Field implementation of botanical repellents for organic management of spotted-wing drosophila (Drosophila suzukii) in small fruit

Final report for GNC19-276

Project Type: Graduate Student
Funds awarded in 2019: $8,266.00
Projected End Date: 03/31/2021
Grant Recipients: University of Minnesota; University of Minnesota
Region: North Central
State: Minnesota
Graduate Student:
Faculty Advisor:
Mary Rogers
University of Minnesota
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Project Information

Summary:

The recent invasion of spotted-wing drosophila (Drosophila suzukii; SWD) is a significant threat to local small fruit production in Minnesota and throughout temperate fruit producing regions world-wide. Mature female SWD pierce the skin of ripening and ripe fruit in order to deposit eggs. Larvae hatch from the eggs within 12 to 72 hours and destructively feed on the interior of the fruit, resulting in soft, unmarketable fruit at harvest. A recent economic analysis showed that conservative estimates of 20% yield loss to raspberry producers corresponds to a statewide loss of $2.2 million annually in Minnesota (Digiacomo, 2019). Increasing numbers of raspberry growers are halting production–tilling plants under as they struggle to manage this pest. Current management strategies are limited to frequent applications of broad spectrum insecticides, the majority of which are synthetic and can’t be used in organic systems. Presently, there is a need for additional management strategies, understanding how these strategies may work together in an IPM program, and studies on the economic viability of these practices. Botanical products such as volatile organic compounds (VOCs; e.g. essential oils) and their role in plant-insect interactions could provide an alternative pest management strategy that could be implemented without additional equipment investment from growers. SWD is known to locate hosts via visual and olfactory cues, suggesting that plant VOCs could mask the scent of SWD host fruit, while also decreasing the use of broad spectrum insecticides which negatively impact the environment, human health, pollinators and other beneficial insects. We propose a research project to study the field implementation of botanical repellents for organic management of spotted-wing drosophila (Drosophila suzukii) in small fruit. We hypothesize that growers could artificially apply VOCs on or around small fruit crops as a natural deterrent for SWD without harming beneficial insects or pollinators. The objectives of this proposed project are to assess the efficacy of botanical VOCs (the individual compound lavender oil, as well as the commercially available organic approved Ecotrol Plus and Sporan products) on SWD infestation in blueberry and raspberry crops. The results of the proposed project will guide future research and result in recommendations for both organic and conventional growers for sustainable management of SWD.

Project Objectives:

Small fruit producers and researchers will both benefit from the outcomes of this research. Learning outcome 1: organic and small fruit producers will learn basic SWD management techniques such as monitoring, identification, biology, and behavior of SWD. Learning outcome 2: organic and small fruit producers will learn how to integrate botanical repellents into their current pest management plans for SWD. Learning outcome 3: Researchers will learn about the efficacy of botanical repellents to prevent SWD infestation in fruit crops. Action outcome 1: organic and small fruit producers will implement appropriate science-based IPM techniques to reduce infestation by SWD and improve marketable fruit yield. Action outcome 2: This research will be used to guide future applied research on SWD behavior and IPM in order to provide additional control strategies for producers. Action outcome 3: Having additional pest management options will lead to an increase in production of small fruit, the supply of local small fruit, and the profits for growers while reducing the amount of pesticide applied to the fruit.

Cooperators

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Research

Materials and methods:

Introduction

The recent invasion of Drosophila suzukii (spotted-wing drosophila; SWD) to North America poses an unprecedented threat to small fruit. In addition to being an invasive species, SWD reproduce rapidly and feed destructively on the interior of soft fruit; these traits have made SWD unlike any other insect pest that producers have had to manage. SWD established in California in 2008 (Bolda et al., 2010) and spread quickly to the Upper Midwest by 2010 (Hamilton, 2010); since establishment, SWD has been responsible for significant economic losses nationwide (Bolda et al., 2010) and locally. Conservative estimates of 20% fruit infestation correspond to an annual economic impact of SWD on Minnesota raspberry producers is $2.2 million in losses, which is approximately 10% of the income generated from raspberries  (Digiacomo, 2019). Current management strategies are limited to frequent applications of broad spectrum insecticides, the majority of which are synthetic and can’t be used in organic systems. 

