SWD Persists, but Research Holds Promise
Professor Oscar Liburd, University of Florida, examines blueberries with a loupe for spotted wing drosophila infestation. The male SWD sports the characteristic spot on the wings, but the female, with an ovipositor, is more difficult to identify.
Photo courtesy of Oscar Liburd.
In 2008, the spotted wing drosophila (SWD) invaded California. By 2011, this tiny fruit fly was damaging soft-skinned tree fruit and small fruits in growing areas throughout the country. The pest is not deterred by borders and has been found in Canada and Mexico.
Only 2 to 3 millimeters in length, this pest can reproduce in two to three weeks during the summer growing season. Unlike other fruit flies, Drosophila suzukii has a saw-like ovipositor to lay eggs in ripening fruits. Within two to three days, the fruit wrinkles, bruises, softens, scars and collapses. Mold may grow and juice may seep out. The larvae, which have been feeding inside the fruit for five to seven days, then begin to pupate. The brownish-yellow pupae last four to five days, and then the adult flies emerge, mate, and start the cycle all over again. Ten generations can emerge and flourish if conditions are favorable.
Since SWD enjoys numerous hosts with ripening stages that progress throughout the season, the management challenge is compounded. This pest especially likes blueberries, blackberries, strawberries, raspberries and cherries. Other soft-skinned fruits, such as apricots, peaches and plums, are also vulnerable. It also targets wild plants, including elderberry, ornamental blackberry and raspberry, mulberry and pokeberry.
Growers may not detect the presence of this nuisance until an infestation is rampant. However, entomologists and other horticultural researchers in various regions have joined forces in attacking SWD at multiple levels.
At Rutgers University, Dr. Cesar Rodriguez-Saona is leading research to identify cues that attract SWD to hosts. Aroma, color, shape and other factors may play a part in selection. The ultimate goal of this multistate effort is the development of strategies that will change the pest's behavior.
Except for spinosyns for chemical control, SWD spray requirements could set back decades of integrated pest management (IPM) progress by eliminating beneficial predators and parasites. At the Connecticut Agricultural Experiment Station, Dr. Richard Cowles directs New England-focused research on lure and trap systems. Refining bait formulations is part of the research. Guidelines for growers to enhance their spray efficiency and possible discovery of a Bacillus thuringiensis strain with high toxicity to SWD could potentially reduce pesticide usage.
Researchers in the north-central region are determining how landscape influences the timing and severity of pest infestations, with raspberries as the focus. Also, specialists from the University of Wisconsin, University of Minnesota and Michigan State University (MSU) are evaluating physical exclusion methods, including high tunnels and netting. If effective, these would be less costly than the current chemical controls.
The researchers are measuring captures by crop stage and growing degree-days. The first and peak captures, as well as the first and peak infestation data, compared by crop stage, help pinpoint management techniques.
Spotted wing drosophila eggs.
Photo by Hannah Burrack, North Carolina State University, http://Bugwood.org.
Several regions have been seeking the most effective traps. One of the most recent evaluations, led by Jana C. Lee of the U.S. Department of Agriculture's Agricultural Research Service in Oregon, found that traps with greater entry area caught more flies, and caught them earlier.
In a multistate study to detect the vulnerable stage of ripeness in berries and cherries, more SWD eggs were laid as Brix levels increased; the insect seems to prefer the elevated sweetness. Preference among cultivars was mixed. Interestingly, all the berries attracted more SWD than green grapes, and few SWD developed on green fruit, wine grapes and overripe berries.
In susceptibility research with cranberries, also a multistate effort, SWD readily fed and developed into adults in wounded fruits. However, ripe, unripe and overripe cranberries did not appear to be suitable hosts.
Although not problematic at present, SWD has been confirmed in tomatoes. By monitoring traps, IPM specialist Marion Zuefle of Cornell University found SWD adjacent to tomato field plots and within a high tunnel. The preliminary study showed that SWD infested cracked tomatoes from the field. The high tunnel tomatoes were intact and without damage. However, Zuefle's lab work demonstrated that SWD, when given no other choice, could lay eggs and develop in intact tomato fruit.
