Evaluation of high tunnels for management of Drosophila suzukii in fall-bearing red raspberries: Potential for reducing insecticide use
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Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is an invasive pest of soft-skinned fruit causing significant damage on a variety of fruit crops in North America, Europe, and Asia. In North America, fall-bearing fruit, such as primocane raspberries, ripen when D. suzukii populations peak and thus are vulnerable to high levels of infestation. In recent years, growers in northern climates have increased raspberry production under protected culture (high tunnels), resulting in season extension, increased yield, and improved fruit quality. High tunnels may be used as a pest management tool by physically excluding insect pests. This study investigated whether D. suzukii can be excluded from fall-bearing raspberries cultivated under tunnels covered with plastic or fine mesh netting, and whether this production technique can improve fruit marketability and serve as an alternative to insecticide application. We found that berries in plastic-covered tunnels had low season-long levels of infestation by D. suzukii (mean = 2 %), compared to netted tunnels (35 %), insecticide-treated open plots (60 %) and untreated open plots (81 %). Our microclimate data show that temperature and humidity levels inside the plastic-covered tunnels were often outside the previously published optimal temperature range for development, mating, and/or oviposition for D. suzukii, and may have therefore limited overall population growth. We conclude that exclusion and modification of microclimate may be effective and complementary pest management strategies for fall-fruiting raspberry and serve as an alternative to insecticide applications, particularly for small-acreage and organic production systems.
KeywordsSpotted wing drosophila Asian vinegar fly Rubus idaeus Invasive species High tunnel
Drosophila suzukii is invasive and causes economic damage on a variety of small fruit crops. Insecticide alternatives are necessary for long-term sustainability.
Plastic and mesh covered tunnels may physically exclude D. suzukii and create an unsuitable climate for pest development. Fruit marketability was higher inside covered tunnels than in sprayed and unsprayed open plots.
Covered tunnels may hold promise to reduce damage by D. suzukii in fall-bearing raspberries and serve as an alternative to insecticide controls.
Small fruit growers in North America, Europe, and Asia are reporting significant economic losses due to Drosophila suzukii (Matsumura) [spotted wing Drosophila (SWD)] (e.g., Asplen et al. 2015). SWD females are known to oviposit in a variety of cultivated small and stone fruit, and red raspberries (Rubus idaeus L.) are a highly preferred crop (Lee et al. 2011). In recent years, growers in the U.S. and Canada have increased production of small fruit, including red raspberries, under protected culture (Demchak 2009). Fruit yield from primocane-bearing raspberry plants grown under plastic covered tunnels can be much higher than uncovered plants, particularly in northern climates (Yao and Rosen 2011). Other advantages of growing raspberries under protection in high tunnels include temperature moderation and season extension, protection from wind and rain, and potentially reduced disease and insect pressure (Hanson et al. 2013). As small fruit crops are susceptible to damage by SWD, and more growers are utilizing high tunnels, it may be possible to adapt these structures to exclude SWD and to provide an alternative to insecticide application. This is of particular interest to organic producers, who are limited to a single effective insecticide for SWD management in the U.S., i.e., spinosad, Entrust 80WP, Dow AgroSciences LLC, Indianapolis, IN (Van Timmeren and Isaacs 2013).
Currently, SWD is the biggest threat to raspberry production in covered tunnels in North America and exclusion is a potential management tactic (Hanson et al. 2013). High tunnels may be fitted with fine mesh insect netting on the sides or end walls to exclude SWD. Fine mesh netting of <0.98 mm has been shown to exclude SWD in blueberries without loss to crop yield or fruit weight in Japan (Kawase et al. 2008). A follow-up study in Quebec showed that no SWD were reared from blueberry samples harvested under similar netting, indicating that fine mesh netting is effective in excluding SWD from ripe fruit (Cormier et al. 2015). Although these crops are often planted under protection (tunnels), and many cultivars can adequately self-pollinate without negative impacts on yield or fruit quality (Tuohimetsä et al. 2014), there are currently no published reports verifying SWD exclusion in red raspberry production.
The purpose of this study was to determine if SWD can be excluded from fall fruiting red raspberry plants by using both fine mesh netting and plastic covered tunnels, and if this level of exclusion is adequate to be used as an alternative to insecticide application by reducing infestation and improving marketability of fruit. Our study focused on a primocane-bearing (fall-fruiting) cultivar, as these fruit are typically more vulnerable to SWD infestations due to high populations of reproductive adults that overlap with susceptible crop growth stages in the Midwestern region of the U.S. This report includes microclimate data, fruit yield, percent infestation of berries, and adult fly captures collected from a single season, in 2015.
