Deadly mosquitoes have landed in the Bay Area. Here’s how to defend yourself

Two dangerously invasive mosquitoes – male and female – have been trapped in Santa Clara County, the latest evidence that the disease-carrying species may be colonizing the San Francisco Bay Area.

The detections follow a similar sighting in Watsonville last week and another in Martinez last August. They have not been spotted in Alameda or San Mateo counties.

“It was expected. It was only a matter of time,” said Nayer Zahiri, Santa Clara County Vector Control District Director. “It was a bit of a surprise, of course, but we were ready.”

None of the mosquitoes, named Aedes aegypti, carried Zika, dengue fever, yellow fever, chikungunya or any of the other deadly viruses they are known to spread. Although the county had occasional cases of these diseases among returning travelers, they were not transmitted by insects.

But the findings are prompting pest control experts to ask residents to drain and clean any water sources where the insect may be breeding. Eggs – sticky and small, like black dirt – are laid on interior walls, above the water line, buckets, plant saucers, birdbaths, clogged gutters, bowls for pets and even bottle caps.

Winter will not eliminate them. The eggs can survive for more than a year, even if they dry out, and hatch as soon as they get wet again.

Native to West Africa, the insect terrified the world in 2015 when it quickly spread the Zika virus across Brazil and more than 50 other countries and territories. Zika can cause debilitating birth defects, including microcephaly, with brain damage so profound that the consequences are only beginning to be understood.

A Santa Clara County vector control officer shows off mosquitoes they captured in an invasive mosquito trap in Milpitas, Calif. Tuesday, Oct. 25, 2022. The mosquitoes will be analyzed in a lab to determine if there are in a non-indigenous Aedes aegypti mosquitoes.  (Dai Sugano/Bay Area News Group)
A Santa Clara County vector control officer shows off mosquitoes they captured in an invasive mosquito trap in Milpitas, Calif. Tuesday, Oct. 25, 2022. The mosquitoes will be analyzed in a lab to determine if there are in a non-indigenous Aedes aegypti mosquitoes. (Dai Sugano/Bay Area News Group)
Captured mosquitoes are shown during the Santa Clara County Vector Control District's demonstration of how its staff retrieves mosquitoes from an invasive mosquito trap on October 25, 2022, in Milpitas, California.  Mosquitoes will be tested to find out if there are any that are not native Aedes aegypti mosquitoes.  (Dai Sugano/Bay Area News Group)
Captured mosquitoes will be tested for non-native Aedes aegypti mosquitoes and to see if they carry disease. (Dai Sugano/Bay Area News Group)

The mosquito was first found in Southern California about two decades ago. Since then, it has spread north through 20 California counties, from San Diego to Shasta, in a pattern that joins the Central Valley Interstate 5 freeway. Based on genetic analysis, California bedbugs were likely introduced from the southeastern United States.

Its expansion into California is causing such concern that vector control officials plan to introduce 2 million genetically modified mosquitoes into Tulare County, with potential expansion into Fresno, San Bernardino and Stanislaus counties. When these lab-built insects mate with wild insects, their offspring die, causing an eventual population crash.

So far, the Bay Area has been largely spared. Although the mosquito has been spotted here before — a few were found in San Mateo and Alameda counties between 2013 and 2015 — they never got a firm foothold.

In Southern California, the mosquito is here to stay. “They’re set because it’s really hard to get rid of them,” said Edgar Nolasco, director of the Santa Clara County Environmental and Consumer Protection Agency.

The mosquitoes found this week were in a commercial business district in northern San Jose near the Milpitas border near the intersection of Dixon Landing Road and McCarthy Boulevard. They do not thrive in the natural areas of the region; they rather prefer life in the city, close to people.

An intensely black mosquito, it is distinguished by its pointed abdomen and two lyre-shaped white bands on its back and white bands on its legs.

The eggs of the invasive mosquito Aedes aegypti are shown under a microscope.
The eggs of the invasive mosquito Aedes aegypti are shown under a microscope. Courtesy of Santa Clara County Vector Control District

California has about 35 native mosquitoes. Most bite in the evening when people tend to be indoors. Most worrisome, a nondescript brownish insect named Culex pipiens, transmits the West Nile virus. It’s usually the one you hear buzzing in your ear at night. But the vast majority of native mosquitoes do not transmit any disease.

These new invaders hunt blood during the day when people are outside. They are aggressive, biting repeatedly. But they don’t travel far. Known for hiding in foliage, they are homebodies, rarely flying more than a block from their birthplace.

The female mosquito was captured first, in one of 40 to 50 small traps set up throughout Santa Clara County in a grid monitoring system.

To attract insects, the traps use dry ice as an attractant. This is because dry ice emits carbon dioxide, which is what humans exhale – so mosquitoes are tricked into thinking the trap is a source of blood.

Out all night, the traps are picked up every morning. The captured insects are frozen in the traps, then sorted by hand and identified, using tweezers, under a microscope.

“As soon as we saw what was in their traps, we put it aside, because it was not a common mosquito that we see every day,” Zahiri said.

The male was later captured during an expanded trapping effort in the neighborhood. Photos were taken and sent to insect experts in Sacramento for confirmation.

Since then, “luckily we haven’t found any other mosquitoes,” Nolasco said. “But that doesn’t mean we’re going to let our guard down. Its eggs can live for up to a year.

How to prevent mosquitoes from breeding near you:

• Drain even the smallest amount of standing water. Cleaning and scrubbing birdbaths, pet dishes, and other containers is a good way to eliminate lingering Aedes aegypti eggs.

• Properly shield rain barrels, cisterns and irrigation drains to prevent mosquito access.

• Repair leaky water faucets and broken sprinkler heads and avoid overwatering lawns and plants.

• Make sure that the water level of the swimming pools is adequate for good circulation and filtration.

Leave a Comment