New pest keeps industry guessing
Spotted wing drosophila is a late-season threat.
A female spotted wing drosophila is about to enter a Contech apple cider vinegar trap. WSU scientists will be trapping for the pest during the coming season and will send out e-mail alerts.
ELIZABETH BEERS, WSU
Will spotted wing drosophila became a threat to cherry production in Washington State, or will harvest be over in many orchards before the insect becomes active?
Dr. Betsy Beers, entomologist at Washington State University, who has been heading a trapping program in cherries in central Washington, said experience with spotted wing drosophila in 2011 was very different from 2010, the first year it was trapped in Washington.
“Populations in our traps were 90 percent lower than in 2010,” she said. “There was a big change from one year to the next.”
In 2010, Washington’s first spotted wing drosophila was caught on June 28 in Mattawa. Flies were soon found in high numbers around the state. Populations increased sharply in August and remained high until early November. Later that month, while the flies were still active, a deep freeze hit the region, shutting down traps. Temperatures plunged in a matter of hours from over 60°F to below zero.
The first catch in central Washington in 2011 was on June 8, but most traps didn’t pick up any spotted wing drosophila until August. Beers said this might be because the pest took a long time to build up again after the freeze. Also, the 2011 growing season was one of the coolest and wettest on record.
Based on last year’s experience, spotted wing drosophila might not be around in high enough numbers to cause a problem until many cherry orchards have been harvested, Beers said, though growers should not count on that yet. After only two seasons of dealing with the pest, it’s impossible to see a trend. Survival of the insect in the 2012 season might be better following a winter with less extreme temperatures early on and snow cover to protect it, she said.
Tim Smith, WSU extension educator, believes that cherries that mature later in the season are likely to be most vulnerable to spotted wing drosophila, and higher elevations are likely to see the greatest pest pressure. “The intensity of the spotted wing drosophila program depends on what time of year your cherries are ripening in relation to the other cherries in the region,” he said during the North Central Washington Stone Fruit Day in Wenatchee.
For example, growers with orchards at the top of Stemilt Hill, near Wenatchee, at an elevation of over 3,000 feet, will need to be more concerned than people lower down.
“In high areas, you’re almost certainly going to have to watch for this every year, because it can and will infest your fruit, especially the last two weeks before harvest,” Smith warned.
As the insect spreads, more is being learned about it. On the East Coast of the United States, it started out in Florida and worked its way north. It has shown up in Michigan and Colorado. It’s also been found in parts of Europe.
A positive outcome of this is that scientists in many different places are researching the pest, Beers said. She is cooperating in a major project involving scientists in Oregon, Washington, and California that received funding of $5.8 million from the federal Specialty Crop Research Initiative.
Beers said scientists are still mulling over the concept of applying postharvest treatments to kill the flies that infest mature cherries left in the orchard. Dimethoate, a pesticide often used after harvest to clean up cherry fruit fly, will also kill spotted wing drosophila, but Beers said there is evidence that as the cherries mature and senesce, other common species of vinegar flies become more competitive than the spotted wing drosophila, so if the spray is applied too late, it won’t impact the target species.
Imidacloprid, which is an effective postharvest treatment for cherry fruit fly, is not as good an alternative for spotted wing drosophila, she said.
Beers and Doug Walsh, WSU entomologist based in Prosser, will be trapping the spotted wing drosophila again during the coming season and have an e-mail alert system to tell growers when and where the fly is caught. To sign up for e-mail alerts or to check for the latest information, go to www.tfrec.wsu.edu/pages/swd.