Recent studies focus on alternative pest management practices for SWD, addressing the potential of sanitation, biocontrol, novel bioinsecticides, and exclusion. These practices vary in their effectiveness, can be expensive, and increase labor. Additionally, there is currently only a single product (Entrust) that has been shown to be effective in organic systems, and pesticide resistance is a concern. Presently, there is a need for additional management strategies, understanding how these strategies may work together in an IPM program, and studies on the economic viability of these practices.  

Botanical products such as volatile organic compounds (VOCs) and their role in plant-insect interactions could provide an alternative pest management strategy that could be implemented without additional equipment investment from growers. Mature female SWD preferentially seek ripening and ripe fruit for oviposition, where they lay eggs inside with a large and serrated ovipositor, unlike other drosophila species which utilize damaged and rotting fruit for oviposition. Microbial yeast and fermentation products release volatile odors, which attracts SWD to fruit crops (Cha et al., 2012). This behavior is exploited for pest monitoring with baited traps. Interesting avenues of study from a pest management standpoint is whether certain volatile odors can have an opposite effect, essentially masking the scent of ripening fruit and preventing adult SWD from ovipositing on the fruit crop. 

Previous laboratory studies on using essential oils as a pest management strategy for SWD have shown that lavender oil, avocado oil, peppermint oil, and thymol (thyme oil) are effective at preventing SWD from landing on treated surfaces but have not been adequately tested in the field (Erland et al., 2015; Renkema et al., 2017). There are concerns about cost but as the essential oil industry continues to grow, the cost may decrease over time. Commercially available, organic approved, botanical VOCs have been developed for pest management purposes in recent years, (i.e., Ecotrol Plus KPA-001 (Keyplex), and Sporan KPA-034 (Keyplex)) with varying combinations of rosemary oil, geraniol, peppermint oil, clove oil, and thyme oil. SWD is known to locate hosts via visual and olfactory cues, suggesting that plant VOCs could mask the scent of SWD host fruit, while also decreasing the use of broad spectrum insecticides which negatively impact the environment, human health, pollinators and other beneficial insects. However, it is not yet understood how to effectively implement botanical essential oils in the field (e.g. volatilization rate, application method, and impact on infestation in small fruit).

We conducted preliminary no-choice and choice laboratory studies on the three proposed experimental treatments (lavender oil, Ecotrol Plus, and Sporan) at the request of a local small fruit producer. In the no-choice experiment, the three experimental treatments did not significantly increase SWD mortality compared to the water sprayed control, and the treatments did not prevent SWD from laying eggs or larvae development (Gullickson et al., 2020). However, in the choice experiment, the two commercial products significantly repelled SWD compared to the untreated control (Gullickson et al., 2020). We hypothesized that growers could artificially apply VOCs on or around small fruit crops as a natural deterrent for SWD without harming beneficial insects or pollinators. The objectives of this study were to assess the efficacy of botanical VOCs (the individual compound lavender oil, as well as the commercially available Ecotrol Plus and Sporan) on SWD infestation in blueberry and raspberry crops in comparison to an organic insecticide, spinosad, and an unsprayed control. The results of the proposed project will guide future research and result in recommendations for both organic and conventional growers for sustainable management of SWD.

 

Methods

Blueberries

The blueberry (Vaccinium corymbosum L. cv. Polaris) experiment took place at Little Hill Berry Farm in Northfield, MN. Two weeks prior to the first harvest, on 15 June 2020, green fruit clusters with more than 10 developing fruits were labeled with a white twist-tie looped around the base of the branch and covered with organza bags to exclude SWD prior to treatment application. Treatments were arranged in a randomized split-block design with three replicates per treatment. Treatments were lavender oil (Lavandula angustifolia L.) (Millipore Sigma, Milwaukee, WI), Ecotrol® Plus (rosemary, geraniol, and peppermint oil) (KeyPlex, Winter Park, FL), Sporan® EC2 (rosemary, clove, thyme, peppermint oil) (Keyplex), Entrust® SC Naturalyte Insect Control (spinosad) (Corteva Agriscience, Wilmington, DE). Plants were sprayed at sunrise (between the hours of 5:00 am and 7:00 am) to coincide with SWD activity and to limit the effects on pollinators. Immediately before spraying, organza exclusion bags were removed, and plants were sprayed at maximum label rate with a single boom CO2 powered backpack sprayer (R&D sprayers, Opelousas, LA) for each treatment (Table 1). 48 hours after spraying, 10 previously bagged fruits from each treatment replication were placed in 1.25 oz. incubation cups for determining infestation, and plants were harvested to measure yield. Data was collected on 2 July, 10 July, 17 July, 24 July, and 30 July in 2020. Spray dates were two days prior to all harvest dates.

