Spotted Wing Drosophila: Small Fly – Big Problem
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.
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.”
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.
“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.”
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.”