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).
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.
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.
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.
Educational & Outreach Activities
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.
This study demonstrated that border sprays provided similar control of D. suzukii as cover spray applications and prevented yield loss; indicating reduced spray application strategies are important management tools for this pest. Border spray treatments resulted in higher numbers of natural enemies and lowered input costs. The hypothesis of minimizing D. suzukii populations with reduced sprays was supported because of likely D. suzukii mobility within field and movement from field margins into cultivated crop (Mitsui et al., 2006; Klick et al., 2014a). Fly movement from field margins into blueberries most likely was reduced with the use of border sprays in the low-pressure D. suzukii year of 2012. The integration of cover sprays may be necessary in a high-pressure D. suzukii year, as in 2013. It is possible more females were trapped in 2013 border spray plots because they are more resilient to insecticides than males (Bruck et al., 2011).
The lack of rain enabled longer residual activity of insecticides (Van Timmeren & Isaacs, 2013), which would be important in fields receiving lower insecticide inputs (i.e., reduced spray strategies).
An unexpected outcome of this research was the effect of trap position in cover spray blueberries. In 2012, traps in borders (~5 m into the field) of cover spray plots had significantly higher D. suzukii counts than traps placed in interiors (40 – 60 m into the field). In addition, higher D. suzukii counts in the borders occurred less than five days after applications, whereas interior traps had zero to very low counts (data not shown). This suggests that placing monitoring traps in borders may not be an accurate indicator of a plot-wide effect from a cover spray. Flies in border areas of cover spray plots may have less insecticide exposure than flies in interior areas. Although border traps do not necessarily estimate plot-wide adult D. suzukii populations accurately, they are still valuable indicators of early pest presence and level of pressure. It is unclear why fly captures in trap positions were similar in border spray plots. Perhaps the cannon sprayer (i.e., border spray) provided better penetration and coverage into border plant canopies than the vertical boom sprayers (i.e., cover spray), improving insecticide exposure to D. suzukii. Another possibility is that cover spray plots were exposed to higher D. suzukii pressure than border-spray plots. Indeed, block 3, which was overhead irrigated, had an outlier trap in the cover spray plot with consistently high counts throughout the harvest seasons.
To reduce crop risk, border sprays can be integrated with cover sprays to overcome higher pest pressure or to ‘clean up’ a field (Vincent et al., 1999; Trimble & Vickers, 2000). Level of insecticide residual activity may also influence crop risk, which varies between production regions, as risk is influenced by weather. For example, western Oregon likely has longer insecticide residual activity (Bruck et al., 2011) than North Carolina because of lower humidity and less rainfall in the summer (Burrack et al., 2013). Other factors possibly impacting insecticide efficacy include the method of irrigation, inactive ingredients such as additives, stickers, spreaders, and ultraviolet light breakdown (Van Timmeren & Isaacs, 2013). Overhead irrigation is suspected of reducing spray residuals and creating a more ideal habitat of higher humidity and moisture for D. suzukii (Tochen et al., 2014a), thus increasing risk.
Two blocks in 2012 were treated with neonicotinoid for a secondary pest outbreak of aphids (Hemiptera: Aphididae), which was not effective on D. suzukii adults in direct-spray laboratory bioassays (Bruck et al., 2011), but has shown efficacy against adult and immature stages in semi-field and field trials (Van Timmeren & Isaacs, 2013) and curatively for post-infested fruit (Wise et al., 2014). Honeydew from aphids can reduce fruit quality (Oatman & Platner, 1972). A single neonicotinoid application for aphid control to the entire border spray plot late in the 2013 season (Table 1) was made after D. suzukii larvae were detected. Larval densities did not change and remained very low after treatment.
More microhymenoptera were found in borders of cover spray plots in 2012 and in borders of both treatment plots in 2013, suggesting possible movement from surrounding field margins into the crop. If natural enemies migrate into the field from field margins (Longley et al., 1997), then border sprays may also affect them. Van Driesche et al. (1998) reported that leafminer parasitism in apple orchards was only higher in second generation parasitoids but not in succeeding generations that were border sprayed (Van Driesche et al., 1998), suggesting there may be a lag effect of border sprays on natural enemy populations. Poor predation of cottony cushion scale occurred in California citrus by predacious Vedalia beetle (Coleoptera: Coccinelidae) when DDT was treated in two rows surrounding treatment plots (four trees) when compared to untreated control (DeBach & Bartlett, 1951), as the beetles were also affected by the border sprays. Another study revealed that parasitism of apple maggot was unaffected by border spray treatments when compared to cover sprays, regardless of plot size (Van Driesche et al., 1998). Border sprays seem to thus affect natural enemies differently and are perhaps dependent on the mobility of the organism. It is important that monitoring occur not just in field borders, but also in interiors, to assess natural enemies, pest pressure, and treatment efficacy on the entire plot.