 

Table 1: Chemical treatments, active ingredients, and application rates applied on blueberry and raspberry plants for managing spotted-wing drosophila.

Treatment (Trade name)

Active ingredient (% A.I.)

Application rate

Unsprayed control

N/A

N/A

Ecotrol® Plus

Rosemary oil (10%), geraniol (5%), peppermint oil (2%)

57.6 oz. / acre

Entrust® SC

Spinosad (22.5%) 

6 oz. / acre

Lavender oil

Lavender oil (100%)

46 oz. / acre

Sporan® EC2

Rosemary oil (16%), clove oil (10%), thyme oil (10%), peppermint oil (2%)

57.6 oz. / acre

 

Raspberries 

The raspberry (Rubus idaeus L. cv. Heritage) experiment took place at the University of Minnesota Research and Outreach Center in Rosemount, MN. Due to the fragility of raspberry fruit, organza bags for SWD exclusion was not feasible. Therefore, organic store-bought fruit was purchased the evening before spraying to serve as sentinel fruit. Prior to placing in the field, the store-bought fruit was inspected for previous infestation to adjust field infestation rates. Treatments were arranged in a randomized split block design with 4 replications per treatment. Treatments were the same as for the blueberry experiment. Immediately before spraying, five organic fruit per treatment rep were placed on a petri dish in the canopy (Figure 1) of each treatment. Plants were sprayed at max label rate (Table 1) at sunrise. 24 hours after spraying, fruit were placed in 1.25 oz. incubation cups for determining infestation. Data was collected on 28 August, 3 September, 11 September, 17 September, 24 September, and 30 September in 2020. Spray dates were 1 day prior to data collection dates.

 

Infestation assessment

Both blueberry and raspberry fruit were collected from the field and returned to the laboratory to determine SWD infestation. All fruits were observed under a dissecting microscope for infestation. Individual SWD eggs were identified by oviposition holes and breathing tubes visible on the exterior of the fruits. The number of eggs per fruit was counted and the proportion of infested fruit was calculated. 

 

Analysis

Yield data for the blueberry experiment was analysed with a two-way ANOVA which included essential oil treatment and replication as factors. Infestation data from the blueberry experiment were not normally distributed due to an overabundance of zeroes presumably caused by SWD arrival after the start of blueberry harvest. Therefore a zero inflated model with a negative binomial distribution was used to determine treatment differences followed by a one-way and two-way ANOVA with Tukey’s post hoc analysis. For the raspberry analysis, a general linear model was used followed by a two-way ANOVA analysis and Tukey’s post hoc test. Essential oil treatment and harvest weeks were included in the two-way ANOVA as treatment variables and the proportion of fruit infested by SWD was the response variable. 

Raspberry fruit in the field
Figure 1: Sentinel raspberry fruit on a petri dish in the raspberry plant canopy 24 hours after spraying. Image credit: Matthew Gullickson

 

 

References:

Bolda, M.P., Goodhue, R.E., Zalom, F.G., 2010. Spotted wing drosophila: potential economic impact of a newly established pest. Agric. Resour. Econ. Updat. Univ. California. Giannini Found. 13, 5–8. https://doi.org/10.1016/j.jff.2015.04.027 

Cha, D.H., Adams, T., Rogg, H., Landolt, P.J., 2012. Identification and field evaluation of fermentation volatiles from wine and vinegar that mediate attraction of spotted wing drosophila, Drosophila suzukii. J. Chem. Ecol. 38, 1419–1431. https://doi.org/10.1007/s10886-012-0196-5 

DiGiacomo, G., 2019. Preliminary results: Economic impacts of spotted wing drosophila for MN raspberry industry. Presentation, Minnesota Fruit and Vegetable Growers Association conference, St. Cloud, MN.  

