Destructive fruit fly causes havoc on soft fruit in Southwest Michigan

By Tony Wittkowski | Business Reporter | The Herald-Palladium

COLOMA — Scanning the six acres of his U-pick farm in Riverside, Chuck McCallum knows he’ll have to spray his fruit in a few days.

The owner of The Extraordinary Berry also knows he’ll have to spray again five days later.

McCallum and other Southwest Michigan farmers have battled a pest for the last five years that has grown in numbers at a terrifying rate. It’s called the spotted-wing Drosophila.

Known as a fruit fly from East Asia, the Drosophila damages several fruit crops – particularly soft-skinned fruit – by inseminating its eggs. The insect was detected in California in 2008, before making its way to Florida, Utah, Wisconsin and Michigan for the first time in 2010. These insects gravitate toward raspberries, blackberries, blueberries and strawberries. In some instances, they go after peaches and cherries.

Mark Longstroth refers to the Drosophila as “The Spotted Weapon of Destruction.”

Longstroth is a fruit educator with Michigan State University who helps trap Drosophila for research at the MSU Extension. He said there’s no easy way to eradicate the insect because of how fast it multiplies.

“It’s a scary pest. It reproduces continuously,” Longstroth said. “Once fruit starts to ripen, it has a host to reproduce in. The population begins to rise in June and July and explodes in August and September. The numbers keep increasing until there’s no more fruit around.”

The population builds to unimaginable levels each year and then comes to a halt in winter. Longstroth said 99 percent of the Drosophila die, but are able to start over in the spring and spread fast because of its breeding cycle.

Drosophila may look like regular fruit flies, but possess one clear advantage in reproducing. The female Drosophila uses its serrated ovipositor – a tubelike organ – to pierce the skin of fruit where it deposits its eggs. Because of its ovipositor, fruit doesn’t have to be damaged or decaying in order for the Drosophila to attack it.

Because the fly can inseminate fruit without there being any sign of damage for days, farmers and researchers are fighting an uphill battle.

The average Drosophila has a two-week life cycle. However, Longstroth said the female lives longer at three to four weeks, which allows it to lay up to 600 eggs.

“If every two weeks you’ve got 100 more flies out there, it can build to pretty high levels,” Longstroth said. “The fly gets in there and eventually it turns the fruit to mush. We start catching them in late May. The first ones come during strawberry harvest.”

On the fly

Because the flies are only a few millimeters long and cannot fly far, natural dispersion between states is unlikely. Human-assisted transportation is a more likely cause of the recent rapid spread, Longstroth says.

McCallum said he came across the Drosophila five years ago and has been seeking help from the MSU Extension.

“When they first appeared, the way we checked was we put an actual berry in a solid solution and the worms would come out of the berry,” McCallum recalled. “You would see how well you were spraying.”

Prior to this pest appearing, McCallum might have sprayed once or twice during the whole picking season. Now he has to spray every five days.

He buys a 1-pound can of spray that costs $220 a pop. It lasts him four or five sprays, which equates to $50 a spray that gets used up in a month.

He uses a tractor with a three-point hitch sprayer, using only two nozzles as he moves up and down the rows of crops.

“You have to stay on top of this one,” McCallum said of the Drisophila’s breeding patterns. “You can’t spray this week and forget about next week. Once the season starts you have to spray continuously.”

Tracking the pest

When a new pest appears, there’s an involved process the MSU Extension goes through to help farmers.

Upon the Drosophila’s arrival, Longstroth helped set up a trap near McCallum’s U-Pick, where it collected pests for about five years. It’s used to catch the Drosophila, which are then examined by the MSU Extension. There are dozens of these traps across the region and the rest of the state.

The trap consists of a large plastic cup container with holes and apple cider vinegar inside to attract the flies. Inside near the top is a sticky card, which provides a surface on which to catch the flies and is normally replaced each week.

“Once the fly flies into the trap, it falls into the water and drowns,” Longstroth said. “It doesn’t keep the population down, but is gives us the opportunity to examine them. The males are easy to notice among other fruit flies because they have spots on the wings. We start catching a few flies in early June and they are dark-colored. Later in June and July we catch lighter-color flies. Those are the ones that hatched this year.”

Bill Shane, senior extension tree specialist with the MSU Extension, said the Drosophila has a great potential for expanding its population. Shane said he’s noticed Drosophila lay eggs when fruit first started to put on color.

That’s why he recommends farmers pick their fruit as soon as they ripen. Home owners with private gardens should also harvest every other day and can store fruit in the freezer after picking. The longer fruit is on the vine, the greater the chance of an infestation.

“This has become relevant in recent years because as the season goes on it becomes more of a problem,” Shane said. “The fall raspberries have become a problem specifically. By that time, there are so many spotted-wing Drosophila, you can’t grow without an ongoing insecticide program. For some people it has caused them to not grow fall raspberries.”

One way to determine if the fruit is infested is its condition. If the fruit is becoming softer at a faster pace than what’s typical or the surface appears dented, the crop should be checked. Shane said commercial growers evaluate the fruit by a salt test. They mix fruit in a bag with a salt solution and let it set for 20 minutes. The larvae, which are skinnier than typical for fruit flies, emerge from the fruit and are left suspended in the water.

A final solution

Longstroth and his colleagues are still studying what pesticides work best and how long they last.

They’ve begun to look for a predator to counteract the five-year infestation. Longstroth referenced the Japanese beetle, which came to Michigan and became a problem. However, he remembers a lot of them died out when natural predators built up to the necessary populations to keep them down.

“It’s been frustrating for the growers because we normally target a specific pest,” Longstroth said. “There’s a very specific control window that’s hard to find. We’ve had to do integrated pest management. That’s what we are forced to do and it’s messing up our other controls for other pests.”

Until another piece of the puzzle falls into place, its more collecting from traps and research. A response team that is focused on the spotted-wing Drosophila was formed to combine the expertise of MSU entomologists, horticulturists, Extension educators and Michigan Department of Agriculture staff. The team is also helping to coordinate research projects to understand how best to protect fruit from infestation by the new pest.

While the response team continues to collaborate, farmers across the country will have to be patient.

“There’s no known process to eradicate,” McCallum said Thursday, while inspecting a batch of raspberries. “Some institutions around the country are looking into finding a predator to attack any pests like the Drosophila. Until then all we can do is keep them at bay and hope for an outcome.”

Contact Tony Wittkowski at twittkowski@TheHP.com or (269) 932-0358. Follow him on Twitter: @tonywittkowski.

(Author’s Note: This article was originally published on July 31, 2016)

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