Impact of Mulches on Management of Spotted Wing Drosophila, Fruit Yield and Quality

Final report for GNC18-265

Project Type: Graduate Student
Funds awarded in 2018: $11,987.00
Projected End Date: 10/31/2021
Grant Recipient: University of Wisconsin - Madison
Region: North Central
State: Wisconsin
Graduate Student:
Faculty Advisor:
Dr. Christelle Guédot
University of Wisconsin - Madison
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Project Information


The North Central region's fruit industry is experiencing substantial damage from the invasive spotted wing drosophila (SWD) fruit fly, with Wisconsin grape and berry growers identifying SWD as their top insect pest. Fruit growers worldwide are facing similar problems, with few effective, sustainable solutions. Management of SWD relies heavily on chemical control, which detriments economic and environmental sustainability, and does not provide adequate control of this pest. Growers apply broad-spectrum insecticides every 4-7 days, raising concerns about consequences on natural enemies, pollinators, secondary pest outbreaks, soil health, and overall land stewardship. Excessive insecticide use threatens the quality of life of farmers, as well as surrounding communities. Growers express a strong interest in alternative management strategies that significantly decrease chemical inputs and costs.

Our proposal, titled "Impact of mulches on management of spotted wing drosophila, fruit yield and quality", examined the impact of plastic mulches on spotted-wing drosophila adult and larval populations and canopy microclimate conditions relevant to the fly. We tested 3 types of plastic mulches including metallic polyethylene, black biodegradable, and white-on-black biodegradable.  We used passive trapping with clear sticky cards to assess adult populations of SWD in the canopy and the salt float method to count the number of larvae in fruit. Canopy temperature, RH, and light intensity were monitored continuously using HOBO data loggers. For a more complete view of the light conditions in the canopy, we measured radiance over 4 days using a spectrometer.

Over the two years of this study, we found that black, white, and metallic plastic mulches reduced adult SWD populations in the canopy by 41-52% and larval populations by 52-72% compared to the grower standard. The mulches did not change canopy temperature or relative humidity, but metallic mulches increased canopy light intensity compared to the black mulch. Radiance in UV spectrum (380-400 nm) was higher in the canopy above all three plastic mulches, suggesting that this could be deterring adult SWD. Future studies will determine whether changes in radiance are associated with the reported reduction in SWD  populations. In 2021, we assessed plant growth, yield, and fruit quality at our research plot established at the West Madison Agricultural Research Station, since collecting yield data and destructive sampling of fruit is not feasible on our grower-collaborator's farm. Data analysis for this objective is ongoing. Overall, plastic mulches are a promising cultural practice for managing D. suzukii since they can reduce adult and larval populations. 

We evaluated the efficacy of our research in achieving the desired outcomes by creating an advisory panel with growers (which met in 2019 and 2020), and conducting pre- and post-study grower surveys online through the Wisconsin Fruit website and at presentations given at the Wisconsin Fresh Fruit and Vegetable Conference. Our results are relevant to North Central region fruit growers experiencing damage from SWD, as well as fruit growers worldwide. Ultimately, we will provide recommendations to fruit growers for achieving more sustainable management of SWD, subsequently reducing insecticide inputs, and increasing environmental and economic sustainability of fruit production.

Project Objectives:

The expected learning outcomes for this research are as follows: 1) researchers will learn how mulch color and reflectivity affect SWD management, fruit yield and quality; 2) researchers will learn how to use mulches to effectively manage SWD and increase fruit yield and quality; 3) 30% of growers surveyed will have increased knowledge of how mulches impact SWD management, fruit yield and quality. The expected action outcomes are that: 1) growers will start using mulches for SWD management; 2) growers that use mulches will achieve more effective SWD management, higher fruit yield and quality, and decrease insecticide applications for SWD.


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Materials and methods:

Objective 1 was conducted on a small commercial farm in Iowa County, WI, USA in 2019 and 2020. Two rows each of cultivars ‘Caroline’ and ‘Polana’ were used. Primocanes from 2018 were cut in mid-December 2018, and straw mulch and chicken manure were applied in February. Rows are approximately 30 m in length and 0.51 m wide. Distance between row centers is 3.05 m.The experimental design was a split plot with four mulch treatments (black biodegradable, white biodegradable, metallic polyethylene, control; Figure 1) replicated in two rows of each cultivar (Caroline, Polana). Blocking occurred by row. There was a total of 16 treatment plots that were all 7.6 m x 1.5 m.

