Exploring natural solutions

Dr. Kelly Hamby, associate professor and Extension specialist, University of Maryland, is determined to help growers of brambles and other soft fruits understand and manage spotted wing drosophila (SWD). Measures such as cooling fruit as soon as possible after harvest to prevent SWD larvae from developing can help preserve quality in harvested fruit, but it doesn’t stop the problem in the field.

While many growers rely on timely chemical applications to thwart SWD, others prefer natural control. Some research has explored entomopathogenic fungi and nematodes, but Hamby said these products don’t provide reliable control.

“There are commercially available nematodes and entomopathogenic fungi that may kill the SWD, but further work is needed to ensure field efficacy and economic returns,” Hamby said. “The current recommendation is to not use nematodes or entomopathogenic fungi.”

Predators provide effective control for many crops. Numerous predators consume SWD, including carabid ground beetles, rove beetles, earwigs and ants. The most effective predators of SWD are arthropods, but arthropods are sensitive to chemical insecticides. Although arthropod predators help limit SWD populations, their contributions aren’t sufficient for growers who offer soft-skinned fruit at fresh markets.

However, parasitoids may help limit SWD damage. These predators, including several native wasps, spell doom by developing within the host and killing it. Unfortunately, native wasps are also sensitive to chemical insecticides.

Specialized parasitic wasps might do a better job dealing with SWD. Researchers sought beneficial wasps in areas where SWD is native, including China, Japan and South Korea. They found two figitid wasps and a braconid wasp that were parasitizing SWD. The wasp frequently seen in soft fruit is the figitid Ganaspis kimorum.

“We got a petition approved to release Ganaspis kimorum for SWD in 2021,” Hamby said. “We have been releasing this wasp in Maryland since 2022. People in the Pacific Northwest were also doing biological control, and they found that Leptopilina japonica, the species we don’t have a permit for, established itself in British Columbia and has probably been there since 2016. They also found Ganaspis kimorum that made its way over here by itself. We have been working throughout the United States, rearing Ganaspis kimorum for releases in different areas.”

Hamby said the goal of importing wasps is to establish a self-sustaining parasitic wasp population.

“We want to establish them in the woods in non-crop hosts like honeysuckles and mulberries they’re feeding on and doing well,” Hamby said. “The wasps find them in the woods and hopefully that will help reduce the [SWD] population in early spring in all the places where we can’t control them.”

USDA provided more than 12,000 wasps for release in Maryland. Ongoing research at the Western Maryland Research & Education Center (WMREC) includes determining whether beneficial wasps have become established. Researchers collect crop and non-crop fruit at sites several times during the year, including before and after wasp releases, to evaluate establishment.

Because Hamby doesn’t use chemical sprays, fruit grown at WMREC has high SWD populations. This provides a more accurate view of parasitoid success.

“We’ve been doing this at our farm and on two diversified fruit farms,” Hamby said. “We have also been trying to determine whether [parasitoids] have become established.”

Beneficial wasps lay eggs in SWD larvae inside the fruit and wait until the SWD pupates to develop. After rearing wasps through the pupal stage, researchers can determine whether the pupa is a wasp or a SWD. Wasps chew their way out of the pupae, leaving tell-tale jagged edges behind.

The released wasp, G. kimorum, has been recovered at all sites. However, only 40 wasps have been found compared to the 12,000 that were released. During this project, Hamby and her team found another wasp that appeared on its own: L. japonica, which is doing well in Maryland and is the primary wasp parasitizing SWD at all sites. Over the last five years, 5,769 L. japonica have been released.

“Wasps are found from the beginning of the sampling till the end of the sampling, so we’re finding them from June to October,” Hamby said. “That means they’re active at the same time as SWD and they’re out there doing well while SWD is active. We’ve found them both in crop and in non-crop hosts. They’re out in the woods and spilling over into crops. Ultimately, we’re hoping these wasps are going to help slow the early season population growth.”

Hamby reminds growers that like all IPM programs, there’s no silver bullet for managing SWD.

“It’s a combination of tactics,” Hamby said. “Timely harvest and horticultural best practices help reduce losses and slow the development of the population over the season., but we’re largely still reliant on insecticides, especially in zero-tolerance situations. There are new management tactics nearing commercialization, and more on the horizon.”

by Sally Colby