All summer long, Harris County uses insecticide trucks to spray through neighborhoods.
But about once a year, they send up planes to do aerial spraying in areas where West Nile virus is more active.
Harris County Mosquito Control Director Dr. Rudy Bueno says starting tomorrow evening, they'll spray about 63,000 acres by air.
The spraying will take place primarily north and west of the 290/Beltway 8 intersection.
"It really helps to supplement our ground operations, because we've been spraying since the first part of June. But Harris County is a large county, I mean it's close to 1,800 square miles. It really helps to have the aerial operation because we get really good coverage in those hot spots, but at the same time it helps to release trucks to those other areas where we're finding West Nile activity."
Dr. Bueno says it's the dead bird count that signifies the spread of West Nile.
"This year it certainly appears to be there's a lot more virus circulating in the bird population, which makes it easier for the mosquitoes to acquire the virus."
Dr. Bueno says anyone concerned about exposure to the aerial spray should stay indoors tomorrow evening. But he says the insecticide degrades very rapidly and doesn't hang around in the environment very long.