Researcher baffled by pest numbers
Trapped spotted wing drosophila 40 times greater than norm
By DAN WHEAT
WENATCHEE, Wash. -- It was the first week of November, about four months past cherry harvest -- not exactly when Washington State University entomologist Elizabeth Beers expected to be inundated with spotted winged drosophila.
But a fruit company fieldman had been bringing in increasingly higher counts of the new Asian pest each week from traps in a cherry orchard near Royal City.
In that first week of November the count peaked at 2,200 flies in one trap alone and slightly fewer in three others, totaling 6,539 for the four traps. That was 40 times greater than the norm of all traps in Eastern Washington in October, which had been a heavy month.
"What in the world is going on?" Beers asked herself. So she went to the orchard to have a look.
There was no known host in sight for the pest to have fed on since the cherry block had been harvested in early July. It was a well-picked, high-value commercial orchard -- "cleaner than most," Beers said -- with virtually no residue from cherries rotting on the ground. There were no wild blackberries or raspberries, known hosts.
The orchard was isolated, surrounded by row crops, sage brush and basalt, none of which is believed to be spotted winged drosophila food. The nearest orchard was about two miles away.
Beers was mystified.
Her best guess was that the infestation came from other orchards where drosophila feeds on rotting apples and pears on orchard floors.
The high count seemed alarming but fit a pattern of increased counts in fall apple and pear residue, she said.
The spotted winged drosophila, first found in Eastern Washington on June 28, probably wintered over and was undetected the prior season, Beers said. It probably will winter over again in very low populations in warm spots like heated garages or shops.
Wintering over gives it a head start over spreading in from western Washington and Oregon next summer, but populations will be so small that none may be detected until next June, she said. It didn't really take off until August this past year.
The main concern is keeping it controlled.
Spotted winged drosophila is threatening the state's Integrated Pest Management program because of its high reproduction rate, multiple hosts and resistance to "softer," or less toxic, pesticides, Beers said.
Beers, at the WSU Tree Fruit Research and Extension Center in Wenatchee, and Doug Walsh, entomologist at the WSU Irrigated Agriculture Research and Extension Center in Prosser, are testing pesticides on the insects in their labs this winter and will increase trapping next summer.
Cherries is the main crop of concern. It has also attacked peaches, plums, apricots, nectarines and pluots. Apples, pears and winegrapes are all more resilient because of tougher skin.