Wet springs, warm-but-not-too-hot summers and an abundance of wild blackberries have led to an infestation of the spotted wing drosophila locally.
Damage caused by the fly, which can ruin berry and cherry crops, "has gotten worse every year and certainly is not going to get better," said Rick Hilton, an entomologist with the Oregon State University Extension Service in Jackson County.
The fly came from Asia four years ago and has infested the entire United States, preferring to lay its larvae on cherries, blueberries, raspberries and blackberries, making them unfit to eat or market, Hilton said.
"They're still increasing in numbers and they have no problem living in the Rogue Valley," says Hilton, noting the wet springs of 2010 and 2011 aided in the spread. "A lot of small growers are having to spray (insecticides) a lot more than they used to."
Vaughn Walton, an entomologist with the OSU Extension Service in Corvallis, said it appears this season will be similar to, or worse than 2012, which was the worst on record.
"Winter and spring temperatures in the Pacific Northwest have been warmer than last year, and heat equals larger populations of spotted wing drosophila," Walton said.
The valley's climate suits the flies when the winters are not too cold, springs are wet and summers aren't too hot for too long, said professor Peter Shearer, an OSU entomologist.
He said the summer's high temperatures slow the insects' reproductive cycles, but "when it cools, they show." Freezing in the winter can kill a lot of the insects, but Oregon hasn't had much of that the past two winters, he said.
Bending over his microscope at the extension lab, Hilton identified many spotted wing drosophila, which are caught in "vinegar traps" — old yogurt containers with holes in the side.
Hilton teaches small-farm operators how to do "mass trapping" and spraying to keep the insect at bay.
Losses in this region have been minimal, he said, because the creatures don't seem attracted to pears, grapes or apples, which are more plentiful here than cherries and berries. Peaches and plums also are vulnerable to the bug.
In his classes for fruit farmers, Hilton teaches that vinegar traps the flies because it smells like rotting and fermenting fruit. Holes should be three-sixteenths of an inch in diameter, just big enough to let in flies and not allow other pests to enter, he said, looking at a trap with hundreds of fruit flies lying dead at the bottom in a pool of vinegar.
The spotted wing drosophila, often commonly called a "fruit fly," is a very small, mostly brown fly, almost gnat-sized. Its clear wings each have spots near their tips, giving it its name.
To limit damage from the fly, growers are encouraged to pick their fruit before it gets too ripe — and to start spraying cherries as soon as they get straw-colored.
The Extension Service recommends sprays of pyrethrin, organophosphates and spinosyn.
"If growers put on the proper control measures, they can control it," said Shearer. "These (chemicals) are about the only way they can."
A group of OSU entomologists plans a research trip to South Korea this fall to capture a particular parasitic wasp that kills the fly by laying eggs in its pupae, said Shearer. They must quarantine the wasps to make sure they don't bring other harmful organisms into this country, he added, or have a negative effect on the environment.
The fly was transported from east Asia in 2008 and in one year spread through Oregon into Canada and then all the way to the East Coast, said Shearer. It is well-habituated in 13 Oregon counties where fruit-growing is common.
The fly has attacked grapes in the East, Shearer said, likely because they get more rain and scientists theorize that makes grape skins thinner and more prone to splitting open.
Organic fruit growers, who do not apply chemical pesticides, have fewer defenses against the fly, but, noted Shearer, are finding some success with Entrust from Dow AgroSciences. Entrust uses naturally occurring soil bacteria to combat the bug.
"They're attracted to red and really home in on cherries," said Hilton. "Then you can't market them ... some growers have been able to cull out infested fruit after harvesting, though."
An OSU statement noted upstate growers spent $198 million combatting the pest and sprayed up to four times more often to control it. The fly could cost Oregon's small fruit and stone fruit industry $31 million a year, according to a report by the Giannini Foundation of Agricultural Economics at the University of California.
More information on the fly is on OSU's website at www.spottedwing.org.
John Darling is a freelance writer living in Ashland. Email him at firstname.lastname@example.org.