Abstract
Day-neutral strawberry (DNS) production is increasing in the Upper Midwest because of its extended harvest season and greater yield over June-bearing cultivars. However, the longer season increases fruit exposure to the invasive spotted-wing drosophila (Drosophila suzukii; SWD), which threatens the production of small fruits and berries, particularly in organic systems. Numerous pest management tactics have been developed for SWD in recent years; however, relatively few studies have investigated the impact of SWD on DNS. Organic DNS growers need information regarding which management strategy is most effective when compared directly. To address this knowledge gap, we established a 2-year controlled field experiment with organic DNS. We applied treatments that correspond with techniques that local growers reported using or that have shown promise for organic raspberries, including increased harvest frequency, botanical-based repellents, and weekly rotations of organic insecticides, which we compared with an untreated control. We hypothesized that noninsecticidal SWD management strategies would result in fewer SWD eggs per berry and a lower proportion of infested berries compared with those associated with an untreated control. We also hypothesized that noninsecticidal management strategies would be as effective and cost less than organic insecticide applications. We collected data regarding labor hours, direct costs, strawberry yield, and SWD infestation in experimental plots on certified organic land in Minnesota in 2022 and 2023. An average of 33% of strawberries contained SWD eggs. The experimental treatments showed inconsistent effectiveness for reducing infestation compared with that of the untreated control plots and had no effect on marketable plant yield over the 2-year period. Thus, the added labor expense of these pest control treatments yielded net returns that were 17% to 21% below the control. Labor-saving alternatives like exclusion netting or postharvest cold treatments, which reduce fruit pest exposure and egg viability without harming nontarget insects, may offer more cost-effective solutions for managing SWD in organic DNS.
Strawberry (Fragaria ×ananassa) production and value are increasing in the United States. Between 2017 and 2022, the number of strawberry farms has declined by 5.5% (from 8964 to 8491) (US Department of Agriculture, National Agricultural Statistics Service 2023b); however, strawberry acreage has increased by 13,300 acres, or approximately 22%, during the same period. The total value of strawberries increased from $2.9 billion in 2017 to $3.2 billion in 2022 (US Department of Agriculture, National Agricultural Statistics Service 2023b). The majority of strawberries in the United States are grown in California and Florida (Samtani et al. 2019). The Upper Midwest, defined as Iowa, Michigan, Minnesota, North Dakota, South Dakota, and Wisconsin, comprise a small proportion of US strawberry production. The Upper Midwest strawberry acreage is reportedly 2222 acres, and there are 1155 farms (US Department of Agriculture, National Agricultural Statistics Service 2023b). The predominant strawberry cultivars grown in the Upper Midwest are June-bearing strawberry (JBS) cultivars; however, growers are interested in expanding the strawberry harvest season with day-neutral strawberry (DNS) cultivars (Samtani et al. 2019).
Early studies of DNS cultivars in the Upper Midwest reported variable performance compared with that of JBS cultivars (Luby 1989). More recent studies of DNS, however, reported that the fruit quality is comparable to that of JBS, and the annual yield per hectare greatly exceeds that of JBS (Anderson et al. 2019; Petran et al. 2017). Total soluble solids, acidity, and flavor are comparable between DNS and JBS (Petran et al. 2017). Advances in breeding and production practices may have compensated for the initial poor performance of DNS in the Upper Midwest. Cultivar evaluations in Minnesota found that day-neutral cultivars, including Portola, Albion, Monterey, and Seascape, produced consistent yields ranging from 22,417 to 42,592 kg/ha per year from July to the end of October in an annual system, which was a substantial increase from the average yield of 7846 kg/ha per year of perennial JBS (Anderson et al. 2019; Petran et al. 2017). The bulk of the harvest season occurs later in the summer, from late July through first freeze, and it lasts approximately 12 weeks in the Upper Midwest, thereby offsetting and extending the strawberry season so that DNS production would not directly compete with established JBS growers. The majority of Midwest strawberry growers market their fruit direct to consumers and retailers, thus capturing a premium above farmgate or wholesale prices. In Minnesota, for example, 90% of strawberry growers surveyed (n = 38) reported marketing their fruit direct to consumers at farmers markets and on the farm through pick your own (DiGiacomo, unpublished survey 2017). Additionally, by selling local DNS at times when there is not as much market competition, farmers may be able to market their fruit at a higher price than what is typical for JBS.
Two major DNS production challenges in the Upper Midwest are increased pest management requirements and labor demands (Samtani et al. 2019). The cultivation of fresh strawberries ranks among the specialty crops that demand the highest labor input (Calvin and Martin 2010). Plants need to be maintained throughout the season, and the harvest season can last 4 months (July–October) in the Upper Midwest. Pest management efforts increase input costs, whether labor or materials, but can improve crop profitability (DiGiacomo et al. 2021; Farnsworth et al. 2017; Leach et al. 2017). A goal of integrated pest management is to suppress or maintain insect pest populations below the economic injury level, i.e., the lowest population density of a pest that will cause economic damage (Pedigo et al. 2021). In Minnesota, farmers report interest in, or are using, several approaches to manage insect pests, including insecticides, physical barriers, botanical repellents, and frequent harvesting, but there is uncertainty about which strategy is most cost-effective.
Of particular concern is spotted-wing drosophila (Drosophila suzukii Matsumura; SWD) because of the phenological overlap between the harvest season of DNS and peak SWD populations (Anderson et al. 2019; Samtani et al. 2019). SWD is an invasive insect pest that oviposits eggs into ripening and ripe soft fruits using a serrated and sclerotized ovipositor (Asplen et al. 2015; Atallah et al. 2014). Larvae feed on the interior of the fruit, which decreases marketability and can lead to distributors rejecting fruit (Ganjisaffar et al. 2023). Strawberries are a host for SWD, although they are less preferred than raspberries (Rubus spp.) and blueberries (Vaccinium spp.) (Bellamy et al. 2013; Walsh et al. 2011). SWD is becoming increasingly challenging to manage on strawberry farms in California, potentially because of developing insecticide resistance (Ganjisaffar et al. 2023; Gress and Zalom 2018). However, there is a gap in the understanding of the impact of effective management strategies for SWD on DNS in the Upper Midwest, where insecticide resistance has not been reported.
