Spotted wing drosophila larvae that hatch from eggs inside the fruit sometimes pop out and walk around on the surface. The spotted wing drosophila can pupate inside the cherry, outside the cherry, or halfway out.
PHOTOS BY ELIZABETH BEERS, WASHIHINGTON STATE UNIVERSITY
The Washington tree fruit industry should be better prepared to tackle the new pest spotted wing drosophila in the coming season, thanks to extensive trapping by field horticulturists last year and results from ongoing research on the pest, says Dr. Elizabeth Beers, Washington State University entomologist.
Specific recommendations for growers are being developed by WSU scientists and will be posted on the Web site at http://extension.wsu.edu by March 15. Beers said notices of the first trap catches of spotted wing drosophila (Drosophila suzukii) in the various growing regions of the state will also be posted on the site.
Cherries are one of the insect’s favorite hosts. It also attacks stone fruits, peaches, nectarines, plums, apricots, and berries. Native hosts include Oregon grape, black currant, blue elderberry, cherry laurel, mulberry, and possibly serviceberry and chokecherry.
Unlike its cousin, the common vinegar fly (Drosophila melanogaster), the new species attacks fruit that is on the tree and not completely ripe or damaged. But, like the vinegar fly, it will infest fruit that falls to the ground and that is where it can build up to large numbers, Beers said.
The spotted wing drosophila originated in Asia and was first detected in the United States in 2008 in California. The following year, it was found in western Oregon and Washington, and British Columbia, Canada. In 2010, it spread to more parts of the United States. It was found in Utah and spread up the East Coast from Florida to Alabama. It also showed up in Michigan’s blueberry districts, which Beers said was surprising and alarming. “If it can survive in southern Michigan, I don’t think it will have a problem surviving in eastern Washington.”
A year ago, it was assumed that eastern Washington would be protected from this new pest by virtue of its cold winters and hot summers. The first spotted wing drosophila in Washington in 2010 was found in a Royal City orchard in late June. Intensive trapping in a number of areas in central Washington revealed a sharp increase in populations in August. Almost every fruit-growing area from Oregon to the Canadian border had some detections. Numbers remained high in October and November, which Beers said is unusual for a pest, since populations normally decline when the weather cools off. Spotted wing drosophila populations did not decline until a hard freeze in late November when the temperature dropped to -4°F in Wenatchee and lower still in other parts of the state. The Oregon and British Columbia fruit industries also saw high late-season populations.
“I am confident that this is going to be primarily a late-season pest,” Beers said, noting that many of the state’s cherries will be harvested before the big increase in populations in the fall, but the August “hump” could affect later-maturing varieties and districts. “Those are going to be more at risk than the cherries harvested in June.”
The flies were found in other stone fruits, particularly plums and apricots, as well as cherries. Fewer flies were caught in the lower Yakima Valley and Tri-Cities areas than in the more northerly Wenatchee, Orondo, and Quincy areas, but Beers said growers should not assume that will be a permanent pattern.
“It was a year of invasion, establishment, and spread, and anything can happen in future years,” she said. “I don’t want people thinking if they grow fruit in the Yakima Valley, they’ll be free of this pest in the future. I don’t know that.”
The male fly is fairly easy to identify from the dark spot on each wing. It also has two dark bands on each foreleg. At a glance, the female looks similar to other drosophila flies, but larger. It takes good magnification to see that it has a toothed ovipositor that is significantly larger than that of other drosophila species.
Female flies lay eggs in cherries, puncturing the skin. Each fly can lay 200 to 600 eggs in her lifespan, compared with the cherry fruit fly’s 50 to 200 eggs. The first sign of an infestation might be the breathing tubes that extend from the eggs to the cherry’s skin.
The larva, which hatches inside the cherry, has two spiracles at the tail end, which distinguish it from a cherry fruit fly larva. Some larvae might pop out of the fruit and walk across the surface. Pupae can pupate completely inside the fruit, halfway out of the fruit, or outside the fruit, Beers said.
The whole life cycle can take very little time. In cold temperatures, a generation might take three weeks or more, but in optimum temperatures, for example in June, the pest can complete a generation in as little as nine days.
In 2010, the Specialty Crop Research Initiative awarded a $5.8-million grant for a major research project on the spotted wing drosophila. Twenty-five researchers at WSU, Oregon State University, the University of California, and the U.S. Department of Agriculture are working on every conceivable aspect of the insect, Beers said.
Dr. Peter Shearer, entomologist with OSU in Hood River, has found in laboratory tests that the pest is susceptible to residues of ultra-low-volume malathion on leaves. Lab tests by Dr. Robert van Steenwyk at UC, Berkeley, indicate that malathion and diazinon, both organophosphates, are quite toxic to the adult. He found that Danitol (fenpropathrin) was the most toxic pyrethroid, and the spinosins Delegate and Entrust were very effective. Provado (imidacloprid) was moderately effective, while other neonicotinoids were not.
Van Steenwyk also did field trials that indicated that a single preharvest spray of malathion would not provide control and that two to three insecticide applications would be needed. However, Beers said the situation might be different in Washington than in California, where the insect overwinters and emerges in high numbers in May.
For Washington, Beers recommends that when the crop is at the susceptible stage (beginning at the pink stage for cherries) and information on the WSU Web site indicates that pest is in the area, growers should begin spraying. Since the pesticides that appear to be effective against the pest have little residual activity, sprays will need to be reapplied every seven to ten days.
She suggests that neighboring orchardists collaborate. Instead of waiting for a positive trap catch in a specific orchard, if the pest is in the area, everyone should think about protecting their crops.
Last season, Beers tested a postharvest control using dimethoate or Provado. The materials did an excellent job of killing larvae in the remaining fruit on the trees. However, they were not effective against larvae of other drosophila species that were also in the fruit.
Monitoring will continue in Washington in the coming season to learn more about the insect’s distribution and population trends. More research trials are also planned.
Beers discussed the pest during winter meetings in Washington.