was educated as a marine biologist and ecologist. He works as a geographic information systems specialist, mapping the pools and water sources where mosquitoes are breeding in the county. He sprays at night to earn overtime.

Caliboso, 54, said she worked as a research entomologist in the Philippines for 26 years and came to Virginia two years ago. She worked with Orkin Pest Control until joining the county's mosquito crew two months ago. She tells everyone to call her "Pi" -- lest they stumble over "Filipinas."

"This is mine and Pi's first time working together," Gallagher said. "Tag team. We're here."

The job, however, isn't easy. Some of the narrow rural roads are difficult, and then there are the 133 "non-participants" -- residents who don't want their property sprayed. The county cannot spray within 50 feet of their property lines.

About 8 p.m., Caliboso and Gallagher came across Doreen Nagourney, holding her cocker spaniel-poodle mix, Bailey. She wanted to know why she never knows when her street is to be sprayed.

The spray itself can be a nuisance, like being in a sudden downpour without an umbrella. The county generally does not know it is going to spray an area until that day, Gallagher explained.

The decision to spray is based on the mosquito count in traps throughout the county, Walker said in an interview. If a trap catches an average of 15 or more mosquitoes a day, the county will spray, she said.

So for the next three hours, the mosquito control truck looked like a traveling disco party of smoke and flashing lights. Inside the cab, Gallagher turned on the radio and bopped his head to rock music to stave off boredom as Caliboso focused on the road.

"One more to go," Gallagher said at 9:45 p.m. "This is the seventh-inning stretch."

They stopped in a roundabout, got out of the truck to check the insecticide and gas for the sprayer, then got back on the road.

Nearly an hour later, they were done. "That's it," Caliboso said laughing. "The night is still young."

In the battle to save outdoor activities from mosquito attack, a crew travels the roads in the evening to spray. Prince William is the only Northern Virginia county using spray every season. County employee Jeffrey Gallagher chats with residents Larry Kidwell and Doreen Nagourney while spraying with partner Filipinas Caliboso in a residential area near Manassas.