Mulches were applied by hand each year in late April when raspberry canes were just emerging from below straw mulch. In each plot, 7.6 m long mulch strips laid on each side of the row, with a ~10 cm gap down the center of the row. The outside edges of the mulch were rolled under so that each side extended 75 cm from the center of the row. All edges were secured with 6” sod staples placed about every 30 cm. Mulch was installed around drip tape, which was installed down the middle of each row by the grower-collaborator prior to mulch laying.

Plots were irrigated identically as necessary by our grower-collaborator. Weeds were removed by hand from the gap between mulches and in the control plots as needed. No insecticides were applied in either year. 

Adult D. suzukii population was assessed using clear sticky cards after adult flies were detected in the raspberry patch. Three Scentry SWD traps (Scentry Biologicals, Billings, MT) were placed in the raspberry patch on May 23, 2019. Traps included Scentry SWD lures and an apple cider vinegar (ACV) killing agent with 100 mL ACV + 1 drop unscented dish soap. The first D. suzukii adult was detected in the traps on June 27, 2019. This prompted sticky cards to be deployed weekly until adult D. suzukii were no longer detected in liquid traps.  Clear sticky cards (Alpha Scents, West Linn, OR) cut to be 15.25 cm2 were placed in the fruiting zone in the center of every treatment plot. Sticky cards were clipped to a wooden stake using a binder clip. The wooden stake was attached to ‘sensor stations’, consisting of a 1.8 m metal U stake with L-shaped shelf supports use to suspend sensors in the fruiting zone. Shelf supports could be moved vertically along the U stake to account for changes in canopy height throughout the growing season. Sticky card height was adjusted as need to stay in the fruiting zone. Sticky cards were replaced every 7 d, and the number of male and female D. suzukii was recorded.

To assess larval infestation of fruit, 36 ripe fruits (~100 g) were randomly collected from each plot. Half of each sample was used in salt floats to determine the number of larvae in the fruit.  The other half was placed in rearing cups and flies were reared to count adult fly emergence and determine what proportion of emerged flies are D. suzukii. The calculated proportion was used as a multiplier to determine actual larval infestation in the fruit.

Rearing cups were assembled by placing 4 oz. portion cups inside of taller 5 oz. clear plastic portion cups (Comfy Package, Brooklyn, NY). Small holes were poked in the bottom of the 4 oz. cups to allow raspberry juice to drain into the 5 oz. cups. A large circular hole (~ 4cm diameter) was cut in the lids of the portion cups. Lids were attached on top of mesh to allow air flow into the container. Rearing cups with fruit were kept at ambient lab conditions for 3 weeks, and then all flies were identified and counted. 

Canopy temperature, relative humidity (RH), and light intensity were monitored continuously starting in early July both years. All sensors were attached to ‘sensor stations’ using shelf supports as described for sticky cards. Sensors were hung in the fruiting zone, and height was modulated by moving the L-shaped shelf supports as necessary. Temperature and RH were measured using HOBO U23 Pro v2 Data Loggers (OnSet, Bourne, MA) attached underneath a 25.4 cm diameter white plastic plate to protect sensors from sunlight and rain. Light intensity was monitored using HOBO Pendant MX Temperature/Light data loggers attached to wooden stakes adjacent to the sticky cards.

Objective 2 was assessed in our experimental plot at the West Madison Agricultural Research Station in Verona, WI in 2019-2021. The plants were planted on May 15, 2019 from bare root plants of the fall-bearing cultivar ‘Caroline’. Plants were established at 0.5 m spacing within the row and 4 m between rows for a total of 6 rows of 37 m long.  Alleyways were planted with orchard grass and are mowed periodically during the growing season. The outer two rows are designated as guard rows and are not covered with plastic mulch. Each plot includes 1.3 cm diameter drip tape with emitters every 5.1 cm placed down the middle of each row. Plots are irrigated three times per week for 30 minutes during the growing season.

