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Experts say that across the Bay Area yellowjackets can be spotted buzzing in larger numbers this year.
The San Mateo County Mosquito and Vector Control District has received nearly double the amount of yellowjacket calls this year compared to prior years. Experts say the mild winters might be one of the leading factors.
As of Aug. 22, the vector control district received over 1,060 calls about yellowjackets since January, said Rachel Curtis-Robles, communications director for San Mateo County Mosquito and Vector Control District. The total is a major jump from the 549 calls it received in 2024 and 442 in 2022.
Recently, the San Mateo County Parks Department has been warning the public of increased yellowjacket populations in local parks including Huddart, Junipero Serra, San Pedro Park and Pillar Point Bluff.
According to Curtis-Robles, the increase has also been witnessed in Marin County and Contra Costa County as well. In San Mateo County, calls about yellowjackets are more common from residents in the southern and eastern region such as San Mateo, Woodside, Portola Valley and Redwood City.
The buzzing insect thrives in warmer weather and climate change could mean that increased yellowjacket activity could become year-round, according to Lynn Kimsey, professor emerita of entomology at the University of California, Davis.
Typically, yellowjackets create annual colonies — founding new nests every spring, but with warmer winters, colonies that would usually die from colder weather, are surviving and continuing to grow throughout the year. Now, colonies are perennial, surviving through all seasons, Kimsey said.
Communities along the Southern California coast are already seeing perennial yellowjackets, she added.
“We’re just seeing this happen in more and more places now in California, because they just don’t get cold enough to shut down during the winter,” said Kimsey.
In temperatures below 60 degrees, yellowjackets aren’t able to fly. She explained that warmer winters allow them a greater chance of survival and the ability to continue to forage for food to feed the nest.
Yellowjackets create nests underground or in protected areas such as utility boxes, attics and between walls. An average nest can house up to 30,000 yellowjackets, according to Kimsey.
She added that humans make it easy for the insects by providing food through garbage and roadside waste.
“Yellowjackets will scavenge anything,” said Kimsey. “They like sugar, protein and dog food.”
As warmer temperatures persist, Kimsey confirmed that increased yellowjacket activity will have an effect on the ecosystem. Although frequently seen buzzing around trash, picnics and barbecues, yellowjackets also provide a service in regulating pest populations.
They are a predator to other insects, often catching and eating house flies, honey bees, flower flies and crickets, according to Kinsey.
But more yellowjackets can also mean an increase in the chances of pets and humans getting stung. Unlike a honey bee, which loses its stinger after attacking once, yellowjackets are able to sting multiple times. Its stinger carries a venom that immobilizes its prey and allows the insects to carry it back to its colony.
Kinsey said however that yellowjacket stings are less painful than honey bee stings which have a venom designed to hurt and last — tailored to defend its nest from animals like bears, she explained.
Staying safe
The San Mateo County Mosquito and Vector Control District is able to help residents who need yellowjacket nests removed, free of cost. Curtis-Robles warns however that technicians will only be sent out if individuals know where the nest is generally located. She recommends looking for an “airport” of yellowjackets flying in and out of the same area.
If a yellowjacket is flying around you, she recommends staying calm, still and not to swat. They are usually just curious to see whether there is a food source nearby, she said.
When picnicking outside, covering your food can help keep yellowjackets away. Before taking a sip from a soda can or sweet drink, it is recommended to check the top of the can or bottle to prevent an accidental sting.
If you are stung by a yellowjacket, the county parks recommends removing the stinger, if it was left in the skin, and to apply cold water or ice in a wet cloth. Individuals allergic to yellowjacket venom should seek medical attention immediately.
The use of hanging traps is also a common method of yellowjacket removal however the district advises that traps be hung on the perimeter of the property rather than in areas where people are eating and cooking. Inside the traps are a sweet scented lure that attracts yellowjackets, so placing them on the edges of the property will draw them away from human activity and also catch yellowjackets that are flying toward the area.
Curtis-Robles said that when the vector control district is called, a technician will typically be able to help within the next business day.
For more information on yellowjackets visit smcmvcd.org/yellowjacket-nest-treatment.