Because of high population pressure and no economic threshold for fresh fruit, SWD is typically managed via repeat applications of broad-spectrum insecticides (Sial et al. 2019; Van Timmeren et al. 2018). Insecticides can negatively impact beneficial insects (Biondi et al. 2012), such as pollinators and natural enemies of pests. Alternative management strategies include increased harvest frequency (Leach et al. 2017), botanical repellents (Cha et al. 2020; Gullickson et al. 2020; Renkema et al. 2017; Stockton et al. 2019; Wallingford et al. 2016), exclusion netting (Ebbenga et al. 2019; Leach et al. 2016; Rogers et al. 2016; Stockton et al. 2020), and postharvest storage (Aly et al. 2017), among others. Spinosyns, including spinosad, are effective organic insecticides that can be used to control SWD (Gullickson et al. 2019; Sial et al. 2019; Van Timmeren and Isaacs 2013; Van Timmeren et al. 2018) and kill insects upon contact or ingestion. Nonpesticidal repellent products may have promise for reducing infestation by modifying host suitability and SWD behavior (Gullickson et al. 2020; Renkema et al. 2016; Wallingford et al. 2016) while conserving beneficial insects such as pollinators and predators (Biondi et al. 2012). Spray applications of Ecotrol Plus (which contains rosemary, geraniol, and peppermint oil) significantly reduced SWD infestation in raspberries compared with that in control plots and was equivalent to spinosad during field trials (Gullickson et al. 2020). This work has not been performed for strawberries, which elicit different volatile organic compounds and tend to be less preferred than raspberries; therefore, we hypothesize this product may be even more effective for strawberries. However, spray applications of botanicals need to be frequently reapplied, thus incurring greater expense and labor (Farnsworth et al. 2017). Previous research of raspberries showed that frequent harvest intervals (ripe raspberries harvested every other day) resulted in reduced infestation compared with that in control plots (harvested twice per week) (Leach et al. 2017). Increasing harvest frequency of fruit can reduce berry exposure to flies and enhance sanitation; however, this has not been tested for strawberries. Increasing harvest intervals will increase labor costs (Leach et al. 2017); therefore, this needs to be considered.
To address these knowledge gaps, we established a 2-year controlled field experiment with organic DNS in St. Paul, MN, USA. We applied treatments corresponding with the techniques that local growers report using or that have shown promise in organic raspberries, increased harvest frequency, botanical-based repellents, and weekly rotations of organic insecticides, and we compared the results with those of an untreated control. Additionally, we recorded direct costs associated with each treatment to assess the economic viability. We hypothesized that noninsecticidal management strategies would be as effective and cost less than organic insecticide applications. The aim of this research was to compare the efficacy of SWD organic control methods and provide strawberry growers with information regarding the economic viability of these practices.
Materials and methods
Plant materials
The experimental plot was located at the University of Minnesota, St. Paul Campus, Agricultural Experiment Station, St. Paul, MN, USA (lat. 45°0′31.09″N, long. 93°18′56.313″W) on certified organic land. The soil (Waukegan silt loam, 4.5% organic matter, pH of 6.9) was tilled as soon as it had thawed and dried sufficiently to operate a tractor on 9 May 2022 and 5 May 2023. After tilling, nine raised beds were shaped to be 10 cm (height) × 1 m (width) × 30.5 m (length) (model 2121-D; Buckeye Tractor Co., Columbus Grove, OH, USA). Bed spacing was approximately 1 m between the rows. A single line of drip tape and white-on-black 1.25-mil plastic mulch were laid over the raised beds (model 2133; Buckeye Tractor Co.). Rows alternated between buffer rows (n = 5) and data rows (n = 4). Each data row was divided equally into four 7.62-m-long sections, with the middle 4.75 m designated as the harvest zone for that treatment replication. Plants arrived as bare-root transplants and were placed in cold storage until the field was ready for planting. Day-neutral strawberry cultivar Cabrillo (2022: Lassen Canyon Nursery, Lassen Canyon, CA, USA; 2023: Crown Nursery, Red Bluff, CA, USA) was planted on 11 May 2022 and 10 May 2023. This cultivar was selected because of its high productivity and quality in major strawberry regions such as California. Plants were spaced every 30.5 cm and planted in offset double rows in the data rows and as single rows in the buffer rows. There were 200 plants per data row and 100 plants per buffer row. There were four replications per treatment arranged in a randomized block design, for a total of 16 treatment plots. Alleyways between rows were planted with annual rye and Dutch white clover (Albert Lea Seed, Albert Lea, MN, USA) after raised bed preparation.
Plots were fertigated using drip tape applicators underneath the plastic mulch at the rate of 5.6 kg·ha−1 of fertility solution per week (Neptune’s Harvest Fish and Seaweed Fertilizer 2-3-1; Neptune’s Harvest, Gloucester, MA, USA). Tensiometers were installed under the plastic mulch at a depth of 30 cm to schedule additional irrigation. All plots were supplementally irrigated when tensiometers averaged −30 kpa. Inflorescences were not removed after planting. Runners were removed weekly during the entire 2022 growing season and until 1 Aug in 2023. Plots were weeded and alleys were mowed weekly during both years.
Berries were harvested between 8:00 AM and 11:00 AM. During each harvest, all fully ripe berries were picked within a harvest zone consisting of the interior 30 plants within each plot replicate; 10 plants were left on each side of the harvest zone to act as a buffer between treatments within the same row. Buffer plants were harvested a minimum of twice per week to reduce SWD reservoir host fruit. Berries were placed in coolers immediately after harvest and weighed for experimental unit yield in the laboratory. Twenty berries in each selection zone were randomly selected, weighed, and divided by 20 to measure the average berry weight for that plot. The percentage of marketable fruit was evaluated throughout the season at each harvest. The US Department of Agriculture (USDA) standards were used to group fruit into US no. 1 and US no. 2 (designated as marketable), and all other fruit were designated as low-grade or unmarketable in this study.
