Reducing Drosophila suzukii Management Challenges: An Alternative to Insecticide Cover Sprays

Final Report for GW13-014

Project Type: Graduate Student
Funds awarded in 2013: $24,750.00
Projected End Date: 12/31/2014
Grant Recipient: Oregon State University
Region: Western
State: Oregon
Graduate Student:
Principal Investigator:
Dr. Wei Yang
North Willamette Res & Ext Center
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Project Information


Since the arrival of Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), field applications of broad-spectrum insecticides have significantly increased to protect susceptible fruit from infestation in berry crop production. These cover sprays have increased production costs, disrupted existing IPM programs, and potentially caused inadvertent environmental and non-target impacts. Field studies were conducted from 2012–2013 to determine whether border treated reduced spray programs could manage D. suzukii, as well as cover sprays, and have less of an impact on non-target arthropods. Three blueberry plots of a mid-late season variety were border sprayed and captures of D. suzukii adults and larvae were compared to conventional cover spray programs. Non-target arthropods were evaluated seven days post-harvest.

No differences in mean adult numbers and larvae of D. suzukii were detected between treatments. Border sprays had significantly more Stethorus spp. No difference in fruit knockdown by border or cover spray was observed.

This reduced pesticide strategy is an additional tool to consider in D. suzukii IPM programs that reduce the amount of spray area, application time, and input costs while conserving natural enemies.


Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is an invasive pest of small and stone fruits in the Americas and Europe (Walsh et al., 2011; Cini et al., 2012; Deprá et al., 2014). Female flies cause direct damage by ovipositing into susceptible ripe fruit (Lee et al., 2011a). Eggs develop into larvae that feed on fruit flesh, rendering fruit unmarketable. To prevent fruit loss and to meet zero to low tolerance infestation levels set by packing plants, growers currently apply broad-spectrum insecticides multiple times during the harvest season (Bruck et al., 2011). They are faced with several challenges, such as knocking mature fruits off of plants, managing insecticide pre-harvest and restricted entry intervals (PHI and REI), impacting natural enemies (Roubos et al., 2014a), and risking possible secondary pest outbreaks, all of which increase production costs. A reduced insecticide spray strategy, such as border sprays, is a possible tool to curtail these challenges (VanEe et al., 2000; Roubos et al., 2014b).

In border sprays (DeBach & Bartlett, 1951), pesticide is applied to crop plants in the field border at the same rate and volume as cover sprays, while leaving the center of the field untreated. Border sprays are typically effective against pests migrating into the field from field margins (Lafleur & Hill, 1987; Ferguson et al., 2000) and against ‘edge oriented’ colonizers (Reardon & Spurgeon, 2003). Border sprays have been used to manage apple maggot, codling moth (Trimble & Solymar, 1997; Trimble & Vickers, 2000), and plum curculio (Chouinard et al., 1992; Vincent et al., 1997) in apple orchards; alfalfa weevil in alfalfa (Roberts et al., 1987); and strawberry bud weevil in strawberries (Kovach et al., 1999). Systemic insecticides have been applied to potato borders for Colorado potato beetle (Blom et al., 2002) and an attract and kill applied to cucumber borders for melon fly management (Prokopy et al., 2003). Possible advantages of border sprays include reduced fruit knockdown within the field, reduction of insecticide inputs, fewer environmental impacts, and conservation of natural enemies (Trimble & Solymar, 1997). A disadvantage includes a greater risk of leaving the center of a treatment plot vulnerable to pest attack.

We hypothesized that reduced spray strategies of border sprays would be as effective as cover applications at managing the highly mobile D. suzukii (Mitsui et al., 2010) and would conserve natural enemies. Mobility of D. suzukii was tested by a preliminary mark-release-recapture study using fluorescent dusts that showed D. suzukii moved approximately 67–87 m in 36 h (J.C. Lee, unpublished). Field margins with non-crop hosts such as ‘Himalaya’ blackberry (Rubus armeniacus F.) may also provide refuge for overwintering adults and a source for invasion of the cultivated raspberry fields when ripe (Klick et al., 2014).

Project Objectives:

The objectives were 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.


Click linked name(s) to expand/collapse or show everyone's info
  • Jimmy Klick
  • Dr. Wei Yang


Materials and methods:

We tested border sprays in a less preferred crop, highbush blueberry (Bellamy et al., 2013).

