Spotted Wing Drosophilia: Small Fly - Big Problem

(Photo Credit: Elizabeth Beers, Washington State University)(Photo Credit: Elizabeth Beers, Washington State University)

Spotted Wing Drosophilia: Small Fly - Big Problem
By David Eddy May 29, 2015


It’s just a fruit fly, for crying out loud. As kids we’d see their like hovering over the family fruit bowl and shoo them away without a thought. But spotted wing drosophila (SWD), Drosophila suzukii, is so much more than that.

Though small like their nonthreatening drosophila relatives — adults are only about 1/16 to 1/8 inch long — it’s the black spot towards the tip of each male’s wing, which earns them the spotted wing moniker.

The other names they are called by fed-up growers — the names that can’t be printed in a respectable magazine — come from an attribute of the females. It is a very prominent, saw-like ovipositor for laying eggs in fruit.

It’s that devastating characteristic that has garnered the SWD a reputation in virtually all of the country’s fruit production areas in just six seasons. It was first found in 2008 damaging fruit in California, and has since spread throughout the U.S.

SWD was originally best known for infesting ripening cherries — though fruit doesn’t have to be over-ripe, green under-ripe fruit don’t do anything for it — but has since become most notorious for attacking berries, especially raspberry, blackberry, blueberry and strawberry crops.

While diverse geographically, SWD was found to have some common characteristics by American Fruit Grower® and Western Fruit Grower™ magazines in a survey of university entomologists from around the country. Here’s a quick rundown.

Be Prepared
Almost all of the entomologists said growers of at-risk fruit can expect to see SWD about a month after temperatures start rising in late spring.

“Keep an eye on it. The best way to know if it’s active in your area is to have traps out in your orchard,” says Elizabeth Beers, Washington State University Professor of Entomology. “WSU has no official recommendation as to how many, but if I had a 10-acre block I wouldn’t want less than 4.”

Keep On Top
When asked what one piece of advice they would give to growers, virtually every pest expert said to be ready, and don’t wait to act.

“Don’t let it get out of control, when the numbers are low you can handle it,” says Mark Bolda, University of California Cooperative Extension Farm Advisor, Strawberries & Caneberries. “But when the numbers are big, you’ve got trouble. And populations can build really fast.”

Pesticides
In virtually all parts of the country, conventional growers are rotating sprays of organophosphates such as malathion, pyrethroids, and spinosad. For organic growers, they just have the one spinosad, Entrust from Dow AgroSciences.

(Photo Credit: Elizabeth Beers, Washington State University)(Photo Credit: Elizabeth Beers, Washington State University)

“Once it’s established in an area, most growers use an aggressive regime,” says Greg Loeb, Cornell University Professor of Entomology. “Spray once every seven days, no more than two consecutive weeks with the same class. Some are more aggressive than every seven days, but that’s depending on the label, too.”

Kelly Hamby, University of Maryland Assistant Professor/Extension Specialist, notes that perhaps some day SWD will be controlled with a parasitic wasp as it is where it came from, in Asia.
“But that will be many years down the line,” she said. “Until then, we are worried about drosophila.”

Cultural Practices
Most researchers say that cultural practices only go so far with SWD, but sanitation and keeping an open canopy for better airflow and more sunshine is crucial.

“We learned that last year, despite the best intentions, you have to make sure to keep the fields clean,” says Hannah Burrack, North Carolina State University Associated Professor and Extension Specialist.

Also, the flies like a nice shaded hangout to spend time in. Really, you’re not going to be able to control the pest with cultural practices alone. On the flip side, you won’t be able to control them with pesticides if you don’t have the cultural practices.”

Pick That Fruit
Perhaps no practice is more important than getting your fruit harvested as rapidly as possible, says Rufus Isaacs, Professor & Extension Specialist in Dept. of Entomology, Michigan State University.

“The longer you leave ripe, or even worse, over-ripe fruit out there, the more trouble you will have with Spotted Wing Drosophila,” he says. “Rapid picking is really useful. It’s not always practical, but think about how you can do it.”



REGIONAL REPORTS

Each of the entomologists surveyed by American Fruit Grower® and Western Fruit Grower™ magazines was asked how the pressure was in the 2014 growing season. They were also asked about how the pest behaves on crops in their region, and were given the difficult task of predicting what they expect during the coming 2015 season. Here’s a brief look at their responses.

Hannah Burrack, North Carolina State University Associated Professor and Extension Specialist:
Last year was shaping up to be typical year. Few if any spring strawberries and highbush blueberries, which are picked from May to mid-June, were damaged. Rabbiteye blueberries, which are picked from June to mid-July and represent about 20% of North Carolina’s blueberry crop, did sustain some damage.

It was a different story for crops that were picked in late July or later, like blackberries, raspberries, and fall strawberries, which in some cases sustained severe damage. We only have a few growers who do those, but they must aggressively manage spotted wing drosophila.

This year we had a similar winter in terms of cold so I would expect a similar year as compared to last year. We haven’t seen any infestations yet (mid-May), which is typical. We expect to start seeing them in the next month or so. Even if there was a lot of mortality in the winter due to cold weather, there is plenty of time for SWD to recover by mid-summer.