Spray treatments were initiated when SWD eggs were detected in the fruit using a dissecting microscope. Sprays were applied weekly at the maximum label rate (Table 1) using a single-boom CO2 pressurized backpack sprayer (Bellspray Inc., Opelousas, LA, USA) calibrated to 234 L/ha. Spray treatments were applied within 1 h of sunset to minimize impacts on hymenopteran pollinators. Plants, mulch, and irrigation lines were removed each year after the first freeze.
Spray application rates for Drosophila suzukii management in day-neutral strawberries using a single-boom CO2 pressurized backpack sprayer (Bellspray Inc., Opelousas, LA, USA) calibrated to 234 L/ha.
SWD populations and fruit infestation
Four SWD monitoring traps (Scentry traps and lures; Great Lakes IPM, Vestaburg, MI, USA) were placed in the corners of the field perimeter to determine the presence and abundance of SWD adults. Traps were deployed beginning on 11 Jun 2022 and 1 Jun 2023, and they were checked weekly for the duration of the growing season. The numbers of male, female, and total SWD per trap were recorded for each week. Lures were replaced once per month.
SWD fruit infestation was measured by randomly selecting five marketable fruit per treatment plot per harvest to visually count the number of eggs under a dissecting microscope. Although the saltwater extraction and floatation method is typically used for determining SWD fruit infestation (Shaw et al. 2019; Van Timmeren et al. 2017; Van Timmeren et al. 2021), we opted for the microscopy method because of the increased accuracy of egg and first larval instar detection (Van Timmeren et al. 2021). Data were recorded for the number of SWD eggs per five fruits to determine both the proportion of infested fruit and average number of SWD eggs per fruit. Data before 1 Aug are not included because of a lack of SWD found before this date, and because spray treatments were not initiated until 15 Aug 2022 and 14 Aug 2023.
Treatments
Four treatments were applied to the strawberry plants. A botanical oil-based deterrent (KeyPlex Ecotrol® Plus; KeyPlex, Winter Park, FL, USA) sprayed weekly increased harvest frequency, a weekly rotation of organic insecticides spinosad (Entrust® SC Naturalyte® Insect Control; Dow Agrosciences, Indianapolis, IN, USA) and Chromobacterium subtsugae strain PRAA4-1T (Grandevo® WDG; Marrone Bio Innovations, Davis, CA, USA) as a positive control check, and an untreated control. All plots were harvested twice weekly (Monday and Friday), except for the frequent harvest treatment plots, which were harvested three times weekly (Monday, Wednesday, and Friday). For the botanical-based deterrent and insecticide treatments, treatments were not applied until SWD eggs were found in the fruit. This was performed to follow label instructions regarding the maximum number of applications for spinosad (three total applications allowed per season for strawberries).
Economic analysis
A baseline enterprise budget (direct costs, indirect costs, gross returns, net returns) was compiled for DNS production with no pest management assumed, representing the untreated control. Overhead expenses, including machinery leases, taxes, utilities, advertising, insurance, interest, and depreciation, were estimated using 2021 to 2022 average values reported in the FINBIN database (Center for Farm Financial Management 2024) for six farms that grow assorted vegetables in Minnesota. Day-neutral strawberries are treated as an annual crop in Minnesota and can be likened to one crop in an annual vegetable rotation when calculating machinery and other overhead expenses.
Direct material and labor costs were estimated by the research team for each treatment based on field observations and valued using local direct-to-consumer market prices. Material inputs included plants, mulch, drip line, fertilizers, alley seed, fuel, insecticides, and harvest containers. Direct cropland rental rates for Minnesota in 2021 to 2022 were obtained from the USDA (US Department of Agriculture, National Agricultural Statistics Service 2022). Direct and indirect costs for the control treatment are listed in Table 2.
Average costs and returns for day-neutral strawberries by treatment, 2022 to 2023.
Labor inputs were tracked in minutes by the research team for each activity and treatment using a stopwatch timer. Labor tasks were grouped into the following nine categories: bed preparation (including drip tape installation, alley planting, and mulch application); planting; flower and runner removal; irrigation and fertigation; pest management; weeding; mowing; harvesting; and postharvest clean-up. Tasks were identified as requiring “skilled” or “unskilled” labor and assigned accordingly to seasoned research staff or first-time student field workers. For this study, it was assumed that skilled workers earned $18.33 per hour for spring-related tasks and $19.28 per hour for fall harvest and clean-up tasks (USDA Farm Labor Report 2022 and 2023a). Unskilled labor was valued at the 2022 to 2023 average minimum wage for “small” businesses in Minnesota, equal to $8.53 per hour year-round.
We did not include postharvest material and labor estimates for storage, handling, packaging, transportation, and marketing efforts because this was outside the scope of our study and can vary substantially by market channel and proximity, thus making estimates difficult. Traditional wholesale markets and institutional channels demand significant resources for pre-harvest and postharvest sorting, grading, packaging, and labeling activities. Alternatively, sales into direct market channels (farmers markets, pick your own, community-supported agriculture) forego many of the wholesale and retail packing and labeling costs but accumulate added expenses in the form of transportation, sales, and advertising (Hardesty and Leff 2009; King et al. 2010). Gross returns were calculated by multiplying yield per plant for each treatment by the observed 2-year average Minneapolis farmers market price equal to $17.64 per kg (DiGiacomo, personal observations). Gross sales per hectare were calculated by multiplying the number of plants per hectare (35,583) by the gross return per plant. Strawberries were not differentiated by fruit quality (size and grade) or priced at a premium for organic certification status.
Net returns as well as break-even price and yield were calculated for the control in both years of the study. Net returns are equal to the income generated after subtracting direct (variable) and indirect (overhead) costs from gross sales. Total costs divided by production yield represent the break-even price, whereas total costs divided by price correspond to the break-even yield. Using the same methods and assumptions, production costs and returns for the three experimental strategies were estimated to explore the trade-offs between treatment benefits, if any, and added input costs.
