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Picky Wasp Attacks Only Certain Michigan Blueberry Bushes

Galls hang from blueberry bushes on WMUK correspondent Joan Donaldson's organic blueberry farm
Joan Donaldson

While fruit farmers continue to fight the invasive spotted wing drosophila fly, Southwest Michigan’s blueberry growers also face a homegrown pest. The blueberry gall wasp is native to Michigan but has experienced a population boom in the past few years - and researchers aren’t sure why. 

Joe DeGrandchamp co-owns DeGrandchamp Farm in South Haven. Over the past few years, he's been finding more and more galls on his blueberry bushes. Galls are brown, kidney shaped growths that form on the stem where the gall wasp lays its eggs, preventing new shoots from growing.

DeGrandchamp says his employees spent four full days this winter picking them off - one by one.

But he says the wasps were picky. They only went after one variety out of the three types of blueberry bushes in his field. They favored a new kind of blueberry called “Liberty” developed by Michigan State University. DeGrandchamp says it was easy to see the wasps’ favorite bushes.

“It’d be six row, six row, six row - and you can almost draw a line, they don’t cross it. So there’s something about this variety they like or the other ones they don’t like,” he says.

The gall wasp’s selective taste has confused experts too. Rufus Isaacs is a researcher in Michigan State University’s entomology department. He says the gall wasp population has boomed north of the Grand Rapids area - especially in Holland - with smatterings in Van Buren and Berrien counties.

Isaacs says right now, the wasp doesn’t cause nearly the damage of the Spotted Wing Drosophila fly. He says, if anything, it’s more of a nuisance. But Isaacs says MSU isn’t sure why the wasps only like to lay their eggs on certain blueberry plants.

“What it is that makes certain varieties resistant and if we could get to the bottom of that, that might really help in developing some newer, non-chemical approaches to controlling this,” says Isaacs.

The good news is, the gall wasp doesn’t infect the fruit - it just stunts the growth of the plant. DeGrandchamp says, the bad news is, right now it’s not easy to control with pesticides.

“When they hatch out of the gall coincides precisely when we have honeybees to pollinate the crop. So anything you would use on the wasp would also kill the honeybee, so you can’t really spray anything at that point,” he says.

Isaacs says farmers can use pesticides, but only after bees have left the field. As a result, DeGrandchamp says even conventional farmers like him have resorted to picking the galls off by hand.

“And I’m not sure we got them all," says DeGrandchamp. "There’s always the low ones - if there’s grass around them, you don’t see them. I would like to say we got 100 percent, but my gut is no.”

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