Advancing Spotted-Wing Drosophila Sustainable Management Techniques for Strawberries in Minnesota

We propose three research and outreach activities for this project: 1) test cultural and behavioral-based management strategies for SWD control and cost effectiveness; 2) facilitate strawberry demonstrations and learning opportunities for growers; and 3) develop case studies with individual farmers about strawberry production, pest control and profitability. These activities relate to the three priority themes of sustainable pest management, marketing and local food systems, and human dimensions of priority invasive-species issues. The methods and approach are listed for each activity below.

Activity 1: Test SWD control strategies and profitability in strawberries

We will conduct rigorous field trial and on-farm experiments to help growers determine the most economically effective SWD control practices for strawberry production and the financial feasibility of growing organic day-neutral strawberries in the Upper Midwest.

A preliminary survey administered by key personnel DiGiacomo shows that farmers primarily use insecticidal sprays for managing SWD but are also interested in non-pesticidal and organic methods if they are economically effective. Our previous preliminary research on raspberries shows promising results for cultural controls including increasing harvest frequency and botanical-based repellents, but these techniques have not been validated or studied in DN strawberries.  Therefore, our cultural control treatments will include: 1) sanitation through harvesting all ripe fruit three times per week and removal of unmarketable and fallen fruit from plants; 2) botanical-based deterrent (Ecotrol Plus) sprayed weekly; 3) a rotation of spinosad (Entrust) and Grandevo (OMRI approved pesticides) as a positive control check; and 4) an untreated control with strawberries harvested twice weekly. We will have four replications (100 ft long rows) arranged in a randomized block design for a total of sixteen plots (50 plants per plot) located at the University of Minnesota, St. Paul campus MN Agricultural Experiment Station. Planting will occur in the spring when soils have thawed and drained sufficiently for tractor activity. Cultural practices will be implemented by initially rotovating the soil, followed by raised bed preparation with a bed shaper, and plastic mulch and drip tape application with a mulch layer. After mulch application, bare-root ‘Cabrillo’ strawberries will be planted in mulch in holes punctured with a dibble bulb and seed planter. This cultivar was selected based on the high yields and fruit quality results from previous cultivar evaluation studies performed by UMN. Alleyway between strips will be planted with annual rye every year after raised bed preparation. 

Plots will be fertigated using drip tape applicators underneath each mulch type, at the rate of 6.1 kg ha-1 of fertility solution per week. As a fertility source, an organically approved fertilizer, AgGrand Organic Series fertilizer, 4N-1.3P-2.5K will be used. Tensiometers will be installed in one strip of each color mulch treatment at depths of 30 and 45 cm to schedule irrigation. All plots will be irrigated when tensiometers average -30 kpa. Inflorescences will be removed until the last week of June to ensure vegetative growth and root establishment immediately after planting. Plots will be harvested twice each week, but only once each week for the first and last harvest weeks of each season, when yields are lower. During each harvest, all berries picked within a selection zone, consisting of the interior 28 plants within each split plot replicate, will be weighed for experimental unit yield as described by Petran (2017). For the control plots, yield from each of the two harvests per week will be combined to calculate weekly yield per plot, and three harvests a week will be combined for the frequent harvest treatment. Twenty berries in each selection zone will be randomly selected, weighed, and divided by 20 to gauge average berry weight for that plot. The percent of marketable fruit will be evaluated throughout the season at one harvest per week as described by Anderson et al. (2019). USDA standards will be used to group fruit into U.S. No. 1 or U.S. No. 2, based on size and uniformity, and all else (designated as low grade or unmarketable in this study). For the purposes of this experiment, USDA grade U.S. No. 1 and U.S. No. 2 together constitute the “marketable quality” fruit category. Spray treatments will be applied once per week at the maximum label rate using a single boom CO2 pressurized backpack sprayer (Bellspray, Inc, Opelousas, LA) and applied 24 hours prior to harvesting.

We will measure the amount of fruit infested by SWD by randomly selecting 10 fruit per plot per harvest to visually assess under a dissecting microscope for the number of SWD eggs. Fruit can be characterized by percentage of infested berries (positive or negative for any eggs). Samples will be analyzed using a GLMM ANOVA on proportion of infested fruit per plot. The model will incorporate fixed effects of treatment, including block number as random factors. Treatment means will be compared using a Dunnet’s post-hoc test.

After the field seasons have concluded, we will study the economic impact of SWD cultural control strategies first by developing an enterprise budget for organic day-neutral strawberries. Enterprise budgets list income, expenses and estimated profitability for one season. The enterprise budget will serve as an economic baseline for organic day-neutral strawberry production in the Upper Midwest where standard harvest practices (2X/week) and routine organic insecticide rotations (Entrust + Grandevo) are assumed.

Marketable yield data needed for the enterprise budget will be collected from the research station and HAFA demonstration plots. Economic data collection techniques and methods will be documented and provided to field researchers in advance of planting. Throughout the strawberry season, data for the research station plots will be compiled by field researchers and recorded in an Excel spreadsheet. Marketable yield data will be collected from HAFA growers using an interview format three times throughout the season (early harvest, mid-season harvest, late season harvest). Translators will be on hand at HAFA to assist with data collection.