Finally, a sensitivity analysis was prepared using variations on price and yield to identify profit opportunities for Midwest growers. Sensitivity analyses can be used to measure business exposure to risk, identify market opportunities, and test the robustness of results for financial feasibility and decision-making (Frey and Patil 2002). A sensitivity analysis was performed in this study by varying marketable yield and DNS prices. Observed marketable DNS yields reported in our study and found in the literature for comparable organic production systems were the basis of the yield assumptions (Bolda et al. 2022; Petran et al. 2017). Similarly, retail market price assumptions were based on observed values in the Twin Cities Metropolitan Area (TCMA). Organic strawberries produced in California were consistently sold in the TCMA for $9.92/kg at grocery store locations in July to Sep 2023. Prices ranging from $16.54/kg to $20.94/kg were observed at TCMA farmers’ markets and on-farm sale locations for fresh, locally grown Minnesota strawberries during the same period (DiGiacomo, personal observations).
Statistical analysis
Strawberry production and SWD treatment data were analyzed with R (R Core Team 2020) using a one-way analysis of variance, followed by Tukey’s post hoc tests for normally distributed data and the Kruskal-Wallis nonparametric test followed by a Wilcoxon rank-sum test for data that did not meet the requirements for normality.
Results
Strawberry production
There were significant differences across all strawberry production variables between the 2022 and 2023 growing seasons (Table 3). Cumulative total yield (g/plant) was 551.50 ± 11.91 g/plant in 2022, whereas during the 2023 season it was 459.95 ± 19.04 g/plant (Fig. 1). Marketable yield in 2022 was 387.14 ± 11.60 g/plant, and it was 343.02 ± 16.76 g/plant in 2023 (Fig. 1). Although both total and marketable yields were lower in 2023 than in 2022, the percentage of marketable fruit was slightly higher in 2023 (74.3% ± 1.1% marketable fruit) compared with that in the 2022 season (70.2% ± 1.5%). Fruit mass was also significantly greater in 2023 compared with that in 2022; strawberries had an average fruit mass of 11.15 ± 0.21 g in 2023, and it was 10.22 ± 0.15 g in 2022. However, there were fewer berries per plant in 2023, with averages of 48.94 ± 1.27 and 40.16 ± 1.33 berries produced per plant in 2022 and 2023, respectively.
Strawberry production (total yield, marketable yield, proportion of marketable fruit, number of berries, berry mass) between 2022 and 2023.
Average cumulative total (dashed lines) and marketable (solid lines) day-neutral strawberry yield among four treatments for spotted-wing drosophila during 2022 and 2023. The day-neutral strawberry yield was different between years (total: P < 0.001; marketable: P = 0.036), but it was not different among treatments in either year. Total and marketable yields were greater in 2022 than in 2023.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05461-24
The SWD management treatments had no effect on any production variable in either year (Table 4). Total cumulative yield (g/plant), marketable yield (g/plant), number of strawberries per plant, and strawberry mass (g) were all similar among the four treatments.
Strawberry production (total yield, marketable yield, number of berries, berry mass) among Drosophila suzukii management treatments in 2022 and 2023.
SWD infestation
The SWD adults were first detected in the monitoring traps on 14 Jul, and they peaked on 8 Sep in 2022. In 2023, the date of first catch was 20 Jul, and it peaked on 24 Aug. Except for one trap on 8 Sep 2022, SWD populations were generally higher in 2023 than in 2022 (Fig. 2). The SWD eggs were first found in strawberries on 12 Aug 2022 and 24 Jul 2023.
The average number of spotted-wing drosophila caught per trap (n = 4) at the Minnesota Agricultural Experiment Station, St. Paul, MN, USA, during 2022 and 2023.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05461-24
Strawberries contained an average of 1.5 to 2.3 eggs between 1 Aug and the end of the season (Table 5). The number of eggs per strawberry was similar among the four treatments in both years, and there were no statistically significant differences between the two years. The average percentage of SWD-infested fruit ranged from 28% to 40%, depending on the year and treatment (Table 5), and it occasionally exceeded 70% (Fig. 3). The proportion of infested fruit was significantly different among the four treatments in 2022, with the insecticide treatment having a lower proportion of infestation (0.28 ± 0.03) than that of the control (0.40 ± 0.03) (P = 0.0217). The treatments had similar proportions of SWD-infested fruit in 2023 (Table 4).
Drosophila suzukii (SWD) infestation (number of eggs per berry and proportion of infested berries) in day-neutral strawberries among SWD management treatments in 2022 and 2023. Data are from 1 Aug through the end of the season in both years to correspond with the peak harvest window and are attributable to a lack of SWD infestation before this time.
The average proportion of strawberries with at least one spotted-wing drosophila (SWD) egg in 2022 and 2023. Average percentage of SWD egg-infested fruit varied from 28% to 40% across different years and treatments, and it occasionally surpassed 70%. Vertical dashed lines represent the dates of spray applications, whereas the solid line represents the average infestation across treatments. Significant differences in infestation proportions among treatments were observed in 2022, with insecticide treatment resulting in less infestation (0.28 ± 0.03) compared with that of the control (0.40 ± 0.03) (P = 0.022); no differences were observed in 2023.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05461-24
Economics
Direct and indirect costs for the control treatment totaled $153,672/ha (Table 2). Indirect costs for the control and all experimental treatments were $3702/ha (2% of the total costs for the control treatment). For the experimental treatments, the organic insecticide generated $164,131/ha in direct costs compared with $164,330/ha for the botanical treatment and $167,999/ha for the frequent harvest treatment. The frequent harvest treatment required the highest level of skilled labor among all treatments, thus skewing direct costs higher compared with those of other treatments. Labor accounted for the majority of direct costs for all treatments. For the control, labor accounted for 86% of direct costs compared with 87% to 88% for the treated plots, on average, over the 2-year period.
Assuming no difference in marketable yield (all treatment yields were assumed to equal the control yield based on Tukey’s post hoc test; Table 4), net returns from the control, $82,600/ha, were significantly higher than net returns from any treatment. Average net returns from the pest control treatments were $64,571/ha for frequent harvest, $68,240 for botanical applications, and $68,439 for the organic insecticides (Table 2). When observed numerical marketable yields are substituted, the results reverse themselves, with the frequent harvest treatment generating the greatest return among insect management treatments, with $78,805/ha for the frequent harvest compared with $52,591/ha and $41,395/ha for the botanical and organic insecticide sprays, respectively. All three pest management treatments, however, net less than the control ($82,600/ha) when observed numerical yields were used in the calculations. The added labor expense of the pest control treatments, which had no effect on pest management or plant yield, resulted in lower net returns than the control in both yield scenarios.