A Specialty Crop Research Initiative (SCRI) project based in Oregon, Washington and California has been developing a comprehensive investigation of SWD coordinated with a plethora of experts. Genomic studies have already provided information on genetic mapping and DNA sequencing. DNA testing can have the added advantage of detecting fruit that contains the larvae, particularly important with fruit being shipped.
The project also involves assessing SWD's plant preferences and the susceptibility to larval infestation at different stages of fruit development, determining the propensity for insecticide resistance, and surveying for natural enemies to develop long-term biocontrol.
The project website (http://www.spottedwing.org) provides information on controls, monitoring and more, plus links to helpful data. Photos of SWD and examples that differentiate SWD damage from normal fruit aging can assist growers in identification. The interactive degree-day model gives calculations for egg-laying and adult emergence periods. The website also provides links to other studies and a link to the SpottedWingFlyBase (http://spottedwingflybase.oregonstate.edu).
These and other projects offer hope, but in the meantime, the experts advise timely management. Since local conditions vary, growers should contact their local extension offices for the monitoring systems and controls best suited to their region. Weekly monitoring, even with simple traps using cider vinegar, is recommended to detect SWD presence, not for control. Identification is essential in order to choose the proper controls.
Spotted wing drosophila.
Photo by Hannah Burrack, North Carolina State University, http://Bugwood.org.
Dr. Oscar Liburd of the University of Florida is encouraged by the vigilance and responsiveness of Florida berry growers. Although present management tools are effective, he notes that new management tactics must be developed to assure long-term control and to reduce the negative impacts associated with the use of insecticides. Liburd also points out that wild hosts cannot be ignored. He has observed plots of wild blackberries where almost every single berry was an SWD host.
At MSU, entomologist Dr. Rufus Isaacs emphasizes that more intensive monitoring, timely application and shorter intervals are critical for the insecticidal control tools available in organic production. Cultural controls are also important to reduce SWD populations.
Dr. Grzegorz Krawczyk of Penn State's Fruit Research & Extension Center underscores the need for sanitation in cultural control. He says, "We believe SWD are overwintering here." Krawczyk urges removing all fruit at the season's end. "Don't create inoculum for the next season. The more removed, the [fewer] problems for next year," he says.
Orchardists with existing equipment and control programs may be in a better position for control management. Dr. Peter Shearer of Oregon State University (OSU) explains: "SWD has not been as problematic in Oregon's cherries. Since we have zero tolerance for the western cherry fruit fly, growers have had to control it during the ripening period." Unfortunately, the western cherry fruit fly controls are not as effective against SWD, so growers had to change their management. Shearer also notes that the SWD insecticides are not as IPM-friendly and are more expensive. Pesticide resistance, which has already been reported in a few areas in California, could present problems. However, resistance is being investigated, and new techniques using genetics could provide better solutions.
There are many university websites growers can look to for more information on this pest. The University of California site (http://www.ipm.ucdavis.edu) lists recommendations for various fruits under the "Exotic & Invasive Pests" heading. The University of Wisconsin (http://labs.russell.wisc.edu/swd) has information on management. Cornell's site (http://www.fruit.cornell.edu/spotted wing) coordinates monitoring and reporting in New York. Frequent updates for North Carolina can be found at www.ncsmallfruit http://sipm.blogspot.com. MSU's site (http://www.ipm.msu.edu/invasive_species) offers fact sheets, crop recommendations and details on how to monitor for SWD.
A free webinar by eOrganic on biologically based organic management strategies for SWD is scheduled for February 11, 2014. Visit http://bit.ly/1kWaJWE to learn more and to register. Presenters include Isaacs; Dr. Vaughn Walton, the lead OSU entomologist for the SCRI SWD project; and Dr. Hannah Burrack of North Carolina State University.
The author is a writer/researcher specializing in agriculture.