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 will assess 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. Overall, plastic mulches are a promising cultural practice for managing D. suzukii since they can reduce adult and larval populations.
We continue to evaluate 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.
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.
This study 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.
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.
We were unable to measure fruit quality and yield at our research plot at the West Madison Agricultural Research Station in 2020 due to COVID, but will be collecting this data in 2021.
Educational & Outreach Activities
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 will give the same survey again in spring 2021.
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.
This will be assessed in fall 2021 after our third field season.
We will send out the second survey for this project in April 2021 and can update this information for our final report after we collect our final data in the 2021 field season.