Washington cherry growers saw higher numbers of spotted wing drosophila in orchards last season. And that’s likely to be the trend in years to come, warns Dr. Elizabeth Beers, entomologist with Washington State University in Wenatchee.
But the good news is that a new trap lure developed by U.S. Department of Agriculture scientists should give growers an earlier warning of when the pest has arrived in their orchards.
Cherries become susceptible to spotted wing drosophila when they turn straw color. Female flies lay eggs in the fruit and larvae that hatch from the eggs feed on the fruit, causing it to soften and decay.
For the past four seasons, Beers and her colleague Dr. Doug Walsh, based in Prosser, Washington, have led a statewide trapping effort to monitor spotted wing drosophila in orchards and vineyards. The results were used to alert growers when the pest showed up in their areas so that if their crops were at a susceptible stage, they could spray.
Typically, pest populations are low through the spring and early summer. In August, about the time that the cherry season is winding down and most of the cherries have been picked and sold, populations increase dramatically.
It’s followed the same pattern in each of the past four years, Beers said. However, there were differences from year to year in how many spotted wing drosophila there were.
Spotted wing drosophila was first found in eastern Washington in late June of 2010 and populations increased sharply in the fall. In 2011, populations were low. A hard freeze in November 2010, had knocked them back to the point where Beers was hopeful that the fly was not going to be an economic pest.
However, the insect came back in greater numbers in 2012, and populations increased again in 2013 following a mild winter. That was the first year that the fly was trapped every month of the year.
“I think 2011 was the anomaly,” Beers reported during the North Central Washington Stone Fruit Day in January. “As this pest becomes established in our area, it’s going to look more like 2013.”
Beers and Walsh will not run their statewide trapping program this year. Beers said it’s no longer necessary to trap in order to detect or survey the distribution of the insect.
“We’re past that phase,” she said. “We know it’s here, everywhere.”
Scientists also know what the general seasonal pattern is, although there will be variations based on the weather. The main reason for trapping now is to find out when it reaches an action threshold for a particular orchardist.
Since the pest appeared in Washington, Beers has been testing various types of traps for monitoring spotted wing drosophila. In general, red traps are more attractive to the fly than yellow, white, black, or gray. She also found that traps worked better when they had more bait and bigger ventilation holes.
In April, the company Trécé introduced a new trap lure that uses chemicals identified by Drs. Peter Landolt and Dong Cha with the USDA in Yakima, Washington.
In trials last year, Beers found the lure more effective than the apple cider vinegar, yeast, or wine and vinegar concoctions tried so far.
Traps with the new lure caught flies one to four weeks earlier than traps baited with apple cider vinegar.
“That’s very exciting,” Beers said. “That, we think, could be the key to using traps as an action threshold. We hope if we can build a better fly trap, we’ll have the confidence to say, ‘Yes, I need to spray’ or ‘No, I don’t need to spray.’”
Beers used the new lure in a trap with water as the drowning solution. She added a little borax to the water to prevent molds from forming. The traps were effective for four months, whereas traps with food-based baits (such as vinegar, wine, yeast, or molasses) had to be changed very frequently, and she was never really sure how long they were attractive to the flies. Beers found that yeast-based traps didn’t work as well in Washington as they appeared to do in eastern growing regions. It’s possible that differences in ambient temperature and humidity affected the growth of the yeast, she said.
Another advantage of the new lure, particularly compared to baits containing yeast, is that the drowning solution is not cloudy, making it easier to identify trapped flies.
“Instead of going through this murk, you have a relatively clear liquid to look at them in,” she said. •