Experimental site and design. A ~26 ha ‘Liberty’ blueberry (Vaccinium corymbosum L.) site in Albany, Oregon, managed using conventional practices, was selected during 2012 and 2013. The site was arranged in a randomized complete block design with three replications in the harvest seasons (seven to nine weeks). Each block had a randomly assigned border spray and cover spray plot (2.8 – 5.3 ha). Blocks 1 and 2 were drip irrigated and block 3 was overhead irrigated. In 2013, weed mat and trellis were installed in blocks 1 and 2 by the grower. Eight clear-cup monitoring traps of adult D. suzukii (Lee et al., 2012) were used per plot, with four placed in the crop border (~5 m into the field on each side of a plot) and four in the crop interior (40–60 m into the field from the border trap).

Monitoring D. suzukii, non-target arthropods, and fruit knockdown. Adult and larval D. suzukii were monitored by trapping and larval extraction methods, respectively, twice per week, as previously described. Non-target arthropods were collected as previously described (yellow sticky cards, leaf collections, sweep net, and vacuum). Beat sheet sampling was not used out of concern for excessive fruit knockdown. Plants near the east and west adult D. suzukii monitoring traps, representing the crop border and interior of each plot, were selected to quantify fruit knockdown from each spray strategy. Within 24 hours prior to application (border and cover sprays), fruit that dropped naturally or from hand-harvest was cleared from the area underneath the selected plant’s canopy, and within 24 hours after spray applications, fruit knocked down by sprayers was collected, weighed (g), and recorded.

Insecticide applications. Growers or crop consultants made all pesticide decisions and applications at the blueberry sites based on biweekly reports of D. suzukii adult and larval counts in the field. Table 1 shows insecticides, rates, cultivars, and mean number of cover and border spray applications per treatment plot at each site. Mean number of sprays per treatment plot versus total sprays was displayed in Table 1 because of differences in application numbers between blocks.

Border spray applications in blueberry were made with a cannon sprayer (J-1000, Jacto Inc., Pompea, Brazil) traveling around the perimeter of the crop and spraying pesticides up to 30 m into the field. Cover spray applications were made with an over-the-row Trellis Boom (Rear’s Manufacturing Company, Eugene, Oregon) sprayer that treated two entire plant rows per pass in 2012 and a track-elevated sprayer (TR4 Tracker, GK Machine Inc., Donald, Oregon) that treated four entire plant rows per pass in 2013. All insecticide applications (Table 1) targeted D. suzukii with the exception of a neonicotinoid applications to control an aphid outbreak in 2012.

Data analysis. All statistical analyses were made in R (R Core Team, 2013) using RStudio (RStudio, 2012) with α = 0.05. Cochran-Mantel-Haenszel chi-squared test for count data was used to determine if sums of each gender were in excess in 2 × 2 tables for each block (Ramsey & Schafer, 2002). Fisher’s Exact Test for count data was used for small sample sizes (Ramsey & Schafer, 2002).

The border spray trial was a completely randomized block design in a two-factor factorial experiment. There were two levels within each treatment (border and cover sprays) and trap position (border and interior traps). Linear mixed effects model fit by restricted maximum likelihood (REML) was used to detect differences in mean adults captured across the harvest season with treatment and trap position as fixed effects and treatment plots nested within blocks as random effects (larval counts were either zero or too low to permit analysis). A drop in deviance test was performed to fit the most appropriate model (Ramsey & Schafer, 2002). The model with the lowest Akaike information criterion (AIC) and non-significant difference between other models was deemed most appropriate using ANOVA. When the full model was most appropriate, linear combinations of coefficients were used to answer treatment and trap position questions. All P-values are reported with 95% confidence intervals (CI) and Bonferroni adjusted when two comparisons were made (α = 0.025). Natural log-transformation was used when normality or equal variance assumptions were not met. Log10 (x + 1)-transformation was used to transform 0 when present in the dataset. Estimates and confidence intervals were back-transformed, in the case of transformed data, and presented in tables along with the original means and standard errors.