Kelly Hamby, University of Maryland Assistant Professor/Extension Specialist:
In the Mid-Atlantic we have a lot of diversified farms. There was heavy pressure because there are small plantings of many host fruit. We have cherries, strawberries, blueberries and raspberries, many with rows of different varieties, which means a lot of the fruit was available a lot of the time. It’s a nightmare situation. Small farmers don’t have the labor or profit margins to stay on top of it.

This year I would expect more of the same. Though it’s been colder longer so heavy damage will likely start a little later in the season. August is when it will likely start to be bad, so the early varieties shouldn’t have too much trouble. If you have only early season strawberries and cherries it should be manageable. But any later season varieties of berries will likely have trouble.

Greg Loeb, Cornell University Professor of Entomology:
In the Northeast in 2014, we had slightly lower pressure and a little bit later arrival than in 2013 and 2012. But by the end of the season we had almost-as-high pressure. There were no reports of damage on blueberries until late in the year. While we had some reports of problems as early as late June in 2013, in 2014 it was mid-July and in some spots farther north it was August.

This year, though it was cold, there was more snow, so more insulation, which means the flies don’t experience -20°F, they are experiencing just -2°F. But there is still debate this far north of what numbers are overwintering. Maybe it’s some migration of flies from farther south. Perhaps storm fronts bring them out and drop them. It’s just a hypothesis, but we have seen that with other insects. Potato leafhopper, a foliar pest on berries and grapes, does that.

Rufus Isaacs, Professor & Extension Specialist, Dept. of Entomology, Michigan State University:
In Michigan, we had less pressure last year. We had a brutal winter the year before and I think that slowed down the initial development in the spring. We also had a cooler summer than normal. The peaks of the population weren’t running as high as in the past. Also the blueberry crop, which is the most affected crop in Michigan, was later and that did help. The other thing that affected it was that we first had it in 2010, so that was our 4th year of experience. In those interim years growers have found out how to control it.

It’s hard to make a prediction for this year. We had another cold winter, though it was not quite as bad as the year before, and this season is looking more normal. But I think the growers are prepared for it.

Besides blueberries, we’ve seen it on day day-neutral strawberries, but most of our acreage is June berries, which ripen earlier. When the day-neutrals start ripening in August they have more trouble. Summer raspberries are a problem, but not as bad as the fall raspberries because they peak in early September.

Elizabeth Beers, Washington State University Professor of Entomology:
Last year the pressure was pretty solid. It came a little bit later than in 2013, which was the highest pressure we’ve ever had. That season was preceded by mild winter, and it just came out so early. We caught SWD every month of the year in some of our traps. It slowed in the winter, and then came right back.

Incidentally, they are limited by high temperatures too. We don’t know exactly at what temperature they become less active, but it’s also related to high humidity. If it’s dry, they are less active at high temperatures. Look at the interior valleys of California. When it gets to be 100°F, they stop being active.

This year, we’ve had another mild winter with the exception of a cold snap in November. But I’m not sure it was enough to knock the populations back. They have the potential to attack Washington cherries. But the only year they were a huge problem with cherries was 2012 when it rained in June and we couldn’t get sprays on.

Mark Bolda, University of California Cooperative Extension Farm Advisor, Strawberries & Caneberries:
Last year the pressure wasn’t overwhelming. In fact, we have the conventionally grown fruit pretty well under control. In strawberries the pressure was minimal. In caneberries it always picks up in mid-July and then was heavy through September or October. It’s sort of like clockwork, always the same time of year.

Though they were first found here, in 2008, we don’t have the problems seen in other areas because along the coast it stays pretty cool and our growing seasons are different. For instance, we grow few blueberries along the coast, and the ones we have are very early, in March and April, before SWD is a problem.



QUOTABLE QUOTES

“Once it’s present, you have to be vigilant. You can’t let it get away from you or you’re going to be in trouble. Extending the spray interval during a critical period can cause problems. I understand growers get busy, but if you say, ‘Oh I can’t get to that spray today,’ you may end up in serious trouble.” — Greg Loeb, Cornell University Professor of Entomology

“Keep an eye on your early varieties if you have multiple varieties. If you leave those fruit in the field, you’re going to have a really hard time the rest of the season. I’ve seen that happen in California, because growers want to move their labor crews to the next varieties that are coming in. Finish that first wave off, clean them up.” — Kelly Hamby, University of Maryland Assistant Professor/Extension Specialist
We’ve seen some really good results from netting. For the smaller-scale grower supplying a farm stand or a farm market who doesn’t want to spray, it’s an option. Though it’s not cheap, exclusion works.” — Rufus Isaacs, Professor & Extension Specialist, Dept. of Entomology, Michigan State University

“Plan ahead, be prepared. Think about what tools you have available, and be on the lookout. Constantly look in your fruit. I’d rather know at the farm level than send my fruit off and have problems found there. It’s unfortunate when you find it on the farm, but you still have options. Once it’s on the truck, you are basically out of options.” — Hannah Joy Burrack, North Carolina State University Associated Professor and Extension Specialist

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A Picture Bev took :

  She found it in the berry she was about to eat!