Spotted wing drosophila pupa protruding from cherry. The pupa is about 1/8 inch long.
The pest, a native of Asia, was first seen in California in 2008. It was found in Oregon and British Columbia in 2009, and turned up in eastern Washington this summer.
The spotted wing drosophila, Drosophila suzukii, is a close relative of the common vinegar fly, D. melanogaster, but is more onerous because it attacks ripening fruit before it is harvested, not just overripe fruit. Its hosts include cherries, peaches, nectarines, plums, various berries, and grapes.
Dr. Peter Shearer, head of Oregon State University’s Mid-Columbia Agricultural Research and Extension Center in Hood River, will do field tests starting this fall to assess its ability to overwinter. The federal Specialty Crop Research Initiative has provided $5.8 million in funding for research to deal with the pest in the western United States.
Shearer will place caged adult flies and pupae in various places in the Hood River Valley, as well as at Mosier, The Dalles, and Milton-Freewater, Oregon. Next spring, he’ll see how many survived. Washington State University entomologist Dr. Elizabeth Beers will do similar field trials in Washington.
First find
Drosophila was found in the Mid-Columbia region in significant numbers in September of 2009. This spring, the fruit industry put out about 90 traps, and the first fly of the season was caught in Mosier on May 10. Flies continued to be caught in the area throughout the season.
Len Coop, entomologist with OSU in Corvallis, has developed a phenology model using published data from Japan. The model predicted that the insect could start to become active this year as early as May 19 in Mosier.
The fact that it showed up in a few areas in the Mid-Columbia region at close to the time it was supposed to, based on the model, makes it seem likely that it overwintered, Shearer said. Most researchers believe that the insect overwinters as a mated female adult that is ready to lay eggs in the spring.
The model uses a lower development threshold of 50°F, which Coop said is relatively high for an insect. In addition to warm days and evenings, the insect needs suitable hosts (ripening fruit) readily available before it will emerge. Overwintering flies can be inactive for as long as 180 days if necessary. “That’s a really long time, and that’s a little bit hard for us to fathom, but they just hunker down and wait,” Coop said.
It first showed up in Washington State this year on June 28 in Mattawa, at the height of the cherry season. Though that was considerably later than the April 21 date predicted by the model, Coop said it is still possible that the pest overwintered in Washington. It could have developed through a couple of generations before it was found, or it could have emerged later than predicted if it lacked suitable hosts.
Beers said few traps were out in Washington early in the season, but the first find prompted an intensive statewide trapping effort starting July 1. Tree fruit industry horticulturists put out traps from Tonasket in the north to the Tri-Cities in the south, and brought flies to Beers’s lab for identification. By the end of August, drosophila flies had been caught throughout central Washington.
Because of its distribution throughout the state, it seems as if it must have been there last year and overwintered, Beers said. Even if only a tiny percentage survived the winter, that would be enough to produce significant populations this year because the fly reproduces rapidly. If it had arrived on fruit, it is unlikely to have been so widespread.
In laboratory studies conducted by Masahito Kimura in Japan, the half-lethal cold temperature (temperature at which half the specimens died) was 32°F. Vaughn Walton, entomologist with OSU in Corvallis, has done lab work that suggests that the insect is killed by winter temperatures of 45°F or lower with a 10-day freezing period at 23°F. Walton said the insect does not lay eggs at below 55°F or above 85°F.
But Coop said lab results don’t always translate into the field. Little is known about where the pest overwinters, but it is probably on, or buried in, the ground. It might find protected places where it is not exposed to severe temperatures.
The study in Japan also looked at the insect’s ability to survive hot summer temperatures and found that the half-lethal hot temperature was 90°F. Coop believes the upper threshold is less critical than the lower threshold in terms of the insect’s activity or survival. “Just because the maximum temperature is over 90, it doesn’t mean it’s not active at other times of the day,” he said.
Even larvae or pupae in the fruit can escape hot temperatures by burrowing to the shaded side of the fruit. And, modification of the orchard environment, such as irrigation and cooling, can also work in the insect’s favor, he said.
Unlike codling moth, which is most active around dusk, drosophila seems to be more attuned to temperature and can be active at any time of day, Coop observed. “I think the spotted wing drosophila is not quite as tied into the sunrise-sunset cycle. I think they can just time-shift and be active at different times of day.”
Generations
It’s been reported in Japan that the insect develops through as many as 13 generations, but Coop said that would be in the subtropical parts of the country, where it could be active year round.
He believes it can take as long as a month to develop through one generation in the Pacific Northwest, perhaps because of the time the adults spend flying around looking for suitable hosts. “Spotted wing drosophila is always jumping from host to host to look for the best meal for its offspring,” he said.
Coop’s model shows that the insect is likely to develop through three to five generations in Hood River, five to seven generations in Wenatchee, and seven to ten generations in Mattawa. Activity should be winding down by the end of October, when the average high temperature is in the 50s, and should end by the first frost.
Whole apples and pears are not thought to be a host, but Coop said the insect is an opportunist and might infest pome fruits that have entry points caused by other insects or birds.
Coop plans to update the model with data from Washington and Oregon this season.
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