Materials and methods
In 2014, plots of the primocane-bearing raspberry cultivar ‘Heritage’ were established at the Rosemount Research and Outreach Center, Rosemount, MN (44.728104N, 93.096785W), using dormant plants. On 16 May, a 3-m-long double row was planted in each plot with 0.6 m between plants and 1.5 m between rows. Plots were arranged in a randomized complete block design with 4 treatments and 5 replications. Replicates and plots were separated by 4.6 m alleys. Treatments consisted of open plots with no cover that received no insecticide applications, open plots with no cover that received insecticide applications, plots covered with a fine mesh netting on the top and ends, and plots covered with plastic on the top and fine mesh netting ends.
In 2015, temporary high tunnel structures were constructed over randomly selected plots in each replicate using standard 5-cm by 8-cm lumber as a base fastened to 60 cm lengths of 5 cm diameter PVC that were pounded into the ground. Three pieces of 3.8 cm diameter electrical PVC conduit were glued together to make a 7.3-m-long piece to use as structural ribs to support the covers. Ends of the structural ribs were placed inside the 5 cm diameter PVC and bolted into place. Additional 3.8-cm-diameter PVC pieces were bolted to the ribs as purlins to provide further structural rigidity. Final size of structures was 3 m long, 5.2 m wide, and 2.3 m high. Covers and ends were attached to the structures using Wiggle Wire® base and wire (Poly-Tex Inc., Castle Rock, MN). The fine mesh covers and ends were 80-gram insect netting (Stone Wall Hill Farm LLC, Stephentown, NY) and the plastic was 6-mil UV stabilized greenhouse poly film (Poly-Tex Inc., Castle Rock, MN). Covers were installed on top of the tunnel structures on 12 June, and ends were not installed until green berries were present on 6 August to allow pollinators access to plots. Once mesh ends were attached, the bottoms were buried with soil. First flowering was recorded on 21 Jul., and the first ripe fruit was recorded on 21 Aug.
Insecticide applications were initiated when the berries in the open plot treatment began to turn from the green color stage to the yellow stage (Perkins-Veazie and Nonnecke 1992). Insecticide applications were made to the open plot treatment with alternating applications of zeta-cypermethrin (Mustang Maxx, FMC Corp., Philadelphia, PA) applied at 0.29 l/ha on 19, 31 Aug and 11, 25 Sep. and spinetoram (Delegate WG, Dow AgroSciences, Indianapolis, IN) applied at 0.28 l/ha on 24 Aug. and 7, 18, Sep. All applications were made using a CO2 pressurized backpack sprayer with a 3-nozzle boom fitted with XR-Teejet 8002 flat fan nozzles (TeeJet Technologies, Glendale Heights, IL) and no screens. The boom was 1.5 m wide and applications were made with 233.8 l/ha of water at 242 kPa to both sides of each treated row.
Once enough fruit had ripened to allow harvest to begin, a one-meter section of row was marked off with flags and all berries that were ripe (had turned red and were easily separated from the receptacle) were harvested. The first harvest was conducted on 27 Aug. and was repeated approximately twice per week until 1 Oct for a total of 10 harvest dates. Each successive harvest was taken from the same meter of row in each plot. Immediately after harvest, berries from each plot were weighed and rated for the proportion of marketable fruit. Unmarketable fruit were classified as berries exhibiting discoloration or damaged druplets caused by disease, insect feeding, crumbly berries (pollination related), or water soaked appearance. All other berries were classified as marketable. The tunnels were entered by unburying and opening one corner of the mesh end. The mesh remained draped over the end of the tunnel while harvests were completed to minimize the chance of SWD entering the tunnel. Tunnel ends were closed and buried after the harvest was completed. Once per week, for a total of 6 collections, ten individual berries were selected arbitrarily from the harvested berries in each plot and placed into individual 30-ml plastic cups (Dart Container Corp., Mason, MI) with a 2.5-cm2 piece of filter paper and a plastic lid. Berries were held at room temperature (22 °C) for 7 days at which time they were assessed for the presence of larvae or pupae. If one or more larvae or pupae were found, the berry was classified as infested with D. suzukii. Berries were kept for one additional week at which time a sample of adult flies that had emerged from the berries were collected and identified to confirm they were D. suzukii.
