Top image: Aedes aegypti mosquito. Photo: U.S. Department of Agriculture (public domain) via Wikimedia Commons

Residents of North Fair Oaks and Redwood City’s Redwood Oaks neighborhood who are up before dawn on Sunday may see a slow-moving truck releasing a fine mist. It is the first neighborhood spraying in San Mateo County’s months-long effort to contain Aedes aegypti, an invasive mosquito that has now turned up in three parts of the county since July.

The San Mateo County Mosquito and Vector Control District will treat portions of North Fair Oaks and Redwood Oaks between 4 and 6 a.m. on Sunday, Oct. 18, weather permitting, according to an Oct. 15 announcement. Truck-mounted equipment will release microscopic droplets containing DeltaGard, a pesticide registered by the U.S. Environmental Protection Agency for mosquito control. A treatment-area map is posted on the district’s adult mosquito control updates page.

What residents need to know

The district says it is not necessary to stay inside or close windows. Applied as an ultra-low-volume spray, the droplets stay in the air no more than 30 minutes, and residents with outdoor fishponds or beehives should cover them. Its FAQ on adult mosquito control says people who prefer to avoid any contact can stay indoors with doors and windows closed until the treatment is done. Questions go to the district at 650-344-8592.

Crews will trap mosquitoes before and after the spraying to measure how well it worked.

Why now

The district has been working house by house since summer. It first found Aedes aegypti in Redwood City’s Friendly Acres neighborhood in July, and in unincorporated North Fair Oaks in September, according to district releases.

“Since early September, our staff have found invasive Aedes aegypti mosquitoes on multiple properties in North Fair Oaks and now in the Redwood Oaks neighborhood of Redwood City,” said Laboratory Director Angie Nakano.

District Manager Brian Weber said staff have visited more than 1,000 properties in the area. “Unfortunately, we have found these mosquitoes on multiple properties across a wider area than anticipated,” he said. “Controlling the adult mosquitoes in the area will help slow – and hopefully prevent – the further spread of these mosquitoes.”

The day before announcing the treatment, the district reported a new detection in the White Oaks neighborhood of San Carlos, where it is now checking properties door to door.

The mosquito, and the risk

Aedes aegypti are small, about a quarter inch, black and white, and bite aggressively during the day, according to the district. They are not native to California but have become established in many counties. The species can spread dengue, chikungunya, Zika and yellow fever, but the district says the risk of disease transmission in San Mateo County is currently “very low.”

The mosquito’s real advantage is how little it needs. It lays eggs on the walls of containers that hold water, including plant saucers, birdbaths, toys, old tires and yard drains, and “need[s] only about a teaspoon of water to grow,” said Communications Director Rachel Curtis-Robles. The district asks residents to dump standing water at least once a week, use EPA-registered repellents, keep screens in good repair and report daytime biting mosquitoes.

The long view

Climate change rarely announces itself through one big event. More often, a species shows up in a place it couldn’t survive before. A mosquito associated with tropical diseases is now being tracked yard by yard on the Peninsula, by a special district funded through property taxes and benefit assessments that most residents have probably never heard of.

The early work has been labor-intensive: technicians knocking on doors, checking saucers and setting traps. Spraying marks a shift from trying to eliminate the mosquito to trying to contain it. The district’s own words, “a wider area than anticipated,” suggest that elimination may no longer be realistic. If so, the next few years will depend on surveillance data, cooperation from residents, and whether the county can keep a “very low” disease risk at that level as the mosquito’s range grows.

Small habits, like emptying a flowerpot saucer every week, are now part of public health in Silicon Valley.

Sources: San Mateo County Mosquito and Vector Control District releases (Sept. 10, Oct. 14 and Oct. 15, 2026) and adult mosquito control FAQ.