The break-even price and yield were calculated on a per-plant basis to assist growers with pricing and production strategies. The break-even price, which is the market price at which strawberries would recover direct and indirect costs, ranged from $11.55 per kg (control) to $14.44 per kg (organic insecticides) when numerical study yields were counted (Fig. 4). Organic strawberries produced outside the Midwest consistently sold for $9.92/kg in 2023 at a major TCMA retailer and $17.64/kg at the largest TCMA farmers market during 2022 to 2023.
Break-even price and yield for ‘Cabrillo’ day-neutral strawberries using the numerical study yield of each treatment.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05461-24
Break-even yields represent the yield required for a grower to breakeven on expenses at a specified market price. When valued at $17.64/kg (the 2-year average TCMA farmers market price), the break-even yield for DNS in our study ranged from 0.25 kg/plant (8540 kgs/ha) for the control to 0.28 kg/plant (9963 kg/acre) for the frequent harvest and other treatments (Fig. 4). Numerical (observed) yields in the study were well above break-even and ranged from 0.34 kg/plant (organic insecticide) to 0.40 kg/plant (frequent harvest). The break-even yield would increase if the market price were reduced.
Regarding the sensitivity analysis, we set the starting price point as $9.92/kg, which is the retail price for organic strawberries produced out of state and marketed at a major TCMA grocery store. At this value, organic strawberries grown in Minnesota do not break even and, in fact, lose money when yield is less than 0.57 kg/plant. As price and yield improve, net returns improve. In the TCMA, direct-marketed organic strawberry prices ranged from $16.53/kg to $20.94/kg. Under these direct-market price assumptions, organic DNS would net a positive net return of $52,678/ha to $106,023/ha when plants yield ≥0.34 kg of berries per plant (Table 6). In fact, DNS begin to yield positive net returns when priced at $13.23/kg with yields of 0.34 kg/plant.
Net returns for day-neutral strawberry controli with various yield and price ($/ha).
The sensitivity analysis demonstrated that locally grown and marketed organic strawberries in the Upper Midwest have the potential to generate lucrative net returns when yields average ≥0.34 kg/plant and berries are priced for direct sale to consumers. The 2-year average yield for untreated berries in our study was 0.38 kg/plant.
Discussion
The four management treatments for SWD did not significantly affect strawberry production, although there was a slight increase in marketable yield for the frequent harvest treatment in both years. The shorter intervals between harvest may have resulted in less over-ripened and environmentally damaged (e.g., sun-scalded or rain- and hail-damaged) fruit (Leach et al. 2017). Our total strawberry yields of 418 to 582 g/plant was comparable to other DNS studies in the Upper Midwest. Previous studies determined that the total yield (g/plant) was between 290 g and 667 g (Anderson et al. 2019; Petran et al. 2017).
Strawberry production varied between the two years, with 2023 having lower total and marketable yields but a higher proportion of marketable fruit. Environmental and other pest conditions during the growing season may have resulted in these differences in fruit production. Strawberry plants have complex responses to environmental conditions (Durner et al. 1984; Hughes et al. 2017; Kadir et al. 2006). Temperature influences strawberry flowering and resource allocation, and even day-neutral cultivars can exhibit long-day flowering response at high temperatures (Durner et al. 1984). Average daytime temperatures higher than 28.9 °C can inhibit DNS flower production (Kadir et al. 2006). From the beginning of May until the end of October in 2022 there were 36 d with high temperatures above 28.9 °C; in 2023, there were 53 d with high temperatures above 28.9 °C. The additional 17 d above the maximum temperature threshold for day-neutral strawberries may have contributed to lower yields in 2023. Kadir et al. (2006) suggested that photosystem II may be irreversibly damaged at temperatures between 30 and 40 °C.
Additionally, DNS produce more runners at temperatures between 26 and 30 °C (Durner et al. 1984), which may lead to fewer resources allocated to strawberry production. Hughes et al. (2017) found that weekly DNS runner removal resulted in higher yields than removing runners three times or not at all, although the labor cost was twice as high. In 2023, we stopped removing runners after 1 Aug to reduce labor hours. The combination of less runner removal as well as potentially more runner production because of warmer temperatures may have resulted in an even greater effect on fruit production. In the Upper Midwest, the impact of climate change (e.g., more days >28.9 °C and higher dew points) could limit DNS production. Comparing climate models for this region and DNS temperature thresholds could provide information regarding whether investing in the DNS industry in this region is viable in the long-term.
Although not the focus of this study, tarnished plant bug (Lygus lineolaris; TPB) damage affected the marketability of the strawberries. The TPB is a major insect pest of strawberries and many other crops (Wold and Hutchison 2003; Zalom et al. 2018). Damage to strawberries occurs when TPB nymphs feed on the flower receptacle with their piercing and sucking mouthparts, resulting in misshapen, small, and unmarketable fruit (Wold and Hutchison 2003). We observed lower TPB counts and damaged fruit in 2023 than in 2022 (Supplemental Table 1), which may have contributed to larger berries and a higher percentage of marketable fruit in 2023 than in 2022. However, TPB management in organic strawberry production is challenging; therefore, future research should investigate strategies to reduce TPB damage to organic DNS in the region. There is increasing interest in growing DNS in high tunnels and other controlled environments. In these settings, fine-mesh exclusion netting may effectively prevent SWD infestations (Ebbenga et al. 2019; Kuesel et al. 2019; Leach et al. 2016; Rogers et al. 2016; Stockton et al. 2020) and damage from other insects such as TPB. Future studies should investigate the economic viability of these production settings compared with that of open-field growing conditions.
Our results confirmed that wild SWD oviposit in DNS in the Upper Midwest, and that the average percentage of infested fruit is 33%, although specific dates exceeded 70%. Ganjisaffar et al. (2023) reported that SWD is becoming more prevalent in California DNS production, likely because of emerging insecticide resistance to spinosad, spinetoram, malathion, and pyrethroids. We observed lower infestation in DNS compared with that reported by other studies in fall-bearing (primocane) raspberries (i.e., 80%) (Rogers et al. 2016) for the Upper Midwest.