Temperature and relative humidity data were recorded in one replicate each for the treatments with a plastic cover, mesh cover, and open plot with no cover and no insecticide applications. Microclimate data were recorded from 12 Jun. until 1 Oct. using a Hobo® RX3000 3G Remote Monitoring Station fitted with temperature and relative humidity sensors (Onset Computer Corp., Bourne, MA). Sensors were placed in a central location of the plot and plant canopy, approximately 50 cm from ground level. Temperature and relative humidity data were used to generate the average number of hours per week where temperatures were ≥30 °C, considered the upper threshold temperature for SWD development (Tochen et al. 2014), and where relative humidity was ≤40 %. Tochen et al. (2015) reported significant reductions in egg production and male and female SWD longevity at 33 % relative humidity under laboratory conditions. However, the humidity levels in our field experiment were rarely reduced to ≤33 % in any treatment; therefore, a level of ≤40 % was selected for comparison between treatments and likely reflects a humidity level that would still impact SWD population growth.
Traps for monitoring D. suzukii adult activity were placed in three of the open plots that received no insecticide applications. Pherocon® SWD (Trece Inc., Adair, OK) traps were used and consisted of a 0.95 l cup with a Pherocon® SWD lure over apple cider vinegar with a drop of liquid soap. Trap contents were collected weekly and returned to the lab for identification and counting under a dissecting microscope. Lures were replaced once per month, and apple cider vinegar was replaced weekly. Traps were set up on 21 May with trap samples collected for the first time on 28 May and approximately weekly until 28 Sep.
Data for yield, marketable and unmarketable berries, and infested berries were analyzed using a one-way analysis of variance (ANOVA) and a protected least significant difference test (LSD, P = 0.05) for mean separation (SAS Institute 2014). Data for percentage of marketable and unmarketable berries and percentage of berries infested were converted to proportions and arcsine transformed for analysis; untransformed means are presented.
Mean (±SEM) yield, marketability, and D. suzukii infestation of ‘Heritage’ raspberry grown in high tunnels and open plots, Rosemount, MN 2015
Percentage marketable berries
Percentage unmarketable berriesa
Percentage infested berries
Netting high tunnel—untreated
107.97 ± 12.69a
89.38 ± 1.72a
10.62 ± 1.72b
34.58 ± 7.59c
Plastic high tunnel—untreated
82.67 ± 9.75a
86.32 ± 3.41a
13.68 ± 3.41b
2.08 ± 1.34d
Open plot—insecticide applicationb
90.60 ± 11.56a
80.42 ± 3.00b
19.58 ± 3.00b
60.20 ± 6.53b
84.70 ± 11.62a
71.32 ± 3.68c
28.68 ± 3.68a
80.93 ± 5.17a
In our study, cumulative berry yield over 10 harvest dates was not significantly different across treatments (P > 0.05), however, both covered treatments (plastic and netting) yielded significantly more marketable fruit than either open plot treatment. Additionally, both covered treatments experienced less SWD infestation than open plot treatments (insecticide treated and untreated). This indicates that the covered treatments may offer substantial protection from SWD, at least initially. We did not achieve 100 % exclusion, and it is possible that SWD adults were introduced into the tunnels when we entered to harvest fruit twice per week or could have been trapped inside the tunnels when the ends were first installed. Although the plastic covered and netted tunnels were constructed similarly and both were entered in the same way at harvest, more SWD infested berries were observed in the netted tunnels than the plastic covered tunnels. This result is likely explained by the microclimate created by the plastic covered tunnel. The microclimate under plastic produces conditions that are not conducive to SWD population growth and sustained infestations, as the daily high temperature in the tunnel often exceeded the maximum developmental threshold of 30 °C, for SWD reared on cherries and blueberries (Tochen et al. 2014). Moreover, Kinjo et al. (2014) found that when adults were mated for 4 days at 31 °C, none of the subsequent eggs that were laid hatched. In addition to temperature effects, when reared on blueberries, adults lived longer and females produced more mature oocytes at 82–94 % RH (Tochen et al. 2015). This indicates that even if tunnels do not offer 100 % exclusion, they may provide an unfavorable habitat for population growth of SWD, thus limiting further oviposition and fruit damage. Although no data exist on SWD developmental thresholds in raspberries, the thresholds from Tochen et al. (2014, 2015) indicate that SWD are better adapted to lower temperatures and higher humidity (optimum 21 °C and 94 % RH), and conditions in our plastic covered tunnels were at or beyond the maximum developmental threshold limits for temperature and minimum thresholds for humidity for extended periods of time (Fig. 2).
