Reducing Drosophila suzukii Management Challenges: An Alternative to Insecticide Cover Sprays
Since the discovery of spotted wing drosophila (SWD), Drosophila suzukii, in the U.S. in 2008, growers minimize crop losses by repeated cover sprays with broad spectrum insecticides, which significantly increased productions costs, disrupted existing IPM programs and potentially caused inadvertent environmental and non-target impacts. We implemented a reduced pesticide application strategy in a mid-late season blueberry field in the Willamette Valley of Oregon. Crop borders were treated with insecticides during the harvest season and showed similar adult and larval SWD captures in 2012 as conventional cover sprays. In 2013, greater fly pressure warranted integration of border sprays with cover spray treatments to prevent infestations. These findings showed that border sprays may be used in SWD management programs during low SWD pressure scenarios (i.e., early season cultivars, low populations) and possibly integrated with cover sprays in higher SWD pressure scenarios (i.e., late season cultivars, high populations). In both situations, extensive monitoring (i.e., sampling adults and larvae twice/week throughout the field) is vital for successful implementation. Further field studies testing these hypotheses are needed to broaden the scope of inference.
The objectives are to determine if border sprays for spotted wing Drosophila, Drosophila suzukii, (SWD) are
(I) as efficacious as conventional covers sprays,
(II) result in less fruit knockdown, and
(III) conserve natural enemies and result in fewer secondary pests.
Preliminary data suggested that SWD overwinter in surrounding field margins and move into crops during ripening. We wanted to observe if SWD invasion was prevented or delayed by spraying only the border of the crop. Field trials were performed in the 2012 and 2013 harvest seasons. We selected a 24 ha ‘Liberty’ blueberry (mid-late season cultivar) site in Linn County, Oregon. The experimental design was a randomized complete block design (RCBD) with a border-spray and a cover-spray (standard “control”) treatment plot per block and replicated three times. Plots ranged from two to five ha and each contained eight SWD traps. Border sprays were made with a cannon sprayer (Photo 1) that traveled around the perimeter of border spray plots and sprayed insecticide approximately 15 m into the crop, leaving the center portion of the field untreated. Cover-sprays were made with a trellis sprayer in 2012 and a raised “over-the-row” vertical boom sprayer with narrow tires in 2013 (Photo 2). Insecticide applications ranged from four to eight, depending on year and treatment, with 608 liters per ha. ‘Liberty’ blueberries, five to six years old, were not trellised in 2012 but trellised in 2013. Fruit knockdown from sprayers was quantified after each application. SWD were monitored twice per week with clear cups containing ACV (5% acetic acid) for adults and salt extraction of 50 fruit near each trap for larvae (Photo 3).
Difference in mean adults (blue) and larvae (red) during 2012 and 2013 harvest seasons are shown in Figure 1. Values above zero indicate higher abundance of adults/larvae in border-spray plots and values below zero indicate higher abundance in cover-spray plots. Approximate insecticide applications are noted above the dates. No statistically significant differences were found between treatments in 2012 and 2013 adults (very low larval counts prevented statistical analysis). Border-spray plots had similar adult captures and no yield loss in 2012, a low SWD pressure year. Economic benefits from border spraying were estimated at $320/ha at this site with 70% reduction in spray area, 87% less fruit knockdown and a 12-fold decrease in spray application time. In 2013, SWD pressure was significantly higher, and two of three border-spray plots were cover-sprayed two to three times as a result of high adult captures or presence of larvae. These supplemental cover-sprays were made in four to five day intervals and successfully averted further damage or yield loss. The center of the single remaining and unaltered border-spray plot suffered complete yield loss on the last day of harvest, as it was not sent to the processing facility due to larval infestation. Similar fruit knockdown between treatments was attributed to the narrower sprayer and trellised rows in 2013. In conclusion, border-sprays may find utility in low SWD pressure situations (i.e., early-season cultivars, low pressure years). Conversely, SWD management during high pressure situations (i.e., late-season cultivars, high pressure years) may integrate border-sprays with cover-sprays to avert yield loss when adult captures become high or larvae are detected. In both situations, extensive monitoring (i.e., sampling adults and larvae twice/week throughout the field) is vital for successful implementation. Non-target effects are still being assessed.
- Photo 3. Student worker, Evan Dishion, collecting berries and servicing SWD traps in 2013 harvest season
- Difference in mean adults and larvae in 2012 and 2013
- Photo 1. Blueberry crop border sprayed with cannon sprayer in preliminary summer 2011 trial
- Photo 2. Cover-spraying blueberries during 2013 harvest season
Impacts and Contributions/Outcomes
To date, this is the first study in blueberries to show possible utility of border sprays in blueberries for SWD management. However, further field studies testing border sprays are needed to broaden the scope of inference. Regardless, some growers in the Willamette Valley are purchasing cannon sprayers to integrate border sprays into their SWD management programs.
This research was presented at the Entomology Society of America (ESA) meeting in Reno NV in 2011 and Knoxville TN in 2012, the Pacific Branch ESA meeting in Portland OR in 2012, the Pacific Northwest Insect Management Conference in Portland OR in 2013, and the North Willamette Horticulture Society meeting in Canby OR in 2013 and 2014 and the Oregon Horticulture Society meeting in Portland OR in 2014.
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