Overall, there were low numbers of eggs found in each strawberry, with an average of 1.5 to 2.3 eggs. The number of eggs per strawberry was comparable to what researchers have found in California (Ganjisaffar et al. 2023). Organic insecticides reduced SWD infestation compared with the control only in 2022, and none of the other treatments reduced infestation in either year. This could be attributable to the short residual activity of organic insecticides and botanical treatments. The conventional insecticides such as pyrethroids and organophosphates can provide between 5 and 14 d of SWD control, depending on weather conditions (Bruck et al. 2011), whereas organic insecticides are effective for less than 5 d (Sial et al. 2019). The volatilization rate of the botanical treatment is unknown, and there is no re-entry interval or preharvest interval associated with the product tested, suggesting short residual activity.
We observed lower abundances of SWD in baited monitoring traps on site in both years of our study (<100 individuals/trap/sampling date) (Fig. 2) compared with the amount of SWD (>1000 individuals/trap/sampling date) collected in other years and in different parts of Minnesota (University of Minnesota Extension 2024). The low abundance of SWD at the experimental location may have contributed to a lack of difference in infestation rates among the treatments. However, other studies have reported that traps do not accurately predict local populations of SWD (Hampton et al. 2014).
One limitation of this study was that the area treated with insecticides did not cover the entire strawberry field and the proximity of treatment plots to each other could have allowed SWD to re-infest the insecticide plots soon after insecticide degradation. Because SWD can fly up to 283 m at a time (Tran et al. 2022), they may have traveled from the control treatment plots, buffers, or other fruit crops nearby to the other three treatments. Therefore, insecticides and repellents may be more effective than we observed in this experiment if they are applied to the entire field and there are limited nearby sources of SWD to reinfest the fields.
The most striking observation in the study was perhaps the high cost of labor. This may not be surprising at first glance, however. Research has found that fresh strawberry production is one of the most labor-intensive specialty crops (Calvin and Martin 2010). However, despite management modifications to conserve labor in 2023, labor rates observed in Minnesota remained almost 2.5-times the rates reported in California, where labor has been shown to account for 36% of direct operating costs (Calvin and Martin 2010). By comparison, labor for the field trials accounted for 86% (control) to 88% (organic insecticide treatment) of direct costs.
In Minnesota, several of the most time-consuming labor tasks (bed prep, planting, mulch and irrigation installation, flower and runner removal, and postharvest clean-up) accounted for more than 50% of labor costs. These costs were distributed over the 5-month growing season, whereas in California these expenses are allocated to the enterprise over a 10-month period, making the marginal costs of strawberry production significantly more competitive. For these and other reasons, fresh strawberry production in Minnesota and other Midwest states cannot compete economically with year-round growers, i.e., California or Florida, in terms of labor efficiencies and cost, particularly after factoring in postharvest handling, packaging, and marketing expenses.
Instead, growers who operate in regions with relatively short production seasons, like the Upper Midwest, will need to market strawberries as a specialty product and appeal to consumer preferences for locally grown and organic characteristics with superior flavor and quality to justify price points well above those of supermarket berries distributed from California and other major strawberry production regions. If we had priced the local strawberries at wholesale values, which are often 50% of retail, then net returns would have been negative at almost any achievable marketable yield. An additional worthwhile study should investigate consumer willingness to pay for locally grown and organic strawberries in the Upper Midwest.
Conclusion
Although DNS have the potential to become a widely produced and sold local fresh fruit in the Upper Midwest, key pest and economic challenges may limit industry growth. Organic insecticide applications inconsistently reduced SWD infestation. The low numbers of eggs in any given fruit suggest that postharvest cold storage may be more economical for preserving fresh fruit for markets than implementing more costly and inconsistent management techniques.
References cited
Aly MFK, Kraus DA, Burrack HJ. 2017. Effects of postharvest cold storage on the development and survival of immature Drosophila suzukii (Diptera: Drosophilidae) in artificial diet and fruit. J Econ Entomol. 110(1):87–93. https://doi.org/10.1093/jee/tow289.
Anderson HC, Rogers MA, Hoover EE. 2019. Low tunnel covering and microclimate, fruit yield, and quality in an organic strawberry production system. HortTechnology. 29(5):590–598. https://doi.org/10.21273/HORTTECH04319-19.
Asplen MK, Anfora G, Biondi A, Choi DS, Chu D, Daane KM, Gibert P, Gutierrez AP, Hoelmer KA, Hutchison WD, Isaacs R, Jiang ZL, Kárpáti Z, Kimura MT, Pascual M, Philips CR, Plantamp C, Ponti L, Vétek G, Vogt H, Walton VM, Yu Y, Zappalà L, Desneux N. 2015. Invasion biology of spotted wing drosophila (Drosophila suzukii): A global perspective and future priorities. J Pest Sci. 88(3):469–494. https://doi.org/10.1007/s10340-015-0681-z.
Atallah J, Teixeira L, Salazar R, Zaragoza G, Kopp A. 2014. The making of a pest: The evolution of a fruit-penetrating ovipositor in Drosophila suzukii and related species. Proc Biol Sci. 281(1781):20132840–20132849. https://doi.org/10.1098/rspb.2013.2840.
Bellamy DE, Sisterson MS, Walse SS. 2013. Quantifying host potentials: Indexing postharvest fresh fruits for spotted wing drosophila, Drosophila suzukii. PLoS One. 8(4):e61227. https://doi.org/10.1371/journal.pone.0061227.
Biondi A, Mommaerts V, Smagghe G, Viñuela E, Zappalà L, Desneux N. 2012. The non-target impact of spinosyns on beneficial arthropods. Pest Manag Sci. 68(12):1523–1536. https://doi.org/10.1002/ps.3396.
Bolda M, Murdock J, Goodrich B, Summer DA. 2022. Sample costs to produce and harvest organic strawberries. https://coststudyfiles.ucdavis.edu/uploads/pub/2022/08/17/2022organicstrawberrycc-final_draft-august_2022.pdf. [accessed 22 Aug 2024].