Erland, L.A.E., Rheault, M.R., Mahmoud, S.S., 2015. Insecticidal and oviposition deterrent effects of essential oils and their constituents against the invasive pest Drosophila suzukii (Matsumura) (Diptera: Drosophilidae). Crop Prot. 78, 20–26. https://doi.org/10.1016/j.cropro.2015.08.013

Gullickson, M. G., Hodge, C. F., Hegeman, A. Rogers, M. (2020). Deterrent effects of essential oils on spotted-wing drosophila (Drosophila suzukii): Implications for organic management in berry crops. Insects, 11, 1–12.

Hamilton, K., 2010. Pest highlights of 2010, Wisconsin Pest Bulletin.

Hampton E, Koski C, Barsoian O, Faubert H, Cowles RS, Alm SR. (2014). Use of Early Ripening Cultivars to Avoid Infestation and Mass Trapping to Manage Drosophila suzukii (Diptera: Drosophilidae) in Vaccinium corymbosum (Ericales: Ericaceae). J Econ Entomol.107(5):1849-57. doi: 10.1603/EC14232  

Renkema, J.M., Buitenhuis, R., Hallett, R.H., 2017. Reduced Drosophila suzukii infestation in berries using deterrent compounds and laminate polymer flakes. Insects 8. https://doi.org/10.3390/insects8040117 

Research results and discussion:

Results and discussion

During the first three blueberry harvests, there was significantly less SWD infestation compared to the last two weeks (F = 80.389; d.f. = 4, 64; P < 0.0001) (Figure 2). This was not surprising as growers and researchers have reported that early ripening blueberry cultivars are able to at least partially avoid SWD infestation (Hampton et al., 2014). For the essential oil spray treatments, there were no statistically significant differences (Χ2 = 0.5949, d.f. = 4, P = 0.9636), although Ecotrol had marginally less infestation compared to Sporan (P = 0.0750) and lavender oil (P = 0.104), but not less than the control (P = 0.34). This is consistent with what we have observed in the past for blueberries at other locations (Gullickson et al., 2020).

Graph with depicting spotted-wing drosophila infestation in blueberries
Figure 2: The proportion of SWD infested blueberry fruit over the course of the harvest season by treatment. During the first three harvests, there was significantly less SWD infestation compared to the last two weeks (F = 80.389; d.f. = 4, 64; P < 0.0001). Additionally, Ecotrol had marginally less infestation compared to Sporan (P = 0.0750) and Lavender oil (P = 0.104), but not less than the control (P = 0.34).

There are no significant differences in the blueberry total yield (g) among the essential oil treatments or spinosad when fruit was sprayed weekly and harvested 48 hours after spraying (F= 0.707, d.f.= 3,56, P = 0.552).  It does not appear, therefore, that there was a phytotoxic effect of the treatments. However, we did not have an untreated control as a comparison because of the layout of the plots and there were not enough untreated plants to get an accurate yield estimate. 

For the proportion of infested raspberries, no significant differences when we analyzed the average proportion of infested raspberry fruit over the whole season with a one-way anova (F = 1.004, d.f. = 4,115, P = 0.409). However, when we included the harvest date in our analysis, there was a significant effect of both treatment and week. 17 September had significantly less infestation compared to the other weeks (F = 16.97, d.f.= 5,114, P < 0.0001) (Figure 3). The lower infestation on 17 September was possibly due to a brief period of colder temperatures with lows near freezing. Additionally, in our two-way ANOVA, Ecotrol and the unsprayed control were significantly different (P = 0.028) with less infestation in the Ecotrol plots. No significant differences were observed between any of the other treatments.  Although Ecotrol had significantly less infestation compared to the other treatments, average infestation rates were still at 71% compared to 84% in the unsprayed plots. 

Graph depicting spotted-wing drosophila infestation of raspberries
Figure 3: The proportion of SWD infested raspberry fruit during the harvest season by treatment. 17 September had significantly less infestation compared to the other weeks (F = 16.97, d.f.= 5,114, P < 0.0001). Ecotrol had significantly less infestation in raspberries (p = 0.028) compared to the other treatments when harvest week was included as a factor in the two-way ANOVA analysis.

