WENATCHEE, Wash. — Spotted wing drosophila may have made a huge increase in Central Washington in 2013, but it is difficult to know how much because trapping changed.
The Asian fruit fly was first detected in the U.S. in California strawberries in 2008. It moved into berries in the western parts of Oregon and Washington and later into Central Washington cherries in 2010.
It has been kept under control with early detection and pesticides. But it was a bigger challenge for cherry growers near Washington’s Tri-Cities and berry growers in the western sides of Oregon and Washington in 2013 because of increased numbers and
multiple rains and winds making it hard to spray pesticides and keep them on,
, said Elizabeth Beers, entomologist at the Washington State University Tree Fruit Research and Extension Center in Wenatchee.
She said it was the toughest year of four in battling the pest in Central Washington. More were caught in peaches, nectarines and apricots than cherries, she said, but cherries is the larger crop.
Trap counts exploded from 61,722 flies caught in 2012 to 791,408 in 2013, Beers said. The number was skewed, she said, because of a change in traps and doubling of the apple cider vinegar bait in half of them. How much of the increase is attributable to increased population versus better trapping is unknown, she said. She believes both were factors. The number of traps was about the same at 359, she said.
The count was 60,381 in 2010, the first year of trapping. It dropped to 13,807 in 2011. Beers believes that most likely was due to a pre-Thanksgiving Day freeze in 2010.
Beers and her colleague, Doug Walsh, WSU entomologist in Prosser, used the Haviland trap in 2012 only. They had to devise shields for the tops to keep rain and irrigation water out, but wind occasionally caught the shields and dumped trap contents, Beers said. Bait also seemed to evaporate and they didn’t think they were getting accurate counts.
Beers and her team designed and used a new trap, dubbed the PBJ trap, in 2013. It was larger, had more evaporative area and a better top.
But aside from better trapping, spotted wing drosophila did increase in 2013 because of a relatively mild winter, she said.
“We’re seeing the effects of establishment,” she said, “but I don’t expect the trajectory to continue ad infinitum. I think it will fluctuate in a somewhat predictable manner due to weather.”
Freezing temperatures of early December hopefully will knock the population back in 2014, she said. “It may only take one big freeze. We had three to four days. I’m guessing that’s enough for a good kill,” she said.
There’s still substantial population in the Orondo area north of Wenatchee but most of the numbers were near the Tri-Cities where growers battled wind and rain in trying apply pesticides, Beers said.
Delegate and Entrust have the longest residual effect, diazinon and Sevin are intermediate and malathion has the shortest effect so can be used closest to harvest, she said. Rotation of pesticides is best to decrease pest resistance, she said.
“If weather cooperates, our control programs are adequate to control this pest,” she said.
Cherries were damaged by rain and it’s difficult to know how much was lost to spotted wing drosophila, she said.
There’s greater market tolerance for spotted wing drosophila than cherry fruit fly but spotted wing is much more prolific with up to 13 generations in a season compared to one for cherry fruit fly, she said.