Bruck DJ, Bolda M, Tanigoshi L, Klick J, Kleiber J, Defrancesco J, Gerdeman B, Spitler H. 2011. Laboratory and field comparisons of insecticides to reduce infestation of Drosophila suzukii in berry crops. Pest Manag Sci. 67(11):1375–1385. https://doi.org/10.1002/ps.2242.
Calvin L, Martin P. 2010. The U.S. produce industry and labor: Facing the future in a global economy. Agric Labor United States Considerations Sel Res. 106:1–57.
Center for Farm Financial Management. 2024. FINBIN. University of Minnesota. Retrieved from http://finbin.umn.edu. (Report number 852761 originally created 2 Feb 2024).
Cha DH, Roh GH, Hesler SP, Wallingford A, Stockton DG, Park SK, Loeb GM. 2020. 2-Pentylfuran: A novel repellent of Drosophila suzukii. Pest Manag Sci. 77(4):1757–1764. https://doi.org/10.1002/ps.6196.
DiGiacomo G, Gullickson MG, Rogers M, Peterson HH, Hutchison WD. 2021. Partial budget analysis of exclusion netting and organic-certified insecticides for management of spotted-wing drosophila (Diptera: Drosophilidae) on small farms in the upper midwest. J Econ Entomol. 114(4):1655–1665. https://doi.org/10.1093/jee/toab087.
Durner EF, Barden JA, Himelrick DG, Poling EB. 1984. Photoperiod and temperature effects on flower and runner development in day-neutral, junebearing, and everbearing strawberries. J Amer Soc Hort Sci. 109(3):396–400. https://doi.org/10.21273/JASHS.109.3.396.
Ebbenga DN, Burkness EC, Hutchison WD. 2019. Evaluation of exclusion netting for spotted-wing drosophila (Diptera: Drosophilidae) management in Minnesota wine grapes. J Econ Entomol. 112(5):2287–2294. https://doi.org/10.1093/jee/toz143.
Farnsworth D, Hamby KA, Bolda M, Goodhue RE, Williams JC, Zalom FG. 2017. Economic analysis of revenue losses and control costs associated with the spotted wing drosophila, Drosophila suzukii (Matsumura), in the California raspberry industry. Pest Manag Sci. 73(6):1083–1090. https://doi.org/10.1002/ps.4497.
Frey HC, Patil SR. 2002. Identification and review of sensitivity analysis methods. Risk Anal. 22(3):553–578. https://doi.org/10.1111/0272-4332.00039.
Ganjisaffar F, Abrieux A, Gress BE, Chiu JC, Zalom FG. 2023. Drosophila infestations of California strawberries and identification of Drosophila suzukii using a TaqMan assay. Appl Sci. 13(15):8783.https://doi.org/10.3390/app13158783.
Gress BE, Zalom FG. 2018. Identification and risk assessment of spinosad resistance in a California population of Drosophila suzukii. Pest Manag Sci. https://doi.org/10.1002/ps.5240.
Gullickson M, Hodge CF, 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(8):1–12. https://doi.org/10.3390/insects11080536.
Gullickson MG, Rogers MA, Burkness EC, Hutchison WD. 2019. Efficacy of organic and conventional insecticides for Drosophila suzukii when combined with erythritol, a non-nutritive feeding stimulant. Crop Prot. 125:104878. https://doi.org/10.1016/j.cropro.2019.104878.
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):1849–1857. https://doi.org/10.1603/EC14232.
Hardesty SD, Leff P. 2009. Determining marketing costs and returns in alternative marketing channels. Renew Agric Food Sys. 25(1):24–34. https://doi.org/10.1017/S1742170509990196.
Hughes BR, Zandstra J, Taghavi T, Dale A. 2017. Effects of runner removal on productivity and plant growth of two day-neutral strawberry cultivars in Ontario, Canada. Acta Hortic. 1156:327–332. https://doi.org/10.17660/ActaHortic.2017.1156.50.
Kadir S, Sidhu G, Al-Khatib K. 2006. Strawberry (Fragaria x ananassa Duch.) growth and productivity as affected by temperature. HortScience. 41(6):1423–1430. https://doi.org/10.21273/HORTSCI.41.6.1423.
King RP, Hand MS, DiGiacomo G, Clancy K, Gomez M, Hardesty S, Lev L, McLaughlin EW. 2010. Comparing the structure, size, and performance of local and mainstream food supply chains. USDA, Economic Research Service. Report 99. https://www.ers.usda.gov/publications/pub-details/?pubid=46407. [accessed 22 Aug 2024].
Kuesel R, Scott Hicks D, Archer K, Sciligo A, Bessin R, Gonthier D. 2019. Effects of fine-mesh exclusion netting on pests of blackberry. Insects. 10(8):249. https://doi.org/10.3390/insects10080249.
Leach H, Moses J, Hanson E, Fanning P, Isaacs R. 2017. Rapid harvest schedules and fruit removal as non-chemical approaches for managing spotted wing drosophila. J Pest Sci. 91(1):219–226. https://doi.org/10.1007/s10340-017-0873-9.
Leach H, Van Timmeren S, Isaacs R. 2016. Exclusion netting delays and reduces Drosophila suzukii (Diptera: Drosophilidae) infestation in raspberries. J Econ Entomol. 109(5):2151–2158. https://doi.org/10.1093/jee/tow157.
Luby JJ. 1989. Midwest and Plains states strawberry cultivars. Fruit Var J. 43(1):20–31.
Pedigo LP, Rice ME, Krell RK. 2021. Entomology and pest management, Seventh. Waveland Press, Inc., Long Grove, IL, USA.
Petran A, Hoover E, Hayes L, Poppe S. 2017. Yield and quality characteristics of day-neutral strawberry in the United States Upper Midwest using organic practices. Biol Agric Hortic. 33(2):73–88. https://doi.org/10.1080/01448765.2016.1188152.
R Core Team. 2020. R: A language and environment for statistical computing.
Renkema JM, Buitenhuis R, Hallett RH. 2017. Reduced Drosophila suzukii infestation in berries using deterrent compounds and laminate polymer flakes. Insects. 8(4). https://doi.org/10.3390/insects8040117.