Blueberry fruit knockdown and loss seem highly dependent on sprayer type and cultural plant management practices, including trellising and pruning, row width, plant size, and cultivar architecture characteristics such as having a compact growth habit. In 2012, an over-the-row Trellis Boom sprayer was attached to a tractor, rows were untrellised, and plants were four years old and one year from maturity. ‘Liberty’ blueberries are notoriously difficult to prune as they produce long shoots that grow into the row middle by harvest, increasing the likelihood of drop. Hand-pickers and deer likely confounded fruit knockdown data, as only marginal differences were detected. In 2013, the grower upgraded to a track-elevated sprayer and trellised the rows, which greatly reduced fruit knockdown in cover spray plots. Plots were treated immediately after fruit was picked to reduce fruit knockdown by the sprayer. The effect of the cannon sprayer on fruit knockdown appeared minimal, and there was no position effect detected in treated borders compared to untreated interiors of border spray plots.
Based on the findings presented, border sprays may be implemented in low-risk and low-pressure situations and perhaps integrated with other non-insecticide pest management tools (Edson et al., 1998; Atanassov et al., 2003). The appropriate plot size for border spray treatments using a cannon sprayer is between 2 and 5 hectares. Extensive monitoring at least twice per week is important to understand the dynamics and effects of the reduced spray treatments on adult and larval D. suzukii populations (Kleiber et al., 2014). Further work is needed to assess the seasonal and long-term impacts of reduced sprays on non-target arthropods, rather than just post-harvest effects.
Pest pressure from D. suzukii varies during the season (Van Timmeren & Isaacs, 2013), with low populations in winter (Dalton et al., 2011), and the highest populations from mid-summer to fall in most regions. In the border spray trial, ten ‘Liberty’ blueberries in containers placed along the field margin of each block as untreated controls did not serve as a good indicator of D. suzukii pressure because of low trap counts. Crop risk is greater in border sprays compared to cover spray strategies because approximately 70% of the crop is untreated at each application. Manipulating temporal and spatial factors that affect D. suzukii populations may reduce this risk. Knowledge of high-density and low-density D. suzukii areas (Klick et al., 2014a) and crop ripening phenology (Lee et al., 2011b) could facilitate integration of reduced spray strategies into a D. suzukii management program, such as reduced spray applications in low fly density areas onto fruit that ripen in early summer when populations are still low.
An economic impact scenario of border sprays was estimated on a 4.05 ha plot with two insecticide sprays compared to the cover spray application method (Table 4). Insecticide cost, machine time, sprayer cost, fruit loss from knockdown in the 2012 border spray trial, and money saved were included in the analysis. Table calculations for border and cover sprays were based on berry economics (Julian et al., 2011). The analyses included: 1) estimated cost of insecticide material to treat 4.05 ha twice. Insecticide savings are approximately 70% in border applications based on area sprayed by each method; 2) the machine time to treat 4.05 ha with an airblast sprayer based on travel speed of 4.8 km h-1 and 0.74 ha treated per hour and the time to treat the same area at 10.9 ha per hour for border sprays with a cannon sprayer traveling at the same speed. Hectare per hour is the product of tractor speed (4.8 km h-1), row width (3 m), and efficiency (50%) over a conversion factor of 8.25 (C.F. Seavert, personal communication). Efficiency is the actual time spent spraying, excluding filling the tank, traveling to field, adjusting nozzles, and associated activities; 3) airblast sprayer (cover sprays) and cannon sprayer (border spray) costs to treat 4.05 ha for labor, variable machine cost such as repairs and maintenance, and fixed machine cost including depreciating interest and insurance. The airblast and cannon sprayers were a 757 and 398 liter unit with power-take-off (PTO), respectively; 4) fruit knockdown loss in cover spray is based on the difference in fruit knockdown between border and cover sprays of 41.9 kg ha-1 and blueberry value of $3.30 kg-1 (average fresh and processed value) in 2012; 5) money saved is the difference in sprayer and insecticide cost between treatments. Analysis in the border spray trial includes savings from decreased fruit knockdown.
Several growers in the Willamette Valley have adopted border spray programs in blueberry production systems based on personal communication with crop consultants and researchers.
Areas needing additional study
To date, this is the first study in blueberries to show possible utility of border sprays in blueberries for D. suzukii 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 D. suzukii management programs.