The Change in Seasons Does Have an Impact on Yellowjacket Behavior
You're in your backyard, doing a little weekend barbecuing and wham... out of nowhere yellowjackets are all around. You start waving your arms and some start stinging you. What's going on? Is it my imagination, or are yellowjackets more aggressive this year?

The short answer is, yellowjackets do tend to behave more aggressively particularly in the fall, but it's actually something that happens every year because food is starting to get scarce and they are starting to come to a new phase of their life cycle.
Yellowjackets can provide incidental pollination and they eat small pest insects.
Contra Costa County is home to four different types of yellowjackets; two that build their nests above ground and two that build their nests below ground. The ones that build their nests below ground are the yellowjackets for which the Contra Costa Mosquito & Vector Control District (District) provides service. District employees focus specifically on ground-nesting yellowjackets because that is where county residents face the most likely risk of coming into contact with and being stung by these insects.

Why do they seem more aggressive now?

Once fall weather starts to cool, it signals the yellowjackets to prepare for winter. That means the worker yellowjackets have to collect food for the queens that will survive through the winter. Yellowjackets will go wherever they need to gather food, so, it's not your imagination if you see yellowjackets at your next socially distant outdoor dinner. They are attracted to your food — primarily the meat and sugary dishes. The question is, what can you do to get rid of them?

According to the District's Yellowjacket Program Supervisor Sheila Currier, the most important thing you can do is put out traps early next year to catch the queens with the goal of reducing the number of yellowjackets in your yard next year.

"The rule of thumb is put traps up at Valentine's Day and take them back down at Father's Day. That way, you have the best chance of catching queens when they emerge and start looking for places to build their nests. Leave the traps up to catch any new worker yellowjackets that emerge and then take the traps back down in June, to avoid attracting yellowjackets from other yards to your yard."

The University of California Statewide Integrated Pest Management Program has more information on what traps work best. Be sure to place any traps as far as possible away from your home including away from windows, doorways and walkways.

The bottom line is it's not your imagination, yellowjackets behave more aggressively this time of the year. But by putting out traps next year, the yellowjackets could be less noticeable in the future.

Learn more about how the District works with Contra Costa County residents to control ground nesting yellowjackets.
Contra Costa Mosquito & Vector Control District, an independent special district and public health agency, is located at 155 Mason Circle in Concord. Contact the District to report mosquito and vector problems online or at (925) 685-9301