Individual non-target arthropods collected seven days post-harvest in 2012 and 2013 were averaged across collection types (sweep, vacuum, beat sheet, yellow sticky cards, and leaf) and all traps within a treatment plot to assess the overall impact of treatments. For example, anthocorids from all collection methods and trap captures within a treatment plot were combined and expressed as mean number per plot during post-harvest. This was done to detect plot level population changes, regardless of collection method, and to boost otherwise low counts.

Only sufficiently large enough counts of non-target arthropods were included in the analysis. If the mean number of a non-target arthropod was less than 1.0 per treatment plot, the arthropod was not included in the analysis because counts were deemed too low to draw any firm conclusions. Fruit knockdown collected after insecticide applications in the border spray trials were averaged across collection periods and for each trap position (i.e., fruit knockdown per trap position within a treatment plot) and analyzed as previously described for D. suzukii and non-target arthropods.

Research results and discussion:

Adults and larvae of D. suzukii. In 2012, gender was not separated based on χ2 test; however, gender was evaluated separately in 2013 because the odds in favor of an excess of females were 0.75 times greater in border spray plots (P-value = 0.026, CI = 0.59, 0.96). No significant differences in mean adult counts were detected between border and cover spray treatments during the 2012 and 2013 harvest seasons of a cultivated ‘Liberty’ blueberry farm in the Willamette Valley, Oregon (Table 2). In 2012, the effect of trap position on trap counts varied with treatment. Adults captured in cover spray plots were estimated to be 68% lower in interiors compared to borders (P-value = 0.017, CI = 0.12, 0.88) (Table 3). At last collection of the 2012 harvest season, two larvae were found in fruit collected from the border (n = 50) and two larvae from the interior (n = 50) of one of the border spray plots. At the end of the 2013 harvest season, one larva was found in fruit collected from the border (n = 50) and five larvae from the interior of border spray plots (n = 200). A single larva was found in fruit collected from the interior (n = 50) of a cover spray plot.

Non-target arthropods: natural enemies and common pests. In 2013, natural enemies captured in border spray plots was estimated to have about four times more Stethorus spp. than in cover spray plots (P-value = 0.045, CI = 1.04, 14.29) (Table 2). Since trap position varied with treatment, there were marginally fewer Stethorus spp. in cover spray interiors than borders (P-value = 0.081, CI = 0.15, 1.48) (Table 3). Although there was no effect of treatment on microhymenoptera (Table 2), trap position varied with treatment. In 2012, cover spray plots had an estimated 51% fewer microhymenoptera in the interior than in the border (P-value = <0.001, CI = 0.38, 0.64) (Table 3). In 2013, interiors were estimated to have 48% fewer microhymenoptera than borders (P-value = <0.001, CI = 0.42, 0.64) (Table 3). No significant differences between border and cover spray were found in the following natural enemies: predatory thrips, lacewings, and predatory coccinelids (Coleoptera: Coccinelidae) and in the following common pests: cucumber beetles (Table 2).

Fruit knockdown. There was marginal evidence of more fruit knockdown in cover spray compared to border spray treated plots in 2012 (P-value = 0.082, CI = -0.33, 19.07). A mean weight of 16.4 g (±6.5 SE) of blueberries was knocked down in cover spray plots and only 5.1 g (±2.5 SE) in border spray plots. In 2013, only 1.2 g (±0.3 SE) and 1.5 g (±0.3 SE) were knocked down in cover and border spray plots, respectively.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:


Title: Reduced spray programs for Drosophila suzukii management in berry crops

Authors: Jimmy Klick, Wei Q. Yang, Jana C. Lee, Denny J. Bruck

Journal: Pest Management Science

John Wiley & Sons Inc.
350 Main Street
Malden, MA 02148


Status: Manuscript currently in review


Oregon State University Extension Publication (in preparation)

Title: Strategies to Reduce Spray Inputs for Spotted Wing Drosophila Control in Berry Crops

Authors: Jimmy Klick, Wei Q. Yang, Jana Lee, Denny Bruck, and Linda Brewer


This research was presented at: Entomology Society of America (ESA) meeting in Reno NV in 2011, Knoxville TN in 2012, and Portland OR in 2014; Pacific Branch ESA meeting in Portland OR in 2012; Pacific Northwest Insect Management Conference in Portland OR in 2013; North Willamette Horticulture Society meeting in Canby OR in 2013 and 2014; Oregon Horticulture Society meeting in Portland OR in 2014.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.