Each year, mulches were applied by hand in late April or early May when raspberry canes are just emerging from the ground. Two mulch strips of 7.4 m long and 0.7 m wide were laid on each side of the row with a ~10 cm gap down the center of the row for canes to grow. All edges were secured with 15.3 cm biodegradable stakes (Eco Turf Midwest, Bensenville, IL) spaced ~ 30 cm apart. Since the plastic mulches restrict the area where raspberry canes could emerge, canes growing outside the 10 cm center strip in the control plots were pruned in June each year. The mulch treatments include black biodegradable (AG Film in 0.9 mil thickness, Organix Solutions, Bloomington, MN), white-on-black biodegradable (AG Film in 0.9 mil), and metallic-on-black polyethylene (SHINE N’RIPE in 1.25 mil in 2019, SHINE N’RIPE XL in 3 mil in 2020-2022, Imaflex, Montreal, Quebec), and a grower standard control, where grass filled in the space between the raspberry canes and the alleyway. The treatments were set up in the same randomized complete block design each year, with all four treatments in each block, totaling 20 treatment plots that are 7.4 m long and 1.5 m wide.

Soil temperature and moisture were continuously logged throughout the growing season in one block using HOBO USB Micro Station Data Loggers attached to two 12-Bit Temperature Smart Sensors (OnSet, Bourne, MA) and two 10HS Soil Moisture Smart Sensors (Onset, Bourne, MA) per plot. One of each sensor was installed on each side of the plots 10 cm from the inner edge of the mulch in the root zone 10 cm below the soil surface spaced equidistantly between irrigation emitters. Soil data was collected in 2019-2021.

To evaluate plant establishment during the first growing season (2019), we took monthly measurements of cane density and length and recorded the end of season biomass. Plant growth was quantified monthly from June 17 to September 19, 2019 as primocane density (the number of primocanes per plot) and tallest primocane length from 8 permanent plants in the center of each plot. Primocane number was determined by counting all primocanes emerging from the base of the crown. Tallest primocane length was measured from the base of the crown to the tallest leaf tip. We also counted the number of laterals on the tallest primocane in September, 2019. End of season biomass from the center 1m of each plot was measured on December 4, 2019 after plants had dried completely. All canes from the center 1 m of each plot were clipped at ground-level and weighed in the field. To quantify plant growth in the subsequent three growing seasons, we measured end of season primocane density in the center 1 m of each plot, length of the tallest 3 canes, number of laterals on the tallest 3 canes, and biomass from the center 1 m of each plot in early December each year in 2019-2021.

To evaluate marketable yield, we harvested fruit from the center 1m of every plot in all 5 blocks every Monday, Wednesday, and Friday, sorting fruit into separate ‘marketable’ and ‘unmarketable’ containers and weighing each container separately. Yield data was collected when there was enough harvestable fruit, around early September until mid-October in 2021.

We evaluated fruit quality twice per season by sampling 25 marketable fruits from each plot in all 5 blocks. In the lab, samples were weighed, and fruit firmness is measured using a 1 mm·s-1 test speed in a Texture Analyzer (TA. XTPlus Connect, Textural Technologies, Hamilton, MA, USA).  The sample is then smashed and sieved through fine mesh to remove seeds and pulp and assayed for soluble solid content (Brix) using 200µL filtered juice pipetted on the stage of a digital refractometer. For each sample, 5 mL raspberry juice is diluted with 60mL DI water (dilution factor = 2) and titrated to pH 8.2 with 0.1 N NaOH using a titrator. Titratable acidity is calculated as percent citric acid. Anthocyanin content is measured using a modified microplate-adapted Folin-Ciocalteu method. Data was collected twice per season in early and late September 2021.

Research results and discussion:

All three mulch treatments reduced the presence of female flies, with females present less often on sticky cards in the black mulch plots than in the control plots. The mulch treatments also reduced the number of female flies, with lower numbers of females trapped in all mulch treatments compared to the controls. The mulch treatments reduced the presence of male flies, with males present less often on sticky cards in the black and white mulch plots compared to the control plots. There was no difference in the number of male flies trapped in any of the treatments. Over the duration of the study, the total number of flies trapped was reduced by 51% in the black and metallic mulch treatments and by 42% in the white mulch treatment compared to the control.  

