Because the mosquito doing it is a daytime species. The Asian tiger mosquito is the one responsible for most of the biting in Vienna, and Fairfax County’s health department ranks it as the county’s number one nuisance mosquito. Unlike the dusk-and-dawn mosquitoes people expect, it is an aggressive daytime biter that spends the hot part of the day resting in cool shade and comes out to bite whenever someone disturbs it. Vienna’s mature tree canopy and layered shrubbery give it an unusual amount of that shaded daytime harborage, which is exactly why a shady, established Vienna backyard can be unusable at noon while a sunny open lot down the street is fine. Treatment here targets those shaded resting surfaces, because that is where the adults actually spend their day.

Neither one treats your yard. Vienna is an incorporated town with its own government, but it does not run a mosquito-spraying service. Fairfax County does operate a mosquito program, the Disease Carrying Insects Program, but it is surveillance rather than treatment. The county sets thousands of traps through the summer and tests them for West Nile and Zika, and it applies larvicide to the stormwater dry ponds it maintains. It states plainly that it does not spray for nuisance mosquitoes and does not treat private property. So on a Vienna lot, whether you think of yourself as a town resident or a county one, the standing water and the biting population on your property are yours to manage.

It can, though not for the reason most people assume. The Asian tiger mosquito does not breed in flowing creeks or along the trail itself. It breeds in small containers of standing water. What a green corridor like the W&OD Trail or the Wolftrap Creek drainage actually adds is continuous shade, moisture and vegetation, which is prime daytime resting habitat for a mosquito that is already breeding in the containers around your house. So a property backing onto that kind of corridor often has more resting adults moving through it. The fix is still on your own lot: remove the standing water that produces them and treat the shaded edges where they rest.

Small pockets of standing water you probably walk past every day. On an established Vienna lot the usual culprits are tree holes in mature trunks, gutters packed with a season of leaf litter, the saucers under planters, birdbaths, tarps and forgotten buckets, and the low spots that hold water after rain. Fairfax County makes the point that a container as small as a bottle cap holds enough water to breed mosquitoes, so the sources are easy to miss. The first and most effective step on any older Vienna property is a walk of the whole lot to find and empty every one of them, because a barrier treatment works far better once the on-site breeding has been cut off.

Roughly early spring through the first hard frost, which in this part of Northern Virginia generally means activity from around April into October or November depending on the year. The Asian tiger mosquito becomes active as temperatures warm and keeps going until a frost knocks the adult population down, with the hottest, most humid stretches of summer being the worst. That is why seasonal protection is built as a series of treatments every few weeks across those months rather than a single spray, and it is also why the early season matters more than people expect. Getting standing water cleared and the first treatment down before the population builds keeps the whole season more manageable than trying to claw it back in August.