Since its arrival in 2008, spotted wing Drosophila (SWD) has spread and wreaked havoc on soft fruit. To provide organic growers with updates on ongoing studies on SWD management, several researchers recently discussed the NIFA-OREI funded project “On-Farm Integration of Organic Management of Spotted Wing Drosophila in Fruit Crops.” The three-year project began in September 2022 and ends in August 2025.

“The spotted wing Drosophila is a devastating, invasive vinegar fly native to southeast Asia,” said Dr. Phillip Fanning, University of Maine assistant professor of agricultural entomology. “It was introduced to the United States in California in 2008, spread across the country and became a global, cosmopolitan pest.”

Male SWD have dark wing spots, while females are distinguished by serrated ovipositors that allow them to deposit eggs in marketable fruit. “This vinegar fly has led to large losses in our fruit industry,” said Fanning. “A 2014 survey showed a 13% crop loss in blueberries, 27% loss in blackberries, 41% crop loss in raspberries (the preferred host) and 9% crop loss in cherries. Given the value of these crops, these are large losses.”

One project goal is utilizing the biocontrol parasitoid Ganaspis kimorum, recently approved by APHIS, into pest management by integrating it into existing crop management practices. The goal is to reduce the economic impact of SWD on organic fruit production across the U.S.

Dr. James Brown, who conducts research at UC-Berkeley, discussed how biocontrol releases can make a difference in managing SWD.

“If we include biocontrols in the management of pests by introducing them to the natural areas that are adjacent to crop areas, biocontrol agents in the natural areas will suppress the populations of pests,” said Brown. “When the season for the cash crop comes in, fewer pests move from the natural area into the crop area. Less pressure in the crop area means fewer sprays, which means reduced development of insecticide resistance.”

Brown said “classical biocontrol” is one potential solution for managing SWD. He described the term as the introduction of an exotic natural enemy to reconnect pests with natural enemies that come from the place the pest originated. Researchers used G. kimorum, with the goal of releasing thousands of the parasitoids to establish a population that would kick in when the SWD population rises above the economic threshold.

Ideally, once established, parasitoid populations would synch with SWD populations and not require additional management.

G. kimorum lay eggs inside the SWD pupae in fruit. When G. kimorum become adults, they emerge from the SWD pupae, fly off and mate. Their offspring then find their own larvae to parasitize.

Since 2022, G. kimorum has been released experimentally in Maine, New Hampshire, New York, New Jersey, Virginia, North Carolina, Michigan and Minnesota.

Another parasitoid, Leptopilina japonica, can also parasitize SWD. “There was an accidental introduction of Leptopilina in British Columbia, and since that introduction, Leptopilina has been spreading across the U.S.,” Brown said. “This parasitoid is similar to Ganaspis in that it can manage SWD populations.”

Researchers are collecting fruit and setting up sentry traps and yellow sticky cards to determine SWD pressure in the area and whether released parasitoids are established.

In Knox, Maine, researchers deployed Ganaspis three times at two sites and recovered the parasitoid from both release sites. Teams are also recovering Leptopilina in significant numbers.

Researchers in Dakota County, MN, did five releases at two sites. They recovered Ganaspis at both sites and at a control site, and also recovered large numbers of Leptopilina. At a Ganaspis release site in Allegan County, MI, researchers are still processing data. Results so far indicate that release sites in Maine and Minnesota had 31% fewer SWD compared to control sites.

A light microscope image of Leptopilina (about 2.5 mm long). Species in North America are being evaluated for biological control of spotted wing Drosophila. Photo by Matt Buffington/ARS

Dr. Cesar Rodriguez-Saona, Extension specialist in entomology at Rutgers, discussed the non-target effects of insecticides with and without a phagostimulant on SWD parasitoids. Phagostimulants are a natural food component that stimulate feeding and attract pests.

“Although we are releasing parasitoids in non-crop habitat, it’s important to understand the non-target effect of commonly used insecticides on parasitoids to better conserve them,” said Rodriguez-Saona.

He described testing with the phagostimulant Combi-protec®, which contains sugars and proteins. “It can be combined with insecticide applications,” he said. “It’s registered for use in some states.”

The addition of a phagostimulant to insecticides increases efficacy. Mortality is maintained with less lethal ingredients, and the formula is more attractive to target insects.

“Combi-protec can be applied mixed with an insecticide and applied over the entire plant canopy or crop field,” said Rodriguez-Saona, “or it can be applied as a bait spray mixed with an insecticide and applied in smaller quantities.”

Rodriguez-Saona reported that insecticides had varying lethal effects. Mustang® Maxx (pyrethroid) was most toxic, followed by Entrust® (Spinosad) and Exirel® (diamide).

In general, Combi-protec increased mortality rates when mixed with Entrust and Exirel, especially for male insects. However, Combi-protec decreased the lethality of Mustang Maxx, which is absorbed through contact, not ingestion.

Fanning’s research focused on the compatibility of nematodes and Ganaspis. He referenced studies on entomopathogenic (EPN – pathogens that kill insects) nematodes on SWD, several of which have been successful.

To explain the potential role of nematodes in managing SWD, Fanning outlined the SWD lifecycle: adult flies lay eggs in fruit and the eggs develop into pupae, sometimes pupating within the fruit. SWD then emerge from fruit and the lifecycle repeats.

“Many of those larvae will drop out of the fruit and into the soil, where they will pupate,” said Fanning. “Then they become ‘wandering larvae’ and wander around the ground for a bit.” The goal is to apply nematodes to the soil to target wandering SWD larvae.

Fanning’s group infested fruit with SWD under laboratory conditions and used four treatments: wasp + EPN, wasps only, EPN only and a SWD control group. After fruit was infested, eggs were counted and placed in the four treatments. At seven days, fruit was transferred to a petri dish with moist sand and held to 15 days. Researchers counted emerged SWD and wasps.

“Nematodes can work with parasitoids,” said Fanning. “The combination of wasp and nematode kills more SWD than wasp alone or EPN alone, with a 56% to 83% reduction relative to SWD-only control.”

Fanning also wanted to know if nematode exposure lowered wasp survival. “Unfortunately, the answer is yes,” he said. “If parasitoids are already there, nematodes might help further suppress SWD. Field trials delivering nematodes via drip irrigation are underway. If the goal is to release parasitoids for establishment, avoiding applying nematodes in the area might help increase establishment.”

SWD is a challenging pest for organic fruit growers due to limited pesticide options. While G. kimorum and Leptopilina are not currently commercially available, classical biological control agents will be important in reducing the impact of SWD on organic growers. However, pesticides and nematodes can have negative impacts on Drosophila parasitoids.

Future projects will be aimed at continued research to determine the best way to establish parasitoids, suppress SWD and integrate biocontrols.

For more information on biocontrols for SWD, visit eorganic.info/spottedwingorganic.

by Sally Colby