The mulch treatments reduced the presence of larvae in fruit samples, with larvae present marginally less in the black mulch plots compared to the control plots. The mulch treatments also reduced the number of larvae in fruit samples, with fewer larvae in fruit from all the mulch treatment plots compared to the control plots. Over two years, the total number of larvae in sampled fruit was reduced 72% by the black mulch, 61% by the metallic mulch, and 52% by the white mulch. The female fly population in the week prior to fruit sampling was not a significant predictor of presence or number of larvae in fruit.

There were no differences among treatments for daily mean, maximum, or minimum temperature or relative humidity in the raspberry canopy in 2019 or 2020. Mean light intensity was not impacted by the mulch treatments, but maximum light intensity was higher in the canopy in the metallic mulch plots compared to the black mulch plots.

The mulch treatments had a significant effect on radiance, which differed in the UV and visible spectra. In the UV spectrum (338-400 nm), the control plots had the lowest radiance, black and white mulches were equivalently intermediate, and the metallic mulch had the highest radiance.  In the visible spectra (401-680nm), the control plots and black mulch were equivalent, the white mulch had substantially higher radiance, and the metallic mulch had the highest radiance.

Data for Objective 2 was collected in 2021, but data analysis is ongoing. This report will be updated to include the results.

Participation Summary
5 Farmers participating in research

Educational & Outreach Activities

1 Journal articles
4 Published press articles, newsletters
8 Webinars / talks / presentations

Participation Summary:

100 Farmers participated
Education/outreach description:

In 2019, we established a grower advisory panel and met at the Wisconsin Fresh Fruit and Vegetable Conference in January 2019. The advisory panel helps determine research questions, plan experiments, problem-solve, and ensure our research is practical and relevant to farmers.  We met again in January 2020 to update the advisory panel on the study's results and get their feedback for the next season. We have not been able to meet in 2021 due to COVID. 

To more broadly evaluate the success of our research objectives, we conducted a preliminary grower survey at the 2019 Fresh Fruit and Vegetable Conference. We gave the same survey again in fall 2021, and results are still being evaluated. 

I have presented the results of our first field season to the University of Wisconsin's Center for Integrated Agricultural Systems in fall 2019, at the Entomological Society of America Conference in November 2019 and 2020,  at the Wisconsin Fresh Fruit & Vegetable Conference in January 2020 and 2021,  at the North Central Branch meeting of the Entomological Society of America Meeting in March 2020, and at the MOSES Virtual Conference in February 2021.

Our research has been featured in a Press Release from UW Madison, two articles in American Fruit Grower, and in Progressive Crop Consultant. I will also write a short article for the Wisconsin Fruit News in 2021 to reach more regional growers.

Project Outcomes

1 Grant received that built upon this project
1 New working collaboration
Project outcomes:

This study demonstrates that plastic mulches are an effective cultural practice for managing D. suzukii in fall-bearing raspberry in the Upper Midwest of the United States, reducing adult and larval populations by up to 51% and 72%, respectively. Applying plastic mulches is a preventative practice that could be used alongside other cultural controls, such as frequent harvesting and field sanitation, for a more robust integrated pest management program. The efficacy of plastic mulches seems to be influenced by agroecosystem, crop, and climate, so plastic mulches should be tested for D. suzukii management in other susceptible crops and climates to confirm efficacy in different regions. Plastic mulches are a promising new tool that could be integrated in management programs for D. suzukii in both organic and conventional berry production systems. Future studies evaluating the use of plastic mulches on a larger scale, as well as the economics of the addition of this management practice to small- and largescale production systems will help promote the adoption of plastic mulches to control D. suzukii by commercial growers.

Knowledge Gained:

This project has provided important evidence that plastic mulches are effective for controlling spotted-wing drosophila. Our grower-collaborator was very excited about the results, and installed the metallic mulch on his farm in 2021 after our research was completed on his farm. We are currently analyzing our survey data to determine how other growers feel about this research and to better understand how we can provide helpful information.

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.