SWD challenges growers

Unlike the common drosophila flies, spotted wing drosophila will attack cherries before they are ripe.

PHOTO BY PETER SHEARER, OSU

After feeling little pressure from the spotted wing drosophila in 2011, Washington State cherry growers battled the pest through the 2012 season. And that’s likely to be the norm in the future, Dr. Elizabeth Beers, Washington State ­University entomologist, warned during horticultural meetings this winter.

The first fly of the 2012 season was caught on April 30 in the Tri-Cities area, and many flies were trapped in May and June before peak harvest. Growers reported fruit ­damage, and 14 flies were caught in ­packing houses.

The spotted wing drosophila (Drosophila suzukii) is a tiny fruit-feeding insect that is related to D. melanogaster, known as the common fruit or vinegar fly. It appeared in the Pacific Northwest in the fall of 2009. Unlike the ­common fruit fly, which attacks only mature fruit, the spotted wing drosophila attacks fruit while it’s still ­growing on the tree.

In 2010, significant numbers of flies weren’t trapped until July and August in Washington, although Beers said that might be partly because there were not enough traps out that first season.

Since then, Washington has been trapping from the Canadian border to the Oregon border. In 2011, relatively few regions caught their first fly before the cherries were off the tree. Beers said this might be attributable to a severe freeze around Thanksgiving of 2010 when temperatures plummeted from over 60°F to -5°F in a matter of hours. There was not much pressure anywhere in the state during the 2011 season.

After a normal winter in 2011–2012, the pest came right back in 2012 and was active well before harvest, putting the crops at risk. Higher populations were seen south of Interstate 90 than in the northern part of the state, which was the opposite of in 2010.

Watershed year

Dr. Peter Shearer, entomologist with Oregon State University, said 2012 was the Mid-Columbia region’s watershed year. Many orchards had fruit damage after not experiencing much of a problem in 2011. An intensive trapping ­program in the area around Hood River, Mosier, and The Dalles in Oregon showed that populations began to build up in October 2011, and high numbers were still being caught in December. Steve Castagnoli, OSU extension educator, who runs the trapping program, now adds salt to the vinegar traps to prevent them freezing. Flies continued to be trapped through the winter and spring, then populations increased dramatically in July 2012, as the district’s cherries started to ripen.

When Shearer visited infested orchards, he learned that they were not using spray programs recommended for spotted wing drosophila. They were using neonicotinoid insecticides like Provado (imidacloprid), which do not control that insect.

“Some of the responses we got were, ‘It worked the year before,’” he said, “but anything would have worked the year before because there weren’t any flies in the orchard. The industry now recognizes they have to use effective materials, and they have to shorten up their spray intervals.”

Shearer has been testing the ­effectiveness of various materials. Since he doesn’t have field populations to work with, he sprays trees with the insecticides, then brings foliage and fruit back to the lab and exposes them to flies.

In tests on leaves, the pyrethroids Warrior (lambda-cyhalothrin) and Danitol (fenpropathrin) were extremely effective and long lasting. Delegate (spinetoram) and Entrust (spinosad) were also effective but had less residual effect. Malathion was effective but only for a short time after application. Sevin (carbaryl) was intermediate.

However, the results were quite different when he exposed flies to treated fruit. Delegate and Entrust were very effective—more so than Warrior and Danitol. Surprisingly, Sevin was quite effective.

Shearer said the surfaces of leaves and fruit hold the products differently because of the waxy layers. Since the flies don’t spend all their time just on leaves or just on fruit, a grower has offered to let him do field trials next year to find out how the products affect the amount of fruit ­damage in the field.

Shearer said control failures in orchards were seen when malathion was used repeatedly at seven- to ten-day intervals. In Oregon, growers are now discouraged from using it as a back-to-back stand-alone spray. They’re advised to use it just once followed by other insecticides.

Fully susceptible

Beers said tests on Bing and Sweetheart cherries show that they are fully susceptible to the pest 21 days before harvest, and growers are urged to begin protecting their crops as soon as the first fly is caught in the region.

“That’s a very conservative approach—we understand that—but until we find out more about the insect and how effective the traps are, we feel this is the best way to go to avoid infestation,” she said.

Spotted wing drosophila females can lay eggs immediately when they arrive in the orchard, whereas cherry fruit fly females spend about a week foraging on the tree before laying eggs, giving the grower a good chance of killing them before they reproduce.

When spotted wing drosophila first moved into Washington, it was clear that some control failures were due to growers using the cherry fruit fly attract-and-kill product GF120 and nothing else, Beers said. While the pesticide in the product, Entrust, has done a good job of controlling spotted wing drosophila, GF120 allowed a fair amount of damage.

“We warned growers, ‘Don’t rely on this technology for spotted wing drosophila,” and that’s still our recommendation,” she said. “But I heard a lot of grumbling about how great the technology was, how useful it was, and how easy and fast it was to apply, and thought it was worth a second look, to go back and see if we could retool it a little bit.”

GF120 consists of a bait combined with Entrust. It is typically applied to trees from a four-wheel vehicle in very small amounts. Cherry fruit fly that are attracted to the bait and come into contact with the pesticide, die.

As a possibility for spotted wing drosophila, Beers has been testing Monterey Ag Bait (which she found to be an effective bait in traps) combined with the same toxicant, Entrust. She modified the sprayer to have six nozzles instead of two and aimed them at the upper, middle, and lower parts of the canopy, in an effort to increase the number of droplets in the orchard. She used six gallons per acre of the combined product, whereas the maximum allowed rate for GF120 is less than a gallon.

Like Shearer, she took treated leaves and fruit back to the laboratory where flies were exposed to them. The new bait combination gave similar control to Entrust, which Beers said was encouraging. While it might never become a stand-alone treatment, it could be a supplement, she said.

Residual effect

The residual effect of pesticides is a key issue because growers have to balance the need to control the pest while meeting maximum residue levels for exported cherries. Materials with the shorter preharvest intervals, such as Sevin and Malathion, had the shortest effective periods. Warrior and the organophosphate Diazinon have the longest residual effects but also longer preharvest intervals.

Beers also urged growers to think about resistance management and avoid repeated use of the same products.

“The first two years, we were in emergency mode, and the most important thing to do was to control this pest,” she said. “We’ve had a couple of years now, and it’s become clear that some materials are popular because of their low MRLs, and it’s going to be easy to overuse them. It’s time to start thinking about resistance management and developing programs that will help prevent it.”