The whir of a large drone echoed over Woodside’s Jasper Ridge Biological Preserve as it dropped pellets packed with mosquito-killing bacteria across Searsville Lake early Thursday morning. The 7 a.m. flight was part of the San Mateo County Mosquito and Vector Control District’s routine work to suppress mosquito populations before they grow out of control.
A century ago, the threat was even more visible. “San Mateo County was so swarmed with mosquitoes that people said their clothing appeared brown from the insects covering them,” Communications Director Dr. Rachel Curtis-Robles said. Malaria outbreaks plagued the Bay marshes unchecked until 1904, when San Mateo County launched one of California’s first mosquito control efforts.
Mosquitoes are the deadliest animal on Earth, spreading diseases like Zika and West Nile virus that kill over 700,000 people every year. “We want people to forget about mosquitoes,” Curtis-Robles said. “But if our work stopped, people would notice within a month and be seriously wondering within six months.”
The mosquito species San Mateo County watches most closely is Aedes aegypti, nicknamed the “ankle-biter” for its habit of biting people’s lower legs during daylight hours. They are a notorious vector for diseases like yellow fever and dengue, which in severe cases can trigger internal bleeding, low blood pressure, and even death.
To keep populations under control, the County relies on biological treatments called bio-rational larvicides that kill mosquitoes while leaving people and the surrounding animals unharmed. Their toolkit includes a combination of Bacillus thuringiensis israelensis and Bacillus sphaericus, bacteria-based products that work like a Trojan horse. “The pellets carry bacteria that are eaten by mosquito larvae and affect their digestive tract, causing them to die,” Operations Director Casey Stevenson said.
The County also distributes free mosquito-eating fish to residents with backyard ponds or neglected pools, another way to strike mosquitoes during their weakest stage: as larvae, before they take flight.
During a recent visit to the County’s mosquito control facility, Stevenson gave a tour of the equipment, pointing to old all-terrain vehicles once used to spray pesticides along the water’s edge. “It was much more dangerous and time-consuming,” he said.
The County phased out those vehicles nearly three years ago, investing instead in helicopters and drones. Helicopters can cover wide areas, but they bring their own problems. “We received complaints that pesticides from helicopters would drift into the surrounding community,” Stevenson said.
Drones, by contrast, are cheaper, quieter, and easier to navigate. The County began adding these unmanned aerial systems to its program in 2021. “We handle the areas with wires, trees, or hazards where crewed aircraft would be dangerous,” Daniel Seaman, senior pilot of Leading Edge Aerial Technologies, said.
Seaman—who served as a California Highway Patrol officer for 22 years before turning his drone hobby into a second career—said the work feels like a natural extension of his commitment to public safety and community service.
Despite their small size, drones are bound by strict flight rules as San Mateo County is home to one of the nation’s busiest airspaces. The drone path often overlaps with airspace governed by San Francisco International Airport, Oakland International Airport, and several smaller local airports, which means the County has to notify air traffic control towers up to two days in advance and keep flights under 50 feet. “We also report every flight to the Federal Aviation Administration,” Stevenson said.
While airspace rules keep drone pilots in check, pesticide regulations bring another layer of scrutiny. Two biologists from the County’s Department of Agriculture, Briana Maldonado and Jorge Zaragoza, monitor operations. Their job is to ensure pesticides are applied safely and according to the label. “We make sure PPE requirements are met and there’s no danger to the public, animals, or property,” Maldonado said.
Oversight in the field is only one piece of the job. Back at headquarters, staff track millions of breeding sites, using mapping software to monitor standing water across the county. But much of their work is still manual. “It would be nice to have an automated way to check catch basins [storm drains] since there are over 40,000 of them,” Curtis-Robles said.
The County has just 22 Vector Control staff to serve nearly 760,000 residents, leading to an immense workload per person. “There are hundreds of thousands of sites around the County that most people don’t even know,” Curtis-Robles said. With such limited staffing, the County depends on residents to share the load. As even a bottle cap of water can host hundreds of mosquito larvae, residents can help by dumping and draining standing water every week, changing pet water often, avoiding overwatering plants, and using an EPA-registered repellent when mosquitoes are active.
Curtis-Robles added that the most important step residents can take is to stay vigilant. “Just a reminder for people to report biting mosquitoes and dead birds,” she said.
Residents can report biting mosquitoes by calling San Mateo County Mosquito and Vector Control at (650) 344-8592 or submitting a service request. Residents can report dead birds to the state hotline at (877) 968-2473 or online.
The County’s next aerial larvicide drop is scheduled for Oct. 2.
