Spotted wing drosophila on the horizon?

OSU Entomologist Amy Dreves checks a monitoring trap in an Oregon strawberry grower’s high tunnel crop for SWD.
Photo by Lynn Ketchum, OSU EESC.

Spotted wing drosophila (SWD) has northern U.S. growers on watch for the fruit fly (Drosophila suzukii) first discovered in California strawberry and caneberry fields in 2008. The invasive pest moved quickly north into British Columbia, wreaking havoc on thin-skinned fruit crops along the way.

Growers reported losses to SWD of 10 to 80 percent of their crops. Soft-skinned fruits suffer internal damage by larval damage after SWD lays its eggs through a serrated ovipositor that punctures ripening berries, peaches, plums, grapes and other crops. The surface wound can be hard to detect in hanging and harvested fruit by growers and packers.

Mark Bolda is the University of California Cooperative Extension strawberry and caneberry farm advisor at the leading edge of West Coast response to SWD. He says, “We are talking about a $2.57 billion reported loss by strawberry, blueberry, raspberry, blackberry and cherry growers in California, Oregon and Washington in SWD’s first year, and this number considers only production value without any market reaction.”

Bolda, co-author of “Spotted Wing Drosophila: Potential Economic Impact of a Newly Established Pest” (, uses a hypothetical average of 20 percent yield loss that increases or decreases by crop and production area.

In August 2009, SWD was identified in traps located within Florida’s strawberry region prior to planting. The assumption is the pest arrived there by product transport.

Growers on watch

“What we do not yet know about SWD is as important as what we do know,” says Kathy Demchak, a senior extension associate with the Penn State University Department of Horticulture. “While SWD could potentially move south to north, we can’t really predict how or when it will arrive. It may move up the coast, or could just as likely be brought into a given area on a shipment of fruit. Regardless, we will be keeping an eye on nearby states and those close to Florida for any reports of SWD detection.”

The Southern Regional Small Fruit Consortium has established an SWD monitoring program in backyard gardens, commercial vineyards and fruit orchards in South Carolina, North Carolina and Virginia. The program uses trapping kits modeled after those used in Oregon. The traps combine one of two sweet fermenting liquid lure solutions with a sticky card.

Nineteen trapping locations are primarily focused on blueberry, blackberry, strawberry, raspberry, peaches and wine grapes as the targeted fruits. Traps have also been set in non-crop hosts including wild caneberries and Prunus species.

North Carolina State University Assistant Professor of Entomology Hannah Burrack leads the monitoring effort. She says, “We have not identified spotted wing drosophila in this area yet, but we suspect we will catch it soon.”

Early detection and collaborating on trapping protocol

In May 2010, Pennsylvania had not yet begun actively monitoring for SWD. Demchak says, “Until fruit is actually present in the field, the likelihood of detecting SWD with traps in field plantings is low. Early in the season during strawberry bloom we’d be more likely to catch pollinators than SWD.”

State Extension Tree Fruit Entomologist Dr. Greg Krawczyk says Pennsylvania Department of Agriculture survey entomologists are constantly on watch to detect new pests.

“It really is just a matter of time before spotted wing drosophila is identified here,” he says. “Fruit flies create an important marketability issue for fruit growers, particularly if there is a possibility of finding larvae in fruit being shipped to market.”

In New York, Kennoth Carnes heads up a network of 16 New York State Department of Agriculture & Markets’ Cooperative Agricultural Pest Survey (CAPS) inspectors checking greenhouses, garden centers and box stores for unwanted pests. CAPS networks with the state’s food inspectors checking produce in grocery stores and at farm markets and farmstands.

Cornell University Entomologist Dr. Greg Loeb sets a sticky card trap to monitor for tarnished plant bugs in a weedy field near strawberry and raspberry research plots at Cornell’s New York State Agricultural Experiment Station at Geneva, N.Y.
Photo courtesy of Cornell University.

Carnes says, “While spotted wing drosophila is not currently on our watch list, if our surveyors see a fruit fly not previously seen, it will sent to our state lab for identification and we actively encourage growers, retailers and consumers to contact us or their local Cornell Cooperative Extension office anytime they see anything out of the ordinary. We especially encourage anyone who is importing product to be diligent in their own product inspections.”

Dr. Greg Loeb with the Cornell University Small Fruit and Grape Extension is monitoring grapes and strawberries research plots and collaborating with Dr. Rufus Isaacs, a berry crop entomologist with Michigan State University, on a trap design to provide first detection of SWD in their states. Loeb expects to use a covered trap with apple cider and a sticky card to make identification easier and less messy.

“We do not typically trap pests in strawberry or raspberry fields so growers will have an initial adjustment to monitoring with traps compared to field monitoring for pests in the crop itself,” Loeb says.

Isaacs says, “We are being vigilant to be as prepared as possible should it be found in Michigan. We have been monitoring its progress since the fly was first reported in California. Once it was found in Florida, our concern level increased significantly.”

