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Michigan cherry growers mobilize against invasive fruit fly

A pesky insect that loves to invade fruit has found its way to northern Michigan’s cherry orchards. Scientists have had their eye on spotted wing drosophila since it arrived in the U.S. from Asia in 2008. 

The bug has quickly spread across the country after wreaking havoc on cherries in California. Now, northern Michigan cherry growers are trying to mobilize against the pest before it’s too late.

An up-close look

Nikki Rothwell is looking through her microscope at the tiny bodies of spotted wing drosophila. She’s using an old-fashioned counter to tally them up.

Credit Northwest Michigan Horticulture Research Center
Researchers Nikki Rothwell (left) and Emily Pochubay examine spotted wing drosophila collected in local cherry orchards.

These little bugs were caught in a trap at the Michigan State University extension lab near Traverse City.

Rothwell and her team have been collecting and studying them since they first started showing up around here about 2012.

Spotted wing drosophila are a type of fruit fly – like the ones you might see around your kitchen in the fall.

But Rothwell says SWD have a couple of key differences from their fruit fly brothers and sisters.

One is their ovipositor – that’s the needle-like appendage female insects use to lay eggs.

“But this ovipositor, instead of being straight, is almost serrated,” she says. “And so it can saw into fruit that’s not perfectly ripe.”

That little saw allows the fruit fly to hang out longer in orchards.

“The first year you have a few,” she says. “The next year, you have more. And so the more you have overall, the more they come out early and the more they hang out later. And what we’re now starting to see, unfortunately, is that overlap – where the population of SWD is starting to rise and we’re not done with cherry harvest.”

Threatened cherries

Josh Wunsch is a third-generation fruit grower on Old Mission Peninsula. I talk with him as he’s surveying the orchard in his Ford Ranger pickup.

Wunsch has seen threats to the orchards come and go. But this one has him concerned.

The larvae of spotted wing drosophila can be seen breaking through the surface of tart cherries.
Credit Northwest Michigan Horticulture Research Center
The larvae of spotted wing drosophila can be seen breaking through the surface of tart cherries.

“There were extensive, perhaps millions of pounds of tart cherries on Leelanau Peninsula this summer that were caught late by spotted wing drosophila,” he says. “And so now everybody is somewhat apprehensive about where that goes in another cycle – if it’s going to actually get into the tart cherry trees.”

So far, the annual infestation of SWD has been happening after cherry harvest in July, so there haven’t been any major losses yet.

But this year, the MSU team found the bugs showing up in June, well before the beginning of the harvest.

They’ve put out traps to try to slow down the advance, but Wunsch says it hasn’t had much effect.

He says insecticides haven’t worked, either.

“If we want to spend a few public dollars on something, maybe we ought to be sending some researchers into Asia, where this thing came from, and find out what controls this thing,” he says.

Searching for a solution

Peter Shearer is leading studies on spotted wing drosophila at Oregon State University.

“We’re looking at little tiny wasps that attack the fly and kill it,” he says.

Shearer has several different types of imported Asian wasps in a special quarantine facility at the university. The idea is to see if one of them has an appetite for spotted wing drosophila.

Shearer says the idea of introducing a non-native wasp to control the fruit fly is controversial.

But nothing else has really worked.

He says the only thing that seems to make a dent on the population is a really cold winter, and this winter is expected to be a mild one.

While scientists hunt for solutions, Shearer says growers might have to change the way they harvest. And they’re going to have to work together.