An Unwelcome Visitor

09/01/2009 02:00:00 AM
Vicky Boyd

A fruit fly, which prefers maturing fruit over decaying fruit, has growers in at last three states on edge as it came on the scene in May and began causing significant damage to central California cherries and berries.

The spotted wing drosophila entered the spotlight this summer when growers along the California coast noticed bruise-like damage to strawberries and caneberries and puncture holes in cherries.

It has been found in nearly two dozen California counties as well as two counties in Washington state and on two residential properties 3 miles apart in Hillsborough County, Fla.

Identifying features

At 2 to 3 millimeters, the spotted wing is about half the size of the Western cherry fruit fly, which is a quarantine pest in Washington but has not been found in California.

Photo by Ed Snow
The spotted wing drosophila fruit fly has been confirmed in California, Washington and Florida.

Nearly all of the 3,000 species within the drosophila fruit fly group prefer decaying fruit on which to lay their eggs. But the spotted wing likes fruit that’s just beginning to mature.

Initially, the pest was called the cherry vinegar fly. But that has been changed to spotted wing drosophila—scientifically known as Drosophila suzukii—to avoid confusion with other vinegar fruit flies.

Damage is caused when the female pierces the skin’s fruit with a serrated ovipositor to lay eggs. In cherries, the ovipositing hole may be visible, but in strawberries it’s not, according to Mark Bolda, a University of California Cooperative Extension farm adviser specializing in berries in San Benito, Santa Cruz, Santa Clara and Monterey counties.

As the larvae feed internally on strawberries, bruises and soft spots develop that pickers should recognize as defects to cull.

The openings also can provide entryways for secondary disease organisms.

The pest is C rated in California, meaning the state will not quarantine an area or impose control measures should one be found, says Norm Smith, an entomologist with California’s Fresno County Agriculture Commissioner’s office. The only restriction is plant material must be apparently free of the fruit fly before transport.

The pest is not rated in Washington but is a general quality concern, says Brad White, Washington Department of Agriculture pest program manager in Olympia. The spotted wing was found in a residential property in Kings County and in a Washington State University raspberry plot in Pierce County.

“We’re not sure if it’s something that’s been around for a long time and someone just noticed it or whether it moved in recently,” White says. “Right now we’re in a wait-and-see mode.”

Flies target cherries

The pest caused significant damage to cherries this season from Gilroy, Calif., north to Sunnyvale, says Bill Coates, a UC Extension farm adviser in San Benito, Santa Cruz, Santa Clara and Monterey counties who specializes in tree fruits and nuts.

“It caused a lot of consternation and sorting of fruit,” he says. “In some cases, growers just quit harvesting. In San Benito County, we run a bit behind, so we had the added benefit of the knowledge and got started spraying early. It wasn’t nearly the pest down here.”

If it behaves like other drosophila fruit flies and has a short reproduction cycle, it will probably have 10 generations annually, Coates says. That could mean three generations during the period when cherries are susceptible.

The pest is native to Japan and China, which both have cold winters. So it’s a good guess that it will be able to survive California’s milder winters, Smith says.

The best detection methods

Because the pest is so new to the United States, researchers have hurried to try to answer some of the unknowns, such as the best monitoring methods and control techniques.

Bolda conducted a field trial this summer in a heavily infested raspberry block. He compared seven different lures or lure mixtures with a sweep net. The goal was to determine how to best trap for the pest.

Photo by Mark Bolda

Each of seven baits or bait mixtures were placed in a 500-milliliter Nalgene bottle with four 7/16-inch holes drilled into the lid and hung about 3 feet from the ground in the hedgerow.

Of the baits, methyl eugenol and a molasses mix captured no flies, Bolda wrote in a blog. GF-120 and a strawberry puree were most effective, capturing an average of nine flies in 24 hours.

The puree appeared more effective during the second trial as it was older and had begun to ferment.

But the sweep net provided the most effective way to evaluate fly numbers in the field, Bolda wrote.

An average of 40 flies were caught during a sweep of 36 feet of hedgerow in a few minutes.

Based on the results, Bolda says baits will continue to be an important sentinel to detect initial infestations. But sweep netting will provide a better assessment of already established populations in caneberries.

Adopting other fruit-fly-control measures

When growers first began reporting damage, control methods remained an unanswered question. Pest control experts hoped that what worked on other fruit flies, such as husk fly and the Mediterranean fruit fly, would work on the spotted wing.

GF-120, which is a mixture of a bait syrup attractant and an organic form of the insecticide spinosad from Dow AgroSciences, is registered for fruit flies.

It is typically applied at 20 ounces of product in 1 gallon of water per acre. Many growers found success with applying GF-120 two times per week during the critical cherry-ripening period.

“Generally, we found that GF-120 on other fruit flies worked well in low to moderate populations,” he says. “In some orchards in Santa Clara County, I think the populations were just too high.”

Under high populations, there are so many flies that the bait is consumed before others have an opportunity to feed on it, he says.

A possible option could be mixing either malathion with Nulure or mixing Entrust, an organic spinosad, with molasses. Growers would then apply the mixture at a higher active ingredient rate and with more water volume, such as 5 to 10 gallons, per acre, Coates says.

For more information about the spotted wing drosophila, visit http://www.ipm.ucdavis.edu/EXOTIC/drosophila.html.

Contact Vicky Boyd at vlboyd@att.net or (209) 571-0414.



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