MSU’s Project GREEEN (Generating Research and Extension to meet Economic and Environmental Needs), a plant-based agricultural initiative, provided emergency funding to establish a monitoring program in Michigan’s main fruit-growing regions in 2010.

Isaacs says, “If this fly is detected, an industry-wide effort will be needed to ensure its impact is minimized.”

Project GREEEN has also funded formation of an SWD Response Team involving MSU research and extension staff, state agencies and commodity group representatives.

Diligent production practices

Nourse Farms of South Deerfield, Mass., propagates nursery stock for berry growers. In its spring 2010 newsletter, Co-owner Nate Nourse wrote, “Spotted wing drosophila (SWD) will change the way we produce and harvest fruit. SWD is now one of two fruit fly species with the potential to cause severe economic damage to all soft fruit crops. The good news is it is fairly easy to control with current insecticides; the bad news is a part of the control procedure requires the removal of all cull fruit. I recommend growers use traps for early detection.”

Nourse adds, “Spotted wing drosophila has impacted California in a huge way. Trapping is the cheapest form of insurance for growers in the east. Growers can put out traps for a few dollars a week and then they will know if they have spotted wing drosophila and can be on top of it.”

Current research suggests that growers can practice a number of field management strategies to reduce exposure to SWD (see sidebar).

Demchak says, “With SWD, finding larvae in the fruit is a main concern. The pest is similar to sap beetles in that adults lay eggs in the fruit and larvae then feed there. We recommend growers use the same cultural controls as they would for sap beetle management. Keep plantings picked clean, and destroy or bury unused fruit to minimize the amount of old fruit left in the planting. Renovate strawberry plantings soon after harvest, any fruit that remains in the field is a good place for problems to multiply.”

Demchak is particularly concerned for u-pick farm operators. “We have a huge number of pick-your-own farms in Pennsylvania and about 30 percent of fruit is left in u-pick fields either because it is too small or the people picking miss it,” she says.

Growers should check state-level use restrictions for spray products.

“Many insecticides will kill this pest, so growers who already spray for other insect problems may be affecting SWD. These pesticides are not yet registered for use specifically for spotted wing drosophila, however,” Demchak notes. “A big difference between various insecticides is how long they last, and, unfortunately, the ones that are safe to use during harvest because they break down quickly don’t have very long residual activity on the fruit flies either.”

Loeb says, “For organic growers, pest control is always more challenging, but they can use exempted pesticide applications and practice good field sanitation.”

Isaacs says using a knowledgeable scout to provide weekly checking for insects, diseases and weeds is a good investment for growers of several types of fruit.

“From what I have learned from researchers working in the West Coast fruit crops, strawberries, cherries, raspberries and blueberries are all at risk, while the tougher apple skin and fruit tissue may make apples less susceptible,” Isaacs says.

What about grapes? Loeb says, “I am not as worried about SWD impact on grapes as I am the softer-skinned fruits.”

Grapes were added to the list of crops at risk in October 2009 when Oregon State Agricultural Entomologist Dr. Helmuth Rogg with the Integrated Pest Prevention Management program confirmed SWD in both table and wine grapes that had been hand-picked in the Salem area. Dr. Rogg hatched the larvae out in his lab to confirm SWD.

Oregon State University Entomologist Vaughn Walton identified SWD in wine grapes in Northwestern Oregon’s Willamette Valley, but no noticeable damage was seen in the harvested crop in October 2009.

Will cooler temperatures or wasps help?

SWD is known to grow in the colder regions of Japan, and on the West Coast moved north into the cooler climate of British Columbia. Loeb says, “SWD is probably able to survive in New York’s growing conditions.”

“While it is likely this pest will show up in Pennsylvania at some point—and may already be here—how well it will survive in our cooler Northern environment is not known,” Demchak says.

“The spread could be hastened by fruit being shipped all over the U.S., but research that indicates the larvae and eggs can be killed by temperatures close to freezing for four days, at least for strains of SWD in Japan, could mean that fruit shipped at cold temperatures may be less problematic than fruit shipped at warmer temperatures,” she adds.

This apple cider trap is in a young field of blueberries that is surrounded by a more mature planting.
Photo courtesy of North Carolina State University.

University of Florida Entomologist Dr. Jim Price is a strawberry crop extension specialist and research entomologist in Hillsborough County, “The Winter Strawberry Capital of the World,” where SWD was first found in Florida. Price says, ”There are minimum temperatures at which the flies are less active. We don’t know the temperature needed to kill them. They are at their best at cooler temperatures and do not fair as well in hot temperatures, however, Florida had a normal hot summer in 2009 and it was in August that we first trapped SWD here. The bottom line is we need to know more.”

