Berry Pest Makes First Appearance of Season

7/19/2014 7:00 AM
By Philip Gruber Staff Writer

An invasive fly that can cause problems for late-season fruit growers has been found for the first time this year in Pennsylvania.

Three males of the spotted wing drosophila, or SWD, were trapped in Adams County near the beginning of July, according to Penn State’s Fruit Times newsletter and Cornell University.

First found in the Northeast in 2011, the fly has been reported around the region this year, from Delaware and Maryland to New York and Maine.

Populations of spotted wing drosophila generally start small and continue to ramp up throughout the rest of the growing season, said Kathy Demchak, a Penn State Extension associate.

“The populations will just continue to build until the first hard frost,” Demchak said. “We’ve found them up until early November.”

Substantial numbers of the flies could appear in the next week or two, so berry growers should be watchful to see if they need to spray, Demchak said.

“The main problem is that the adult females are capable of laying eggs in the fruit,” she said.

Most fruit flies can lay their eggs only in old fruit that would not be harvested anyway, but spotted wing females’ serrated ovipositor, a body part that implants the eggs in the fruit, is bigger than its counterparts’ ovipositors.

As a result, spotted wing drosophila can lay its eggs in fruit that would otherwise be marketable, potentially affecting yields, Demchak said.

Larvae have not been found in any fruit in the state yet this year, she said.

Blueberries, ever-bearing raspberries and blackberries face the greatest risk of SWD damage. The fly can put its larvae in tree fruit too, but “it’s been at the very tail end of harvest or (in) fruit that probably wasn’t marketable anyway,” Demchak said.

Because SWD tends to be a problem later in the season, black raspberries, June-bearing strawberries, summer raspberries and early blueberries escaped the pest this year, Demchak said.

SWD populations so far seem “a little bit later and a little bit lower” than last year, Demchak said.

Populations often swell in late July with moderate temperatures and high humidity, a bump that this year is about 10 days later than in 2013, she said.

It is possible the cold winter delayed or killed some of the overwintering SWD, Demchak said.

A hot, dry spell can cause populations to plateau or drop slightly during the season, she said.

Growers can identify spotted wing drosophila by the single dark spot on the male’s wings that gives the fly its name. The spot is a little bit forward of the wing tip, Demchak said.

Several other fruit flies have spots on their wings. Penn State has a fact sheet, found at, that helps growers distinguish SWD from other species, Demchak said.

Female spotted wing drosophila look very similar to other flies. “The only way you can identify them is by looking at their ovipositor,” she said.

“We really encourage growers to look for the males” because if males are there, females are likely to be present too, she said.

Growers seem to be doing OK managing the pest so far. Farmers should be trapping, watching for pest alerts and checking their fruit for larvae, Demchak said.

Some traps can attract SWD, but also draw sap beetles, houseflies and earwigs, making it a pain to find any SWD, Demchak said.

Penn State is testing a trap with an apple cider vinegar lure, a commercial trap from Trécé, and a trap with a merlot/apple cider vinegar mix, she said.

Some people have reported success with the various lures, but it is hard to draw conclusions because SWD has not shown up this year at the university’s Rock Springs farm, where the traps are being tested, Demchak said.

“We’re encouraging growers to also use cultural methods to prevent populations from building too quickly,” Demchak said.

That means, for one thing, keeping the fields as cleanly picked as possible.

Pick-your-own operations should encourage customers to collect unmarketable fruit in separate containers rather than drop it in the rows. Growers can then bag the bad fruit and let it sit in the sun for a week, she said.

“They can bury it, but they need to bury it pretty deep,” she said. SWD has been known to emerge from 2 feet underground.

In any case, they should not compost the fruit as they usually would, she said.

Growers should also consider switching to earlier varieties so they can harvest before SWD becomes active. Raspberries and blackberries can be harvested more frequently to prevent old fruit from building up in the field, Demchak said.

If growers need to spray for SWD, they should rotate pesticide classes to avoid resistance buildup, Demchak said.

“SWD’s multiple generations per year makes resistance development more likely than usual, and the only thing that we need less than SWD is pesticide-resistant SWD,” Demchak and entomologist David Biddinger, who has done much of Penn State’s research on SWD, said in their Fruit Times article.

“We don’t want growers to go out and spray just-in-case type of sprays,” Demchak said.

Such a spray program could wipe out some beneficial organisms and hurt pollinators. Most sprays have limits on how much can be used, and farmers do not want to get into the situation where they need to spray but have already used up their quota for the season, Demchak said.

Thorough spray coverage is important, including the fruit and the lower canopy where the dorosophila might go to get away from the heat during the day, Demchak said.

According to research from Rufus Isaacs of Michigan State, the most effective sprays for SWD nationwide last year were Imidan, Lannate, Mustang Maxx, Danitol and Delegate. Malathion, Brigade and Hero were also effective.

Organic growers can use Entrust, Demchak said.

In a recent Rutgers Blueberry Bulletin, university entomologist Cesar Rodriguez-Saona and integrated pest management agent Dean Polk suggested blueberry growers should avoid continued use of neonicotinoids if they face both aphids and SWD. That class of pesticides does little against SWD and can cause resistance issues, they said.

Blueberry growers exporting to Canada can use Lannate if aphids are a problem, or Danitol if aphids are unimportant. For growers serving the domestic market, the spinosyn Delegate or any pyrethroid should work against SWD, they said.

If growers find a red raspberry that looks marketable but has juice in the interior cavity, “that is a key sign that they should be checking the fruit for any signs of young larvae in there,” Demchak said.

Blueberries with a pinprick surrounded by a soft area can indicate the presence of larvae. So can fruit that has an abnormally short shelf life, though that could also be the result of other diseases caused by the wet season, Demchak said.

Fruit should be refrigerated as close to freezing as possible. “That can kill off at least a portion of any larvae that could be present,” she said.

The Pennsylvania Department of Agriculture is no longer asking for samples when SWD is detected, but growers can submit suspected spotted wing drosophila to their county Extension office if they want, she said.

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