Seasonal, weighted average retail prices for strawberries, available bi-weekly for conventional and organic strawberries from USDA Agricultural Market News Service, will be applied to marketable yields to calculate gross returns. Material inputs used for the four treatments will be recorded by researchers at the experiment station and valued using market rates. Labor required for planting, mulch and irrigation, maintenance/pruning, repellent and insecticide applications,  and harvest and packing will be compiled primarily by researchers at the experiment station. We will test the labor efficiency of research staff by observing the time required by HAFA growers to prune/maintain plants and to harvest berries. We suspect that HAFA growers will be more efficient than researchers when harvesting due to grower experience. Fixed and variable costs (input materials and labor) will be deducted from gross returns to arrive at net profitability.

A partial budget analysis will be applied to explore how net returns and overall profitability change when cultural controls (sanitation and organic repellents) are used in place of the standard organic practices (2X/week harvest and Entrust + Grandevo insecticide rotation). The partial budget method accounts for changes in marketable harvest, labor, and input costs. Fixed costs remain unchanged. We will test the rigor of results by applying a sensitivity analysis where price, yield and harvest rates are varied to account for differences in sales channels, SWD infestation, and grower experience, respectively. A rigorous study on enterprise profitability will further our understanding of SWD’s economic impact on strawberry production and the economic viability of day-neutral strawberry production in the Upper Midwest.

Activity 2: Facilitate strawberry demonstrations and learning opportunities for growers

We will engage new audiences and provide research-based support to historically underserved grower communities around managing day-neutral strawberries. We will partner with HAFA for on-farm research and demonstration during the first two years of this project. This will occur via three different modes: 1) demonstration plots at HAFA; 2) training presentations and IPM workshop; and 3) farmer trials.

A demonstration plot (1/3 acre) will be established at the HAFA farm containing four different DN varieties (Seascape, Albion, Cabrillo, San Andreas). This will provide an opportunity for emerging farmers to observe how the varieties perform and differences in yield, quality, and pest pressure. The UMN GRA (Gullickson) and undergraduate research assistant will help establish the plantings and visit the demonstration plot to collect yield data and monitor for SWD in fresh fruit via salt extraction tests twice per month during the growing season. As part of a pre-existing program, HAFA is providing training sessions on best management practices for growing DN strawberries with Annie Klodd (UMN Extension Educator) in January 2021 & 2022. The graduate research assistant (Matthew Gullickson) will co-present on integrated pest management alongside Extension Educator Annie Klodd during the January 2022 session. The training presentation will be adapted to be interactive and visually descriptive for efficient and effective training for Hmong farmers. Furthermore, each training will be conducted in conjunction with a HAFA staff to translate training materials. Each summer during the growing season, Gullickson and Rogers will host an IPM workshop on strawberry pests onsite at HAFA, using the demonstration plot, with translation services provided by HAFA staff. Additionally, individual growers (up to 6 per year) will be invited to grow small plots of DN strawberries on their own sites to record their experiences with these cultivars and practices. In addition to the training sessions provided above, HAFA will provide one-to-one follow up sessions on their farm plots, evaluations with our Farm Manager and Farm Advocate, with opportunities to reach out with follow questions and concerns. Each participating farmer will receive strawberry starters. University researchers will host planning meetings with HAFA staff in March prior to the growing season and in November at the end of the season to better understand practices that work for growers, help estimate production costs and profitability, help answer questions, and listen to grower feedback to ensure that knowledge is exchanged both ways. Farmers will be reached through electronic, print, and word of mouth communications. HAFA will contact all 140 farmers on their farmer-member list and use Hmong community connections, Hmong radio programs, and Hmong social media accounts and groups. These recruitment methods have potential to reach over 600 Hmong farmers in the Twin Cities metropolitan and Southwestern Minnesota areas.

Activity 3:  Develop case studies with individual farmers about strawberry production, pest control and profitability

We will create Extension-ready grower case studies by interviewing the HAFA farmer collaborators and other Minnesota farmers using non-chemical techniques on SWD management strategies and day-neutral production practices, featuring multi-language producer profiles and field trial results.

Feedback provided by 315 MN fruit and vegetable growers in a 2018-2019 survey conducted by UMN Extension (Klodd and Hoidal 2019) suggests that growers prefer learning from other growers. We will create three Extension-ready grower case studies that highlight effective, profitable control strategies for SWD and the production of DN strawberries in the Upper Midwest. The case studies will combine field trial research data with new grower interviews, photographs and infographics using a new template developed by a graphic artist for this project. The case study template will be designed so that it can be re-used by Extension for future case studies on invasive pest management, alternative crops and innovative management practices.  Each case study will answer basic questions such as “Does it pay to spray for SWD?”  Each case study will be 2-4 pages and made available electronically for easy download in English and Hmong. Each case study will include a grower interview and supporting research results. Photos taken on site at the research site and HAFA grower plots will be included.

The primary audience for the case studies are Upper Midwest fruit and vegetable growers representing 608,438 acres (2017 Census of Agriculture) and the Extension educators who serve them. We also anticipate widespread use of the case studies by many of the nation’s 33,919 fruit growers and 2,900 Extension offices.