With one full season’s experience under their belt, western state entomologists and Extension specialists have refined trapping and treatment recommendations for the invasive spotted wing drosophila for the upcoming cherry season.
But they’re quick to admit there’s still a lot about this newcomer that they just don’t know, such as treatment thresholds, population dynamics and alternate hosts.
To help answer some of those questions, the U.S. Department of Agriculture earlier this year awarded a $5.8 million, 4½-year multistate grant to fund development of an integrated pest management program for the pest. Oregon State University is the lead institution. Also participating are the University of California, Washington State University and the Agricultural Research Service.
Much of the early research into SWD was funded by commodity groups, such as the California Cherry Advisory Board and the Washington Tree Fruit Research Commission.
Different trap and bait preferences
One of the questions researchers say they hope to answer in the near-term is the best trap design and bait. Most traps consist of a container filled with a liquid bait attractant and with openings to allow the fly access to the bait.
“There are clearly as many different designs of traps as people looking at this in the research community,” says Rufus Isaacs, an entomology professor and Extension specialist at Michigan State University in East Lansing. He is heading up the state’s SWD response team funded by a Michigan Specialty Crop Block Grant and the university’s Project GREEEN.
Traps picked up SWD for the first time in Michigan last season after most of fruit had already been harvested.
This season, Isaacs says he will be joining a West Coast group comparing five or six different trap designs and baits.
“So I think by the end of this summer we will have a much better idea which is the best trap for specific regions of the country,” he says.
California researchers, for example, favor a 1-quart white yogurt container because of its sturdiness, inexpensive cost and availability, says Bob Van Steenwyk, a University of California Cooperative Extension pest management specialist in Berkeley. The white background color also makes it easier to see if you’ve caught flies.
Last year, UC recommended drilling 15 to 20 3/16-inch holes around the sides to provide fly access to the bait.
This year, Van Steenwyk says they’re recommending covering the top with 1/8-inch hardware cloth or screening instead of drilling the holes. It appears the open-top design catches more flies, he says.
But it also opens up the trap to flooding by rain. To protect the traps, hang them below a rain shield, such as the top cover of a commercial codling moth trap, to shed the water.
For cherry growers, Van Steenwyk recommends filling the containers with about 1 inch, or 4 ounces, of a mixture of unflavored apple cider vinegar to which 1 tablespoon per gallon of unscented dish detergent has been added.
Mark Borda, a UC Cooperative Extension farm adviser in Santa Cruz, recommends a bait made from fermented mixture of baker’s yeast and sugar to berry growers along California’s Central Coast.
“We kind of settled on the apple cider vinegar and the yogurt container because everybody can find apple cider vinegar and it’s not expensive,” says Joe Grant, a UC Cooperative Extension farm adviser in San Joaquin County. “Putting yeast in it does improve the attractiveness, but it doesn’t make enough of a difference to justify the added hassle and expense.”
Isaacs, on the other hand, is using a clear 32-ounce deli container into which 10 3/16- inch holes have been drilled near the top. He favors apple cider vinegar also because of its availability and ease of use.
“Yeast has shown in some West Coast trials to catch flies earlier than the apple cider vinegar,” Isaacs says. “But other studies in the East last season found that the type of bait had no significant effect.”
He will be working with MSU colleagues to use the same design across a monitoring network in Michigan, which will consist of traps in blueberries, blackberries, strawberries, cherries and a few tomato fields.
Results will be posted weekly on a central website, http://www.ipm.msu.edu/SWD. htm, so growers can monitor the situation.
Washington State University researchers also will monitor a trap network in its fruit production areas this season. The first trap catch in each region will be posted on its website, http://extension.wsu.edu/swd/.
Growers also can sign up to receive the alerts.
As the website points out, “Until we have a better understanding of what trap catch means, we are NOT recommending that you use the traps as an indicator of the need to spray on a block-by-block basis.
Once trap catch has occurred in a region, it is likely present throughout that region.”
Tweaking the recommendations
The University of California published in January what it is referring to as provisional management guidelines for this cherry season.
They can be viewed at http://bit.ly/i8hjMW.
“The treatments have been tweaked based on what we found works better and to take stock of some MRL restrictions on some materials,” Grant says of the UC guidelines.
For example, UC guidelines now recommend beginning treatment when the earliest varieties in the orchard—which may be pollinizers—are straw colored rather than green.
“It appears that the fruit really doesn’t become attractive to egg laying until it’s at least yellow in color,” Grant says.
That recommendation was based on results of earlier laboratory tests conducted by Oregon State University. Van Steenwyk says researchers will conduct field trials with caged flies this season to see if that is indeed true.
Preliminary research also has shown varietal differences in susceptibility, with fruit that matures early and is soft at maturity being more susceptible, Van Steenwyk says. Therefore, Early Burlat is more susceptible than Black Tartarian, which is more susceptible than Bing.
Once the fruit enters the susceptible stage and flies are present, growers should begin treatments and continue them on seven- to 14-day intervals, depending on label recommendations, through harvest.
The UC guidelines also warn growers to play close attention to treatment re-entry and pre-harvest intervals as harvest nears.
In eastern Washington, SWD control can be integrated into a cherry fruit fly program because many of the materials that control SWD also control the cherry fruit fly. But the reverse isn’t always true, according to Extension recommendations.
Taking a hard look at control
Joe Valente, who grows cherries near Lodi, Calif., says he hasn’t had a real problem with SWD.
Most of his crop had already been picked when the fly first started causing significant damage to California’s Central Valley cherry crop in Valley in 2009. Because of the damage that some growers reported in 2009, Valente says he adopted an aggressive spray program in 2010.
“Because there were so many unknowns out there, it was probably more than we had to,” he concedes. “The program we did last year wasn’t, and I hate the word sustainable, but it wasn’t sustainable. This year, we’re looking more at the products we’re using.”
Valente hung vinegar-filled traps last year, beginning when the fruit was straw colored, and only caught one fly. But he plans to continue trapping this year.
This season, Valente says he’s already met with some of his packers who gave him a list of preferred crop protection products. At the time of treatment, he has no idea whether his fruit will be sold domestically or exported.
Different countries have different MRLs that packers have to take that into account.