Invasive wasp adds sting to end-of-summer attacks Alien pest joins explosion of yellow jacket population

Vernard Lewis, an entomologist at the UC Berkeley cooperative extension field station in Richmond, points out a wasp nest. Vernard Lewis, an entomologist at the UC Berkeley cooperative extension field station in Richmond, points out a wasp nest. Photo: Michael Macor, The Chronicle Photo: Michael Macor, The Chronicle Image 1 of / 5 Caption Close Invasive wasp adds sting to end-of-summer attacks 1 / 5 Back to Gallery

Dive-bombing yellow jackets are out in full force, ruining barbecues and family picnics throughout California, but another stinging pest may also be responsible for spoiling the outdoor party buzz.

The European paper wasp, which is spreading quickly across Northern California, is at least partially responsible for an increase this year in yellow jacket complaints, particularly in the Sierra foothills and the Sacramento-San Joaquin River Delta region, according to experts.

The invading alien insects, which were first detected in the Sacramento area around 1989, have combined with a huge population of native western yellow jackets to form what is essentially a gauntlet of stinging wasps.

"The European paper wasp, which is about the same size (as the yellow jacket) but more slender, has built up to enormous numbers in some communities," said Lynn Kimsey, a professor of entomology and director of the Bohart Museum of entomology at UC Davis. "They have been making their way out of the Sacramento area for the past 20 years."

Kimsey said the wasps, which are often confused with yellow jackets because they have virtually the same yellow and black markings, have moved outward from Sacramento along river beds and water ways into the Sierra Nevada and along the delta toward San Francisco.

Warm, dry winter and spring

But paper wasps aren't the only stinging pests raising havoc in the Golden State. The bald-faced hornet, umbrella wasp and German yellow jacket have all been reported bumptiously buzzing around the state. All of them wield saber-like stingers that can be repeatedly and painfully poked through human skin.

The overall wasp population has exploded this year because warm, dry weather over the winter and spring allowed more queens to survive and lay eggs, according to the experts.

Western yellow jackets, which typically build their nests inside rodent burrows or tree hollows, are the most common wasps in California and reach their peak population this time of year. As many as 5,000 individuals can make up a single colony.

The insects generally seek out caterpillars, aphids and even honeybees to sting, carve up and feed to their young, but they also enjoy fresh carcasses and, often, barbecue meat. Crime investigators have even called local apiculturists for advice on how to shoo swarming yellow jackets away from homicide victims.

The flying predators switch to eating mostly carbohydrates in the late summer for energy, said Andrew Sutherland, the Bay Area pest management adviser for the University of California cooperative extension. That often translates to raiding soda cans at picnics and, as a consequence, the plunging of stingers into mouths and tongues.

"What happens now in late summer and fall is you have the largest density in the nest, so they are all foraging for sugar and carbohydrates," Sutherland said. "The search for sugar changes their behavior a little bit so they are more likely to come in contact with humans."

The situation appears to be worse than usual in the Bay Area, said Vernard Lewis, an urban entomologist who works out of the UC Berkeley cooperative extension field station in Richmond, where he doesn't have to go far to encounter stinging wasps.

Population explosion

"Where I normally eat my lunch is one of the biggest yellow jacket nests I've seen in years," Lewis said. "It's not just here. I'm getting reports from the Berkeley campus and from Richmond, Antioch and Rodeo. Something is up. It's not just yellow jackets. It's other pests, too, like cockroaches. It's the most I've seen in at least 10 or 15 years."

Paper wasps have not yet reached the Bay Area in large numbers, Kimsey said, but they are known to adapt to urban areas more readily than yellow jackets and are clearly on the move. They have been a particular problem in the Sacramento Valley, Sierra foothills and Lake Tahoe regions, according to the experts.

In August, 345 wasp nests were removed in South Lake Tahoe, more than anyone can remember. That's compared with 232 nests in August 2012, said Karen Bender, the supervising environmental health specialist for El Dorado County.

"We expect the trend to continue through the month of September," said Bender, whose department is averaging 10 complaints a day about rampaging wasps. "I've been to several picnics lately, and they just come out swarming. They sting and it hurts, and they are a nuisance."

Bender said western yellow jackets are mostly to blame, but many of the nests that vector control officials removed were from paper wasps. The species, which is native to Europe, North Africa and parts of Asia, was introduced to the East Coast of the United States in the late 1960s and '70s. It is not known how it got to Northern California, but the nests are normally built underneath overhangs and can be hard to detect.

The invasive wasps have smaller colonies and are generally less aggressive than the native yellow jackets - they don't poach barbecue food or climb into soda cans - but Kimsey said their habits often bring them close to humans. She said she has seen paper wasp nests almost completely covering the sides of buildings. They particularly like curved Spanish tile roofs, she said.

Defending their nests

The insects have been known to enthusiastically defend their nests against hapless pedestrians, utility workers, shoppers and cafe-goers.

"Two things are going on: You've got this introduced species that's going through a population explosion and you have a really mild spring, which allows more wasps in general to survive," Kimsey said. "It's good for them, bad for us. It also explains why cold winters are good. There are certain things you want to kill off during the winter."