Q. How can I control yellowjackets that are feeding on ripening raspberries, ruining the fruits and making harvesting almost impossible? I put out traps that catch hundreds, but the woods harbor a steady stream of reinforcements.—Kathy Hunter, Wallowa, Oregon
A. Although they prey widely on flies, caterpillars, and other pests, yellowjackets (and other social wasps) can be a bit too social when their foraging habits bring them close to people. They can be particularly troublesome in late summer when their colonies reach maximum size and prey becomes scarce.
"Just three or four species of yellowjacket wasps account for the majority of problems with these otherwise beneficial insects," says retired Oregon State University extension entomologist Jack DeAngelis, Ph.D. The most troublesome species worldwide are the common yellowjacket wasp (Vespula vulgaris) and the German yellowjacket wasp (V. germanica). Both are highly invasive pests. The other two troublemakers, the western and eastern yellowjacket wasps, are more regional. DeAngelis explains that pest yellowjackets supplement their normal diet of live prey with scavenged (dead) food, while nonpest species do not. "This behavior has two consequences: Scavenger species make larger nests, and foraging worker yellowjackets come into contact with people more often when there is exposed food outdoors."
While traps can be used to temp-orarily displace yellowjacket activity, DeAngelis says, the best approach is to control individual nests. For gardeners willing to use a powerful pesticide, he recommends bait stations laced with Onslaught, which foraging worker wasps will carry back to their colonies to feed to the queen and developing brood, destroying the entire nest. DeAngelis notes that although it's not strictly organic, this method eliminates the need to locate individual nests and poses little risk to the environment by targeting only scavenger species of yellowjackets. "Even nonpestiferous wasp species are not affected," he adds. In addition to eliminating pest colonies in the current season, insecticidal baits may reduce the numbers of colonies in the following year by killing queens that would otherwise overwinter. Instructions for using bait stations may be found on DeAngelis' website LivingWithBugs.com.
"Another option would be the scented lure traps that contain n-heptyl butarate," DeAngelis says. "The real usefulness of these devices is as temporary decoys to draw bothersome wasps away from an area." DeAngelis recommends using two or three traps per acre, placing them at least 20 feet away from human activity and leaving them up as long as they continue to attract wasps.
Originally published in Organic Gardening Magazine August/September 2013.