Wasps ruin cherry crop

The European paper wasp looks similar to a yellow jacket, but has a narrower body and longer hind legs.

Cherry growers in British Columbia, Canada, are battling a pest that last year forced several of them to walk away from their crop before ­picking.

The European paper wasp (Polistes dominulus) was first reported to be causing damage in the province in 2003, but it became a serious problem in the Creston Valley three years ago, reports Duane Holder of FarmQuest Consulting, Ltd., in Creston.

In 2008, damage was less severe, but last season, ­damage was serious again and several orchards in the Okanagan Valley were devastated.

The wasp is found in Washington State, but has not been reported as a pest of cherries, according to Dr. Peter Landolt, entomologist with the U.S. Department of Agriculture in Yakima, Washington. He thinks that’s because Washington cherries mature sooner than in British Columbia, before the wasps build up to high numbers. However, the European paper wasp has damaged wine grapes, which mature much later in the year. Parts of some vineyards in the lower Yakima Valley and Walla Walla areas have had severe damage.

The wasp, which has the coloring of a yellow jacket, but a thinner body and much longer hind legs, originated in the Mediterranean region, Holder said. It was first reported in the eastern United States in 1981 and has since moved across North America. Orchards that adjoin natural vegetation and have buildings such as barns and sheds nearby seem more vulnerable than contiguous orchards. The wasps build nests in secluded places, such as under eaves and soffits and in bird houses.

Unlike some other species of wasps, the paper wasp is not resident in orchards, Holder said. Overwintering queens move into orchards in early spring, at around the green tip stage, as they begin to establish nests and develop areas for the colony to expand. It might be a month or so before worker wasps appear. Later, new queens emerge and move on to establish more nests.

Fruit damage doesn’t occur until about seven days before harvest. As the cherries mature, the wasps begin to forage on the fruit. They will attack good cherries, open them up to consume the juice, and sometimes leave only the pit.  Growers can lose their crop to the wasps at a rate of 5 percent a day until it’s not worth harvesting, Holder said.

Preharvest intervals limit growers’ ability to control the wasps with pesticides during that period. Only a small percentage of the wasp population comes into contact with sprays that growers generally apply for cherry fruit fly, such as Sevin (carbaryl), and the wasps do not seem affected by the residues.

Holder said there’s a possibility that the pyrethroid Ripcord (cypermethrin) will be available this season in British Columbia under an emergency registration that would allow its use to control both the European paper wasp and the spotted wing drosophila, which is also a recent pest of cherries.

Holder is heading a project funded by the Okanagan Kootenay Cherry Growers’ Association and the British Columbia Agriculture Council to find a way to tackle the wasp problem.

Last year, he used Wasp-Hornet-Yellow jacket (W-H-Y) traps at various densities throughout the season to monitor and trap the wasps.

He was successful at capturing queens in the spring with W-H-Y lures and various types of attractants, such as sugary substances and beer, but later in the season nothing seemed very attractive to them. “We catch other species of wasps quite well, but this species seems to have a different diet and is not quite as interested,” he said.

Bald-faced hornets and yellow jackets are damaging the fruit, though he believes the paper wasp is the ­primary culprit.

While the European paper wasp typically feeds on protein sources, such as caterpillars and other insects, its diet may change over time, he suspects.

Holder, who is working with entomologists Howard Thistlewood and Gary Judd with the Pacific Agri-Food Research Centre at Summerland, said they would like to be able to control the wasps early in the season. Mass trapping of wasps during the summer does not seem ­feasible because of the large number of worker wasps and the fact that the current attractants are not effective ­during that period.

This year, he planned to increase the trap density for overwintering queens in areas where there are known infestations to better pinpoint the time of emergence. He is also working to create public awareness so that residents near orchards can help with wasp control. He believes they could be important in minimizing the threat.

He is testing other potential baits and lures, and will also test some essential oils as potential repellents.

“Ideally, we would have an attract-and-kill technology: one, a bait we can attract them with and, two, a registered insecticide that will destroy the nest, rather than destroy the insect,” he said. “We’re looking into that. That’s more of a long-term solution.”

Landolt said that in Washington the European paper wasp is displacing the German yellow jacket and bald-faced hornet. It is most abundant in places where there are humans around and builds nests in a wide variety of locations, such as inside old structures or cars.

It is also taking the place of native paper wasps, possibly because the European species is more flexible in terms of finding good places to build nests. It also is active earlier in the season and multiplies faster.

Landolt confirmed that, unlike native paper wasps, the European species is not attracted to standard wasp traps. Coincidentally, he is about to begin a project for the U.S. Navy to develop attractants for other species of paper wasps that are causing problems for servicemen and residents in the Pacific Islands. He hopes the results of his work will be helpful in tackling the European paper wasp problem in the Pacific Northwest.