A turbocharged, invasive vinegar fly from Asia — that unlike its American cousins prefers ripening fruit to rotten — is causing growing turmoil for those who farm Michigan's signature fruit crop, tart cherries.
The spotted wing drosophila — called SWD for short by researchers and affected farmers — only made its way to Michigan around 2010, after having been found in California the year before. The tiny vinegar fly (don't call it a fruit fly; that's similar but a little different) only a few millimeters in length, likely made its way in cargo ships from its native range of eastern Asia. As it doesn't fly far, it probably got a lift on human transportation to move from the West Coast to Michigan and other states where it's now found.
SWD are fond of all kinds of thin-skinned fruit. And it was only about two years ago that it became apparent to tart cherry growers that this was going to be a problem.
Unlike native types of fruit and vinegar flies, which require mushy, rotting fruit, the SWD has an ovipositor — a tube-like organ through which female insects lay their eggs — with a serrated edge, enabling it to cut through not-yet-ripe fruit.
"This one starts when the cherries just start turning a little different color," said Jim Bardenhagen, who operates an 80-acre tart cherry farm in Suttons Bay.
A cherry that's been visited by SWD often has telltale pockmarks. When the eggs hatch, tiny white maggots appear.
If that sounds disgusting, the tart cherry industry agrees. It has a zero-tolerance approach to SWD-infested cherries. If the eggs or maggots are spotted, large segments of a harvest are tossed.
"It's increasing all the time. It's quite a concern," Bardenhagen said. "It multiplies very fast, so it's causing us to spray a lot more to protect the fruit as you approach harvest."
Tart cherries are a particularly delicate crop. They spoil quickly, so are most often converted into concentrate for use in juices, jams and pie fillings. Having a pest that causes more spraying and attention, particularly close to harvest time, adds cost to an industry with thin margins.
"Every time we go through the orchard, it costs money," said Phillip J. Korson II, executive director of the Lansing-based Cherry Marketing Institute, a trade organization funded by growers. "It's not only the time and material it takes to apply. The closer you get to harvest, the more fragile that fruit is. You don't really want to take a tractor through the orchard if you can help it."
And if cherries are found infested, fruit can't just be shaken to the ground, Bardenhagen said. It necessitates covering trees, applying pesticides and picking the fruit to dispose of it in a way that helps limit the spread of SWD.
"So we're paying for a harvest and getting nothing for it," he said.
Another problem: Traditional pesticides used by the growers often contain restrictions on their use just before a harvest, to reduce pesticide residues on the fruit.
Michigan State University has a team working with the cherry industry to research and find ways to control the invasive pest.
"It's really hard to manage once it gets going," said Julianna Wilson a fruit tree and integrated pest management specialist in MSU's Department of Entomology.
Among the natural checks on native species of fruit and vinegar flies are parasitoid wasps that lay their eggs on or in caterpillars or flies. SWD, however, can encapsulate the wasp egg inside of them and survive, Wilson said.
"What we really need are parisitoids that have evolved with this pest, that the fly can't survive," she said.
Some researchers at other universities are studying natural enemies to SWD in Asia, but any moves to introduce them here to fight the pests must be thoroughly studied to avoid unintended consequences to local ecology, Wilson said.
In the meantime, researchers are looking at better ways to combat the pest that can be used right up to cherry crop harvesting time.
"We're working on better monitoring, things where we can detect the insect earlier," Wilson said. "We're looking at its biology ... the biology is so important. If you don't really understand how it's reproducing, what conditions it's thriving in, the timing, you're not going to be as effective in controlling it. We're also looking at using poisons where they'll make the biggest impact and do the least harm to the environment."
The Foundation for Food and Agriculture Research, a nonprofit established through bipartisan congressional support in the 2014 Farm Bill, has awarded $300,000 to MSU to combat SWD. The grant is part of the foundation's Rapid Outcomes from Agriculture Research program, designed to mitigate impacts from emerging agriculture threats through applied science. MSU, the Michigan Cherry Committee and the Michigan State Horticultural Society are matching the foundation's grant.
"Michigan is America's biggest tart cherry producer," Wilson said. "We have to solve this problem here."
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