Conclusion

A greater understanding of SWD’s response to aversive botanical odors may yield new management strategies for farmers. Botanical repellents in the form of applied essential oils or fragrant interplanted crops may provide SWD management through aversive stimuli. Although botanical repellents may provide incomplete protection against SWD, any reduction in pest pressure will be beneficial to growers and may be used in integrated pest management programs for this pest.

Ecotrol might be effective while SWD populations are low, but quickly becomes ineffective as the population grows. Since Ecotrol had marginally better performance than spinosad, there is potential to decrease the amount of broad-spectrum insecticides, thereby protecting pollinators and other beneficial insects. Temperature could also have an effect, since these botanical oils volatilize faster when it is warmer. The results of the project demonstrate that certain botanical volatiles may provide an alternative to broad-spectrum insecticide application that is also suitable for organic management of SWD but more research is needed to optimize their effectiveness before they can be recommended to growers.  

Participation Summary
1 Farmer participating in research

Educational & Outreach Activities

2 Online trainings
2 Published press articles, newsletters
5 Webinars / talks / presentations

Participation Summary

83 Farmers
90 Ag professionals participated
Education/outreach description:
  1. Viau, S. (2021, in prep). Essential oils: exploring a health craze as a pesticide alternative for Minnesota berries. News article on University of Minnesota College of Food, Agriculture, and Natural Resource Sciences website.
  2. Gullickson, M. G. (April 2021). Insect repellents for berries: Can spraying essential oils protect ripening berries from invasive spotted-wing drosophila? Fruit Extension blog article. URL: https://fruit.umn.edu/news/insect-repellents-berries 
  3. Gullickson, M. G. (March 2021). Chemical ecology of spotted-wing drosophila:
    Behavioral responses to olfactory stimuli and implications for organic pest management. Applied Plant Sciences Seminar Series, University of Minnesota. Attendees: 28
  4. Gullickson, M. G., Rogers, M. (February 2021). Field implementation of botanical deterrents for organic management of spotted-wing drosophila. Virtual poster presentation at Growing Stronger Virtual Conference. Awarded second place in the poster competition.
  5. Shanovich, H., Gullickson, M. G., Toninato, A., Rogers, M. (February 2021). Invasive insects in organic fruit. Panel discussion at the Growing Stronger Virtual Conference. Attendees: 83
  6. Rogers, M. (January 2021). Spotted-wing drosophila management: what works, what doesn’t, and new things to try. Virtual presentation at MN Fruit and Vegetable Growers Association conference. Attendees: 43
  7. Gullickson, M. G. (April 2020). Drosophila suzukii organic management for small fruit production. Virtual research and outreach presentation for growers hosted by Dr. Thaddeus McCamant of Central Lakes College.  Attendees: 19

 

Project Outcomes

1 New working collaboration
Project outcomes:

As a direct result from this project, Rogers and Gullickson are conducting a preliminary follow-up experiment in the summer of 2021 to determine whether interplanting plants with SWD repellent compounds reduces infestation. Knowledge gained from this SARE Graduate Student Grant was used to help inform two additional grant proposals: USDA CPPM Developing Ecological-Based Pest Management for Lygus lineolaris and Drosophila suzukii in Small fruit in the Upper Midwest. Submitted spring 2021 and NC SARE R&E Advancing Sustainable Management Techniques for Spotted-Wing Drosophila in Strawberries in Minnesota. Submitted spring 2021.

Although it is too early to know the extent that growers will utilize this research, the high amount of attendees at virtual conferences this year indicates that growers are interested in learning about IPM and sustainably managing SWD. We also hope that this study will lead to an increase in regional production of small fruit, the supply of local small fruit, and the profits for growers while reducing the amount of pesticide applied to the fruit.

Knowledge Gained:

The expected outcomes of this project were to share information on SWD identification and damage assessment, SWD monitoring, the biology and behavior of SWD, and methods for integrating botanical repellents pest management plans for SWD with farmers and researchers. Through our education and outreach activities, we engaged more than 173 participants, although some of these may have been repeat views. We were unfortunately unable to hire and train an undergraduate researcher or conduct on-farm demonstrations due to the COVID-19 pandemic, which limited our outreach efforts to some extent. From a research perspective, we learned about the efficacy of botanical repellents at preventing SWD damage which will be extremely useful in guiding future applied research on SWD behavior and IPM.

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.