Mercenary wasps battle fruit flies in Michigan cherry orchards, blueberry patches

SUTTONS BAY, MICH. – Hundreds of tiny samba wasps are now presumably wreaking havoc on a type of invasive fruit fly that has for years caused crop damage across Michigan – at least, that’s what growers and researchers are hoping.

Michigan State University scientists are studying how well samba wasps from Asia released this year into tart cherry orchards in Leelanau County and blueberry fields in three southwestern Michigan counties will target invasive spotted-wing drosophila. Success of this biocontrol measure will also be a matter of whether the wasps can cope with the state’s harsh winter climate.

Growers and researchers are enthusiastic about the opportunity and hopeful the samba wasp experiment succeeds.

“We have to figure out if these things survive in nature, because we aren’t going to be able to rear and release every year. If they don’t establish themselves then we couldn’t put a dent in a population doing that. But if they get established and build their own population, then that’s going to help us tremendously,” said cherry grower Jim Nugent of Suttons Bay.

Nugent’s up north Sun Blossom Orchards deep in the heart of Michigan’s fruit belt hosted the first round of samba wasp trials this year.

The invasive spotted-wing drosophila flies spoil crops by laying eggs inside healthy fruit like cherries and berries, where larvae can remain during ripening and cause sour rot or fungal disease. Samba wasps are parasitoids that fight against that crop damage by laying eggs inside and ultimately killing spotted-wing drosophila larvae.

Nugent explained how tart cherries are more susceptible than sweet cherries to that crop damage because spotted-wing drosophila numbers remain low enough while the sweet cherries are ripening. But a seven-day life cycle for the pest insect means that by the time tarts are ready for harvest, the spotted-wing drosophila population presents a much greater threat, he said.

The invasive fruit fly is among other challenges tart cherry growers face such as climate change-fueled damaging weather and unstable market prices because of imported cherries.

Other types of soft-skinned fruits that ripen in late summer and autumn, like raspberries and blackberries, can be completely overwhelmed by spotted-wing drosophila, Nugent said.

“I raised my hand in a heartbeat for this one because it really is critical to the future of the cherry industry and we could probably say the blueberry industry in Michigan, as well. To have nature help us take care of a good deal of this problem,” he said.

This summer, researchers released hundreds of samba wasps at Michigan fruit farms after a lengthy quarantine, evaluation, and permit review by state and federal agricultural authorities. The overall goal is to use fewer insecticides to defeat the invasive pest that threatens fruit production not just in Michigan, but across the United States.

The samba wasp project is part of a national effort to fight the spotted-wing drosophila, including similar release experiments on the West Coast. Scientists say they won’t negatively impact native fruit flies and don’t sting humans.

“We’re still in the beginning stages of trying to know if they’re going to work. But the thing that’s exciting about them is that I hope that we’re going to find out that they will overwinter,” said Nikki Rothwell, Michigan State University Extension fruit specialist at the Northwest Michigan Horticultural Research Center just northwest of Traverse City.

Researchers will know if the samba wasps established a breeding population when they use sentinel traps next spring and summer.

Rothwell said samba wasps are a more sustainable option to pesticides if they can adapt to Michigan’s climate and reduce the population size of spotted-wing drosophila.

Researchers believe the samba wasps will spend winter inside the pupae of spotted-wing drosophila, said Julianna Wilson, MSU assistant professor of entomology.

“They attack the larvae, and then the larvae keep living and crawling. So, they have an egg inside them of this parasitoid. It’s like, kind of creepy. It’s like an alien a little bit,” she said.

“The larvae develop into pupae as if they were going to become spotted-wing drosophila. And then instead of them, the parasitoid consumes that developing pest and then emerges as an adult.”

More samba wasps will be hatched, reared, and released next year at even more fruit farms, Wilson said.

An additional hope is that samba wasps will attack spotted-wing drosophila in abandoned orchards and in wild-growing fruit, she said, which would additionally reduce pressure from the invasive pest on active fruit farms.

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