An employee of Alpine Helicopter Service Inc. loads pesticide into the helicopter during one of its landings at Mori Point in Pacifica. Brendan Bartholomew/Special to S.F. Examiner

San Mateo County fights West Nile virus with pesticides

Peninsula residents might notice low-flying helicopters executing tight maneuvers over parts of San Mateo County during the next few months.

Dispersing the dry pesticide methoprene, which is harmless to humans, the aircrafts are part of the San Mateo County Mosquito and Vector Control District’s effort to stop the spread of West Nile virus by attacking the mosquitoes that carry the deadly pathogen.

The three Peninsula locations being treated are marshy wetlands on Sharp Park Golf Course in Pacifica, Mills Field at San Francisco International Airport, and part of Stanford University’s Searsville Lake site near Portola Valley and Woodside.

On a recent morning in Pacifica, a 1979 Bell 206B owned by Alpine Helicopter Service made numerous passes over the golf course, flying low enough that it appeared to dip below the tree line, making such dramatic turns that it sometimes seemed to tip on its side in midair.

A ground support crew kept members of the public at a safe distance during the several landings the helicopter made on the walking trail at adjacent Mori Point. And while the technician who refilled the aircraft’s pesticide containers wore a facemask, none of the other district personnel wore protective gear, and the golf course was not closed during the operation.

District Assistant Manager Brian Weber, who was among the ground team, explained the solid granules being dispersed contained the chemical methoprene, which is essentially nontoxic for mammals but prevents mosquito larvae from growing to adulthood.

The Sharp Park Wetlands are a habitat for the endangered California red-legged frog and San Francisco garter snake. Megan Caldwell, the district’s public health education and outreach officer, said her organization works closely with the U.S. Fish and Wildlife Service to avoid impacting sensitive species.

Methoprene only interrupts mosquito life cycles, Caldwell explained, and does not affect other insects. She said the district alternates between methoprene and other products to prevent the mosquitoes from developing a resistance to it.

A 2005 study found the lobster population in the Western Long Island Sound watershed in New York and Connecticut declined after methoprene was used there. But Caldwell noted the mosquito district’s activities would not jeopardize crustaceans because the freshwater marshes being treated are isolated from San Francisco Bay and the ocean.

Nobody has ever been diagnosed with West Nile in San Mateo County, and, according to the California Department of Public Health, nobody in the state has tested positive for the virus this year. But last year, there were 31 deaths in California attributed to West Nile, with a total of 801 confirmed human infections.

The San Mateo County Mosquito and Vector Control District normally learns of the presence of West Nile virus when a dead bird turned in by a citizen tests positive. That prompts the district to place mosquito traps in the area and test the mosquitoes it catches.

So far this year, one West Nile-positive bird was found in Redwood City and two in Menlo Park. However, no mosquitoes from Menlo Park have tested positive for the virus.

Educating the public is a crucial part of the district’s mission, Caldwell said, explaining that any pool of standing water is a potential breeding ground for mosquitoes.

“We have to depend on the public to keep an eye on their property and let us know when there’s a problem on public property,” Caldwell noted.

Residents can aid mosquito mitigation efforts by dumping and draining containers of standing water, Caldwell said. To avoid mosquito bites, she recommends wearing long sleeves and pants, along with mosquito repellent, especially during the dawn and dusk hours when mosquitoes are most active.

Additional helicopter treatments are scheduled for July 22, Aug. 12 and Sept. 2. If weather permits, all three sites will be treated on each of those dates.mosquitoesSan Mateo CountyWest Nile virus

If you find our journalism valuable and relevant, please consider joining our Examiner membership program.
Find out more at

Just Posted

Riders should expect big changes when Muni rail returns in August

Reconfigured routes will mean fewer, longer trains through tunnel

New audio of couple calling police on SF man bolsters racial bias claims, supe says

Pacific Heights incident spurred CAREN Act to outlaw discriminatory reports to police

SFUSD students may start the school year at home

Staff report recommends starting with distance learning in the fall, transitioning to hybrid model

Universities fight new immigration restrictions on international students in court

Local colleges are scrambling to keep international students in the country as… Continue reading

SF library plans to reopen with pickup and drop-off services

Since March, all 28 library locations in San Francisco have been closed… Continue reading

Most Read