Researchers study yellow jacket behavior

Sept. 21, 2000

It's September in Wisconsin - time for the last hot summer days, the first football games and the peak of wasp season.

Beginning in late August, the German yellow jacket replaces the mosquito as Wisconsin's most despised pest. Anyone who's eaten outside in late summer knows how quickly this wasp can crash your picnic or tailgate party.

The yellow jacket problem became severe enough this year that university officials issued a "bee alert" for fans attending a Badger football game and posted warnings on the score board.

Often mistaken for bees, German yellow jackets belong to a group of social wasps that cooperate to build and defend their queen and colony, according to UW-Madison entomologist Robert Jeanne. But these pests - who view your bratwurst and coke as theirs - are way too "social" for most of us.

A German yellow jacket nest starts in spring with a single queen. By September a nest may number 3,000 or more foraging workers, says Jeanne. As colonies grow during summer, the numbers of German yellow jackets zooming around farm markets, picnic grounds and football games become a serious problem.

Yellow jackets are aggressive when threatened. Unlike a honeybee, which stings only once, a yellow jacket can sting repeatedly. The sting can be life-threatening to those who are allergic to it. A native of Europe, the German yellow jacket reached Wisconsin in 1979. Its arrival sparked a sharp increase in emergency room treatments for stings.

"German yellow jackets kill small soft-bodied insects and are aggressive scavengers with a taste for protein and sweet foods," Jeanne says. Foragers return to their nest with food, which they feed to adult and larval wasps.

Jeanne and his students at the College of Agricultural and Life Sciences

The Wisconsin entomologists showed that foraging German yellow jackets can recruit other workers from their nest to a food source, although they are not nearly as good as bees are at recruiting nest mates. "We found that those new recruits use odor as a clue to locate the food that other wasps are bringing back to the nest," Jeanne says.

Although there are commercial wasp traps that drown the pests, they are unlikely to control German yellow jackets, according to Jeanne. "Nests are common in urban areas," he says. "Killing even a thousand yellow jackets may not make much of a difference."

Destroying individual nests can reduce wasp numbers locally, he says, but finding and eliminating nests may be tricky. The native eastern yellow jacket nests underground; German yellow jackets nest below ground and also in building cavities. Some nest entrances are far up on the sides of buildings.

Because of its nesting and feeding habits, the German yellow jacket now dominates Wisconsin's 12 other yellow jacket species in urban areas. Jeanne says the other species include the misnamed bald-faced hornet, which builds large, cone-shaped nests in trees. Native yellow jackets are still common in rural areas.

Another way to control the pest is to identify a potent bait that will deliver biological control agents or toxins to yellow jacket nests, Jeanne says. Scientists have been searching for the perfect yellow jacket bait for almost 30 years without much success. In 1998, a USDA entomologist in Washington State discovered the strongest attractant yet, a combination of isobutanol and acetic acid. Isobutanol is a component of fermenting molasses and acetic acid is vinegar.

Scientists know that yellow jacket species - and even different populations of the same species - sometimes respond differently to attractants and baits. Therefore, Jeanne and his student Sarah Day studied how Wisconsin's yellow jacket species responded to the isobutanol and acetic acid mixture. They also tested other potential attractants, including compounds chemically similar to isobutanol and a series of compounds isolated from ripe fruit. Jeanne and Day are publishing their research in the Journal of Environmental Entomology.

The Wisconsin research confirmed that German yellow jackets are more attracted to the isobutanol and acetic acid cocktail than any other known compounds. The entomologists also found that the mixture was an effective lure for the eastern yellow jacket and the bald-faced hornet, but not for honeybees or bumblebees.

"The fact that the isobutanol-acetic acid combination is an attractant in Washington and Wisconsin suggests that it may be attractive to the German yellow jacket throughout the world and could be used in abatement programs," Jeanne says.

While baiting is an appealing approach for controlling yellow jackets, Jeanne is concerned that widespread use of one bait could select for yellow jacket populations that do not come to that bait.

In the meantime, there is nothing like a Wisconsin winter for controlling insect pests. "From here on out, every frost we have will start to deplete the number of workers in these yellow jacket nests," Jeanne says.

The yellow jacket research was supported by state funding to the College of Agricultural and Life Sciences and a Hatch grant from the College.