—by Matt Milkovich
On Day 2 of the 2024 Great Lakes Fruit, Vegetable and Farm Market EXPO, Michigan State University tree fruit entomologist Julianna Wilson told a “tale of two biocontrol agents” — the samba wasp and ronin wasp.
Wilson spoke during the morning Cherry session. She said the samba wasp (Ganaspis brasiliensis) and ronin wasp (Leptopilina japonica) are both parasitoids of the spotted wing drosophila (SWD), the invasive fruit fly that can devastate North American fruit crops, especially soft-skinned cherries and blueberries.
Populations of both wasps, native to Asia, were brought to U.S. Department of Agriculture facilities a decade ago and studied under quarantine. The samba wasp, which parasitizes SWD exclusively, was approved for mass rearing and release in 2021. The ronin wasp was not allowed to be released, because it also parasitizes other drosophila flies. The ronin wasp has been found in the wild in Michigan and other states, however, and researchers are now allowed to mass-rear and release the parasitoid as an SWD biocontrol agent, Wilson said.
“The good news is we have both species here, and we’re maintaining colonies of each,” Wilson told the EXPO audience.
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 consume them and use the pupae case to protect themselves while they develop into adults.
MSU researchers reared and released a small number of samba wasps in 2022. The wasps were released in “postharvest scenarios” in the wooded edges of cherry and blueberry plantings, where SWD often take refuge and where researchers hoped the wasps would establish populations, 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, but researchers hoped the silver lining would be greater establishment of the pest’s natural enemies. They released more samba wasps that year, but at fewer sites. They hadn’t recovered any samba wasps by Dec. 11, but they were still processing samples, Wilson said.
They were still processing samples from ronin wasp releases, too, but had already found wasps in two out of 14 cherry sites, she said.
“That’s really exciting,” Wilson said. “It seems like it’s building up in number.”
MSU entomologists will continue to rear and release the two biocontrol agents, and will continue to monitor their population establishment. They hope to see some impacts on SWD populations eventually, but it’s going to be a long-term effort, Wilson said.
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