Fruit Pest Research Extends into Winter
Increased populations of fruit flies causes concern
The discovery three years ago of spotted wing drosophila in a Moscow homeowner’s back yard cherry trees led entomologists from the University of Idaho College of Agricultural and Life Sciences to sample far and wide for the fruit-damaging vinegar flies.
They found substantial populations in northern and southwestern Idaho. The discoveries led to widespread concern because the tiny red-eyed flies are capable of damaging a diversity of commercially-grown introduced and native fruits.
Spotted wing drosophila, typically called fruit flies, were present but in low numbers throughout much of the 2015 summer, said Jim Barbour, an entomologist and Parma Research and Extension Center superintendent.
In northern Idaho, populations grew quickly as summer advanced and remained high even after most common fruits had come and gone, said Moscow-based CALS entomologist Stephen Cook.
“They were pretty different until late in the season at least. Ours have gone way up,” Barbour said.
“They were different early on. But he may be collecting more than I am now,” Cook said.
As October ended and before the first frost, fruit fly numbers grew to nearly biblical proportions, with 50,000 to 60,000 flies in a trap, some 4,000 to 6,000 of them spotted wing drosophila.
Even after the frost, both entomologists planned to maintain a few traps to get a sense of how many flies might overwinter, and where.
Cook checked two traps on the Moscow campus in December. To his surprise he found each trap held a few: four in a trap near Sweet Avenue and nine from a trap near the livestock pavilion.
Barbour checked two traps near the Parma center in December and found small numbers in each despite low temperatures that approached zero. When the weather turned cold, Barbour said, “We thought they would just disappear.”
The two traps were active from Nov. 16 to Dec. 9. One was in a research orchard and the other in a commercial orchard. One held 17 flies and the other 12.
After the initial discovery in 2012 in Moscow, scouting showed the fly was in southern Idaho, too, but the population was fairly small.
In more extensive trapping in 2013, fly numbers boomed in southern Idaho, Barbour said. A similar effort in 2014 showed few flies in southern Idaho, however. In 2015, the numbers were small throughout much of the growing season, then grew rapidly toward the end.
The question is where are those initial flies coming from? Are they coming in from somewhere else and building slowly or, or are they always here but in small numbers because they’re overwintering poorly?
With so many flies at the end of fall, the next question is whether the flies are overwintering in wild areas or commercial orchards.
Cook wants to know more about wild sites to know whether they serve as refuges for flies, allowing them to escape spraying. Even if pesticides clear orchards, flies from wild areas may move back in and repopulate quickly.
Or it may be that the flies are infesting wild fruits from Himalayan blackberries at low elevations to huckleberries at higher elevations and lots of other possibilities in between, Cook said. That would be important because those fruits are often highly valued and culturally important for people and ecologically important for wildlife.
Both researchers found the first spotted wing drosophila about the end of May in Moscow, and in orchards down south. Numbers in both areas stayed small for a while.
Both are working with Utah State University entomologists to learn more about the flies.
“When I got to 100 flies in one trap it was like a jackpot,” Cook said.
Numbers grew more slowly in southwestern Idaho in the summer, probably because the typical hot, dry climate is inhospitable to the flies. They prosper in moister, milder conditions, Barbour said.
The flies’ impact on commercial fruit operations in Idaho appears minimal so far, but those who grow cherries, blackberries and raspberries in particular may want to pay close attention to future crops.
The thing that makes the spotted wing drosophila worthy of concern is that it is able to lay eggs in young and tough-skinned fruits. And it can infest a large array of fruits, even vegetables.
The fly’s discovery in Idaho didn’t occur long after it was first reported in the U.S. in California in spring 2009. The fly is native to Korea and Japan.
The next step for Barbour and Cook will be figuring out more about why fly populations were so high late in the season and what happens through the winter. Beyond that, they will assess how the flies are affecting native fruits.
“Here we are, it’s going to be the middle of November and we’re getting thousands of flies in traps where I don’t see any fruit near them anymore,” Cook said in November.
“If they’re active, there has to be a food source somewhere,” Barbour said.