Florida Department of Agriculture and Consumer Services (FDACS) Division of Plant Industry Entomology, Nematology and Plant Pathology Bureau Chief Dr. Greg Hodges says, “We had a pretty cold winter in Florida this past year, but we are not sure if that has impacted the lack of SWD in our strawberries this spring. We found a few flies in the cull berries left in the field, but no damage to the crop. Next winter could be an entirely different situation.”

Bolda says, “Constant temperatures of up to 35 degrees Fahrenheit for 96 hours or more resulted in total mortality of SWD eggs and larvae in the Japanese research. This has been anecdotally confirmed in tests in 2009 in California, however, constancy of temperature is critical and that would be a challenge for fruit shippers and sellers. The point is that we cannot rely on a cool strategy to address the suppression of SWD in harvested fruit.”

Researchers and growers are searching out other control options. Demchak says, “At least one species of parasitic wasp is reported to control spotted wing drosophila populations in Japan.”

Oregon State University has received a $5.8 million matching funds USDA grant to study SWD.

In Georgia, USDA APHIS has begun research to evaluate how levels of irradiation used to assure the mortality of SWD impact shelf life and quality in blueberries and peaches.

The following Web sites have spotted wing drosophila fact sheets, blog notes and pest alert warnings:

What is Spotted Wing Drosophila?

Spotted wing drosophila (Drosophila suzukii), so named for the single spot that appears on both wingtips of the male adult fly, is a fruit fly native to South East Asia.

The insect has spread to Japan (1916), Korea, Russia, Spain, Hawaii (1980s) and California (2008), and was identified in 2009 in Oregon, Washington, British Columbia and Florida.

Most of the 3,000 species of fruit flies eat rotting fruit left in fields or aging on your kitchen counter. SWD may prefer fresh fruit. The female SWD uses a serrated ovipositor to bore tiny holes into soft fruits to lay seven to 16 eggs a day, 1 to 2mm deep.

Temperature is a factor in the growth rate of SWD. At 54 degrees Fahrenheit, it takes 50 days for an egg to grow to adulthood; at 77 degrees Fahrenheit, 8.5 days; beyond 85 to 86 degrees Fahrenheit, the males become sterile. Hot dry weather discourages SWD activity in general.

SWD damage inside fruit from larval infestation can be difficult to detect. Infested fruit that looks fine may be discovered by consumers.

Economic damage by SWD can be extensive. The October 2009 Oregon State University Extension News reported a one-third crop loss of the cherries grown from Davis to Modesto, California, and up to 80 percent losses in some peach orchards in the Willamette Valley of Oregon with up to 20 percent losses by some Oregon raspberry and blueberry growers.


Early Warning Systems and Strategies

Researchers advise that trapping is the best method of identifying whether or not SWD has reached your area. In California, Mark Bolda uses one package of Baker’s yeast, 4 teaspoons of sugar and 12 ounces of water to create a rapidly fermenting liquid that serves as a most-effective lure for SWD.

A bait-toxicant combination (GF120) with a sticky card added is also effective.

SWD does not like the hot sun, so place traps in shady areas for best catch opportunity.

Best management strategies, including the following suggestions, are evolving as researchers in several states study SWD:

  • Use monitoring traps for early detection so that effective management programs can be implemented early in the production season.
  • Trap with baits set to draw SWD away from production areas.
  • Practice good field sanitation: remove cull fruit and fruit left in u-pick fields to reduce breeding sites and kill immature SWD; traps are not as sweet a lure as an accumulation of rotting fruit.
  • Dispose of unmarketable fruit to prevent SWD access.
  • Use state-approved preventative spray products. A March 2010 update from Bolda notes Delegate provided three weeks of control in California fields. Florida approves the use of malathion, diazinon and pyrethrum-based products targeted at adult flies. Organic growers can use broad-spectrum PyGanic or Entrust. Multiple applications may be required with heavy infestation. Rotate treatment products to slow resistance build-up
  • Harvest at optimum ripeness and move harvested fruit to market quickly.
  • Watch for updates from Extension, e.g., a 1939 Japanese report indicated a parasitic wasp might be a natural control option; the California Department of Food & Agriculture is looking for other natural enemies of SWD.
  • Watch for news of SWD impact in various fruit crops. Grapes were added to the list of crops at risk in October 2009 when Oregon State Agricultural Entomologist Dr. Helmuth Rogg confirmed SWD in both table and wine grapes
  • Watch for shipping and handling strategies. Literature dating from 1939 in Japan indicates SWD may have a temperature sensitivity. Bolda says, “Constant temperatures of up to 35 degrees Fahrenheit for 96 hours or more resulted in total mortality of SWD eggs and larvae in the Japanese research. This has been anecdotally confirmed in tests in 2009 in California, however, constancy of temperature is critical and that would be a challenge for fruit shippers and sellers. The point is that we cannot rely on a cool strategy to address the suppression of SWD in harvested fruit.”

The author is a freelance writer who keeps horses and sheep on a 100-acre farm in Mannsville, N.Y.