Renkema JM, Wright D, Buitenhuis R, Hallett RH. 2016. Plant essential oils and potassium metabisulfite as repellents for Drosophila suzukii (Diptera: Drosophilidae). Sci Rep. 6(October 2015):21432. https://doi.org/10.1038/srep21432.
Rogers MA, Burkness EC, Hutchison WD. 2016. Evaluation of high tunnels for management of Drosophila suzukii in fall-bearing red raspberries: Potential for reducing insecticide use. J Pest Sci. 89(3):815–821. https://doi.org/10.1007/s10340-016-0731-1.
Samtani JB, Rom CR, Friedrich H, Fennimore SA, Finn CE, Petran A, Wallace RW, Pritts MP, Fernandez G, Chase CA, Kubota C, Bergefurd B. 2019. The status and future of the strawberry industry in the United States. HortTechnology. 29(1):11–24. https://doi.org/10.21273/HORTTECH04135-18.
Shaw B, Cannon MFL, Buss DS, Cross JV, Brain P, Fountain MT. 2019. Comparison of extraction methods for quantifying Drosophila suzukii (Diptera: Drosophilidae) larvae in soft- and stone-fruits. Crop Prot. 124(May):104868. https://doi.org/10.1016/j.cropro.2019.104868.
Sial AA, Roubos CR, Gautam BK, Fanning PD, Van Timmeren S, Spies J, Petran A, Rogers MA, Liburd OE, Little BA, Curry S, Isaacs R. 2019. Evaluation of organic insecticides for management of spotted-wing drosophila (Drosophila suzukii) in berry crops. J Appl Entomol. 143(6):593–608. https://doi.org/10.1111/jen.12629.
Stockton D, Wallingford A, Rendon D, Fanning P, Green CK, Diepenbrock L, Ballman E, Walton VM, Isaacs R, Leach H, Sial AA, Drummond F, Burrack H, Loeb GM. 2019. Interactions between biotic and abiotic factors affect survival in overwintering Drosophila suzukii (Diptera: Drosophilidae). Environ Entomol. 48(2):454–464. https://doi.org/10.1093/ee/nvy192.
Stockton DG, Hesler SP, Wallingford AK, Leskey TC, McDermott L, Elsensohn JE, Riggs DIM, Pritts M, Loeb GM. 2020. Factors affecting the implementation of exclusion netting to control Drosophila suzukii on primocane raspberry. Crop Prot. 135(February):105191. https://doi.org/10.1016/j.cropro.2020.105191.
Tran AK, Kees AM, Hutchison WD, Aukema BH, Rao S, Rogers MA, Asplen MK. 2022. Comparing Drosophila suzukii flight behavior using free-flight and tethered flight assays. Entomologia Exp Applicata. 170(11):973–981. https://doi.org/10.1111/eea.13222.
US Department of Agriculture, National Agricultural Statistics Service. 2022. Farm labor. https://downloads.usda.library.cornell.edu/usda-esmis/files/x920fw89s/pv63h9083/gq67m157z/fmla1122.pdf. [accessed 22 Aug 2024].
US Department of Agriculture, National Agricultural Statistics Service. 2023a. Farm labor. https://downloads.usda.library.cornell.edu/usda-esmis/files/x920fw89s/v405tw18s/dn39zk84n/fmla1123.pdf. [accessed 22 Aug 2024].
US Department of Agriculture, National Agricultural Statistics Service. 2023b. Fruit and Tree Nuts Yearbook Tables, Table D-8-Strawberries: Acreage, production, season-average grower price, and value, United States, 1980 to date. https://www.ers.usda.gov/data-products/fruit-and-tree-nuts-data/fruit-and-tree-nuts-yearbook-tables/#Berries. [accessed 10 Jul 2024].
University of Minnesota Extension. 2024. Historical SWD Trap Data. FruitEdge. https://fruitedge.umn.edu/historical-swd-trap-data.
Van Timmeren S, Davis AR, Isaacs R. 2021. Optimization of larval sampling method for monitoring Drosophila suzukii (Diptera: Drosophilidae) in blueberries. J Econ Entomol. 114(4):1–11. https://doi.org/10.1093/jee/toab096.
Van Timmeren S, Diepenbrock LM, Bertone MA, Burrack HJ, Isaacs R. 2017. A filter method for improved monitoring of Drosophila suzukii (Diptera: Drosophilidae) larvae in fruit. J Integr Pest Manag. 8(1):1–7. https://doi.org/10.1093/jipm/pmx019.
Van Timmeren S, Isaacs R. 2013. Control of spotted wing drosophila, Drosophila suzukii, by specific insecticides and by conventional and organic crop protection programs. Crop Prot. 54:126–133. https://doi.org/10.1016/j.cropro.2013.08.003.
Van Timmeren S, Mota-Sanchez D, Wise JC, Isaacs R. 2018. Baseline susceptibility of spotted wing drosophila (Drosophila suzukii) to four key insecticide classes. Pest Manag Sci. 74(1):78–87. https://doi.org/10.1002/ps.4702.
Wallingford AK, Connelly HL, Dore Brind’Amour G, Boucher MT, Mafra-Neto A, Loeb GM. 2016. Field evaluation of an oviposition deterrent for management of spotted-wing drosophila, Drosophila suzukii, and potential nontarget effects. J Econ Entomol. 109(4):1779–1784. https://doi.org/10.1093/jee/tow116.
Walsh DB, Bolda MP, Goodhue RE, Dreves AJ, Lee J, Bruck DJ, Walton VM, O’Neal SD, Zalom FG. 2011. Drosophila suzukii (Diptera: Drosophilidae): Invasive pest of ripening soft fruit expanding its geographic range and damage potential. J Integr Pest Manag. 2(1):G1–G7. https://doi.org/10.1603/IPM10010.
Wold SJ, Hutchison WD. 2003. Phenology of Lygus lineolaris (Hemiptera: Miridae) in Minnesota June-bearing strawberries: Comparison of sampling methods and habitats. J Econ Entomol. 96(6):1814–1820. https://doi.org/10.1093/jee/96.6.1814.
Zalom FG, Bolda MP, Stockton DG. 2018. Lygus Bugs (Western Tarnished Plant Bug). UC IPM Pest Management Guidelines: Strawberry. UC ANR Publ. 3468.