Our plastic-covered tunnel temperature and relative humidity trends are similar to previous studies and reflect the typical tunnel environment characterized by greater differences in daily high and low temperatures, little difference in nighttime low temperatures, and lower RH than what is recorded outside the tunnels in open field plots (Xiao et al. 2001; Ogden and van Iersel 2009; Wien 2009). When compared to open field production, raspberries grown in high tunnels tend to produce more total berries and more marketable berries (Demchak 2009; Hanson et al. 2011; Yao and Rosen 2011), and this was also true for our study. Plastic covered tunnels offer protection from wind and rain as well as lower disease incidence (Hanson 2011) and alter insect pressure (Demchak 2009). In addition to infestation by SWD, which was verified by incubating a subsample of berries in the lab, unmarketable fruit resulted from discoloration, physical damage due to wind or insect feeding, and crumbly berries due to poor pollination or virus. For growers already using high tunnels, it may be practical to adapt tunnels with fine mesh to exclude SWD. However, preventing adults from entering the tunnels during harvest may be difficult. It is possible for SWD to enter via small holes, near entrances or edges, and placing traps inside the tunnels may help indicate early colonization or potentially provide the ability to trap out a significant portion of the population inside the tunnel. In addition, the tunnels in our study were experimental and smaller than a commercial tunnel, and it is possible that temperature and relative humidity may vary depending on location within the tunnel. Wien (2009) found that micro-environmental spatial variations inside a commercial-sized (38 m length, 9 m width, 4.8 m height) plastic-covered high tunnel were slight, with air temperatures measuring only 2 °C lower than in the center of the tunnel, but this could vary.
As integrated pest management (IPM) strategies are developed for SWD in raspberries, it is important to consider short and long-term implications that a given strategy will have and how well each strategy works relative to each other. The use of insecticides for SWD management poses risks to non-target insects is limited for organic production and increases the risk of residue on fruit (Cini et al. 2012; Diepenbrock et al. 2016). Due to the rapid population growth of SWD, it is not surprising that open plots that were treated with insecticide still had high levels of berry infestation compared with covered treatments. A recent report of field efficacy of insecticides on SWD in day-neutral strawberries by Cowles et al. (2015) demonstrated that when bifenthrin, an insecticide with high efficacy ratings against SWD (Bruck et al. 2011), was applied twice per week, for a total of 15 applications, SWD recovery from fruit still averaged about 5 flies/kg of fruit and exceeded 20 flies/kg on one sample date. This level of insecticide application was approximately 3 times more than the seasonal maximum active ingredient allowed on the label for strawberries. However, small plot trials, with untreated checks or unharvested portions of plots, may reflect extreme pressure on insecticide residues and efficacy as flies are able to develop in unharvested fruit left in treated or untreated plots, and likely account for the high populations of SWD we found in nearby traps. Subsequently, this scenario creates the potential for continual infestations among nearby plots, or small plantings in a small-farm setting. Ideally, on a commercial farm, sanitation would also occur as part of an IPM or organic-certified program, to further reduce pest pressure.
Pest management for SWD is challenging for berry growers due to the widespread distribution of SWD, the wide host range for soft fruit, high reproductive potential, invasive status, and lack of natural biological control (Asplen et al. 2015). Our work shows that exclusion and the potential for modification of microclimate via tunnels may be an effective IPM tools and a practical alternative to insecticide applications. This management approach should be particularly well suited for small-acreage and organic production systems. More research is needed to investigate the economics of exclusion as a management strategy in different crops in different climates and at different scales. Exclusion netting and support frames will result in additional expenses, but this cost may be offset at least partially by increased revenue of marketable fruit yield over time. Furthermore, research should explore how the high-tunnel benefits can best be combined with trapping and other sustainable management strategies such as cultural and habitat management techniques, attract and kill options, as well as reduced insecticide use.
Author contribution statement
The authors contributed to the design of the study and data collection. EB analyzed the data, and EB and MR wrote the manuscript. The authors contributed substantially to edits and approved the final manuscript.
We thank Suzanne Wold-Burkness, Theresa Cira, Connor Mikre, Ignasi Riera Vila, and Anita Gorder for field and technical assistance.
This work was supported through funding from USDA NIFA Regional Integrated Pest Management Competitive Grants Program—North Central Region award number 2013-34103-21338, the University of Minnesota Rapid Agricultural Response Fund, and the Minnesota Agricultural Experiment Station.
Compliance with ethical standards
Conflict of Interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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