Parasitic Wasp on the Hunt for SWD

One year after the release of a parasitic wasp that attacks the spotted wing drosophila (SWD), scientists from Oregon State University Extension and the USDA Agricultural Research Service are finding some encouraging signs.

“We were able to recover Ganaspis brasiliensis from some of the sites where we released it,” said OSU Extension Entomologist Vaughn Walton, “which is encouraging.”

OSU Extension Entomologist Vaughn Walton said researchers are finding signs that a parasitic wasp that scientists released last summer is parasitizing the spotted wing drosophila.

Walton and Jana Lee, a research entomologist with the USDA Agricultural Research Service in Corvallis, started releasing the wasp last June, putting between 7,000 and 8,000 into the environment at four locations.

“We are making releases strategically,” Lee said, noting they are aiming for areas with high densities of non-crop hosts, such as wild Himalayan blackberries.

The hope, Walton said, is once significant populations of the wasps are established, they will move into blueberries and other crops susceptible to the tiny fruit fly.

Walton and Lee plan additional releases this summer and plan to continue monitoring SWD to see if the wasps are parasitizing the pest.

Walton has been working on the biocontrol strategy for the SWD since 2010, when he led a team of researchers that discovered Ganaspis brasiliensis in South Korea, which is part of the spotted wing drosophila’s native range.

Walton and Lee obtained permits from the USDA Animal and Plant Health Inspection Service to begin rearing the wasps in 2021 and have been doing so since the beginning of 2022.

The wasp is highly selective in its activity, Walton said, attacking only the tiny fruit fly, a trait that helped the scientists obtain the permit to rear the parasitoid.

“They are very specific, and they should be self-perpetuating,” Walton said.

Walton said it may take a while for Ganaspis brasiliensis to build to high enough numbers to help control the drosophila, but when they do, the effect will be significant.

“This will have a huge impact,” Walton said. “Growers are really interested and are excited about a biocontrol agent that will work along with cultural management tools to decrease SWD and not cost them any money. It’s a natural resource available to them. We think this is going to change things.”

The spotted wing drosophila, or Drosophila suzukii, was first found in Oregon in 2009. Today the pest causes an estimated $500 million in damage annually to soft fruit crops across the nation. Management costs for the pest in blueberry alone are estimated at $100 million annually.

“Spotted wing drosophila is very difficult to control. It has a very high reproduction rate with many generations per year and because of that, when using pesticides, they have to be applied constantly, sometimes two or three times a week,” Walton said.

Another encouraging sign in the pursuit of biocontrol agents is researchers have found a second parasitoid, Leptopilina japonica, that is attacking the spotted wing drosophila. “It just started appearing in multiple locations where we never found them in the past,” Walton said.

“It provides a little bit more diversity to the insects that are attacking Drosophila suzukii,” Walton said.

In the end, Walton said, the parasitoids probably won’t be enough to control the SWD. Research has shown that SWD populations can be reduced by up to 65 percent when Ganaspis brasiliensis is present, which, while impressive, still leaves significant numbers of spotted wing drosophila to attack fruit. It will take a combination of cultural and chemical controls to keep the pest at bay, he said. But even if growers can eliminate one pesticide application a season, it can provide savings of up to $100 per acre per application.

Combined with the other control strategies, Walton believes growers now have the potential to significantly reduce the use of pesticides in control programs.

“After 13 years, I think we are in a place where we have a true IPM program where people can start sampling; and when they find damage, they have softer alternatives available,” Walton said.