—by Matt Milkovich

Michigan State University tree fruit entomologist Julianna Wilson shares tales about spotted wing drosophila management during the Great Lakes Fruit, Vegetable and Farm Market EXPO in December. (Matt Milkovich/Good Fruit Grower)
Michigan State University tree fruit entomologist Julianna Wilson shares tales about spotted wing drosophila management during the Great Lakes Fruit, Vegetable and Farm Market EXPO in December. (Matt Milkovich/Good Fruit Grower)

Julianna Wilson had tales to tell about the spotted wing drosophila, the invasive fruit fly that’s become a major pest of North American berries and cherries, when she spoke at the 2024 Great Lakes Fruit, Vegetable and Farm Market EXPO in December.

The Michigan State University tree fruit entomologist’s first tale focused on two biocontrol agents, the samba wasp and ronin wasp, and their potential to manage SWD populations. Another was a tale of two years, 2023 and 2024, and the climatic differences in Michigan that led to low SWD infestation the first year and high infestation the second year. 

Biocontrols

The samba wasp (Ganaspis brasiliensis) and ronin wasp (Leptopilina japonica) both originated in Asia, where they parasitize SWD. The wasps followed the fruit fly to North America and are starting to establish populations in the U.S. 

Both wasps parasitize SWD by laying their eggs inside SWD larvae that are developing in fruit. The wasp larvae allow the SWD larvae to develop into pupae, then they consume the SWD, using the pupae case to protect themselves while they develop into adults, Wilson said. 

MSU researchers reared and released a small number of samba wasps in 2022. The wasps were released after harvest along the wooded edges of cherry and blueberry plantings, where SWD often take refuge, she said.

They didn’t recover any samba wasps in 2022, but they released 37,000 wasps at 30 sites the following year and recovered wasps from five of them (none of them cherry sites, however), she said. 

SWD infestation was “horrible” in Michigan in 2024, Wilson said, but researchers hoped the silver lining would be greater establishment of the pest’s natural enemies. They released more samba wasps that year, though at fewer sites. They hadn’t recovered any samba wasps by Dec. 11, but they were still processing samples. They were still processing samples from ronin wasp release sites, too, and had already found wasps in two out of 14 cherry sites. 

“That’s really exciting,” Wilson said. “It seems like it’s building up in number.”

The role of climate

Michigan saw very low SWD infestation in 2023. The SWD population didn’t start to surge until late July that year, well after cherry and blueberry harvest had started. In 2024, however, there already was an abundance of SWD in the field by late June, which contributed to the poor quality of last year’s Michigan cherry crop. 

So, why the difference? For one thing, 2023 had more days below 41 degrees Fahrenheit (SWD’s activity threshold) than did 2024, which delayed the pest’s population development. Greater rainfall in 2024 probably increased SWD populations that year, as well, she said. 

That difference made Wilson wonder: Can winter temperatures and spring rainfall amounts be used to predict SWD populations for the coming season? And can that prediction be used to guide spraying decisions? Perhaps growers can apply fewer sprays in lower-risk years. 

Wilson’s lab will tackle these and other questions with funding help from the Agricultural Climate Resiliency Program, a new partnership supported by MSU, the Michigan Plant Coalition and the Michigan Department of Agriculture and Rural Development. 

Any chance to use fewer insecticides could help avoid the looming specter of SWD developing insecticide resistance. California researchers are starting to find evidence of resistance to pyrethroids in SWD populations, as well as organophosphates and spinosad. MSU researchers haven’t yet detected SWD resistance in Michigan, but the genetic potential is there and could start to be expressed with heavy use of insecticides over time, Wilson said. 

Another way to avoid resistance is by finding viable alternatives to sprayable insecticides. A Missouri company, Agragene, now produces sterile SWD males through genetic engineering. When sterile males mate with wild SWD females, the females’ eggs don’t develop. Sterile SWD males aren’t yet registered for use, though there have been trials in Oregon, and MSU researchers hope to study them soon, she said.

Attract-and-kill technology — a bait station set out early in the season that kills a lot of SWD before they start damaging fruit, for example — offers another potential alternative, but it’s been hard to find a bait or odor source strong enough to overcome SWD’s desire to lay eggs in fruit. With funding assistance from the U.S. Department of Agriculture, Wilson’s lab is studying yeast-based, species-specific insecticides that could be combined with known attractants to be more alluring to SWD, she said.