Investigating mulches to suppress the invasive swede midge in urban agriculture

Our initial objective was to evaluate the effectiveness of black plastic, cardboard, and reflective mulch as barriers to swede midge emergence. This cultural control practice has shown promise in conjunction with other IPM strategies at reducing swede midge populations in subsequent planting of brassicas. Our research questions were 1) To what extent do different mulches affect pupation and adult emergence of SM?, and 2) Does the mulch act as a physical barrier preventing access to the soil for pupation? To answer these questions, we ran a series of trials in the greenhouse, followed by two field trials in May and July 2024. These experiments are described below.

Materials and Methods

Insect Colony
An endemic colony of swede midges was established at the Plant Growth Facilities at the University of Minnesota. Beginning in late May 2024 swede midge eggs and larvae were collected at two community gardens in Saint Paul. 4 to 6-week-old sentinel cauliflower plants (Brassica oleracea var. Botrytis) were grown from organic ‘Snow Crown’ F1 seeds (Johnny’s Selected Seeds, Winslow, ME, USA), and were strategically planted in areas of known swede midge infestations where adult midges laid eggs for 1 to 2 weeks, and replaced throughout the growing season. Infested plants were collected and transported to a greenhouse and placed inside a 15.7” x 15.7” x 23.6” pop-up insect meshed cage (Restcloud, China) where the insect completed their life cycle, i.e., eggs matured into larvae, larvae matured and dropped to the soil for pupation, and the next generation of adults emerged. Swede midge was positively identified via morphological characteristics, using a Leica dissecting microscope (Leica Microsystems, Wetzlar, Germany). New sentinel plants were added to the cages every 1 to 2 weeks. In addition, fresh 4 to 6-week-old cauliflower plants grown in a greenhouse were placed into cages 3 times per week, where newly emerged and mated midges could complete their life cycle.

Greenhouse Studies

Mulches to prevent larvae access to the soil for pupation. In our initial proposal, we intended to test cardboard mulch; however, we eliminated it due to not having organically approved material for this project. We tested black and reflective plastic mulches to compare against a bare ground control. We then ran a total of 5 trials to answer the question, ‘Can mulches disrupt the life cycle of the swede midge by reducing larval survival and preventing access to the soil for pupation?’

            The experiment was set up in a completely randomized design, with four replicates per mulch treatment. Each experimental unit consisted of the following: 12” W x 12” L x 12” H pop-up insect meshed cage (RestCloud, China), with a black plastic 12-inch diameter pie pan (T.O. Plastics, North America), Lambert LM-ORG organic growing mix, a peat-based substrate composed of sphagnum peat moss, perlite, and compost (Lambert Peat Moss Inc., Riviere-Ouelle, QC, Canada), a cauliflower plant, and appropriate treatment. Thirty-seven 9-day-old late instar larvae or pre-pupae were carefully selected with a pre-moistened fine-tipped paintbrush and placed on the growing point of 4-week-old plants. Each trial ran consecutively for ten to twelve days, starting one week apart. After 7 days, 3” x 3” pre-cut double-sided yellow sticky cards were placed on metal clips on stands at the base of the plant, and the number of adults was counted daily for 12 days. We hypothesized that the mulch treatments would equally prevent larvae access to the soil for pupation, thereby reducing subsequent generations.

Mulches to prevent the emergence of new swede midge adults from the soil. For this set of greenhouse trials, we aimed to test the effect that mulches had on preventing the emergence of new adults from the soil. We used 10.94" W x 21.44" L x 2.44" D, 1020 no-hole trays (T.O. Plastics, North America)  and planted a ‘Snowcrown’ cauliflower seedling on one side and then dispersed 10-day-old larvae on the opposite end of the tray, ~50 cm. We ran 3 consecutive trials with bare ground as the control, black, and reflective plastic as the treatments, with 4 replicates per treatment. Seven days after infestation, 3” x 3” cut-out yellow sticky cards (Olson Products, Inc., Medina, Ohio) were placed at the plant canopy, and midges on cards were counted 21-30 days later. Air and soil temperature sensors were placed in individual cages at the beginning of each trial to assess potential impacts of temperature on emergence. We hypothesized that the mulch treatments would prevent the emergence of swede midge adults from the soil because of the distance from the planting hole.

Analysis of adult emergence was performed using R (4.4.2., R Core Team, R Foundation for Statistical Computing, Vienna, Austria) using a negative binomial regression with number of adults who emerged per sticky card as the dependent variable, and mulch type as the independent variable, using the MASS, dplyr, and tidyverse packages.

Field Studies

In 2024, early and late planting trials were conducted on certified organic land previously infested with swede midge at the University of Minnesota’s Saint Paul campus. The description here is true for both trials, with differences stated in the following sections, ‘early planting’ and ‘late planting.’ The side-by-side plots each measured 60’ by 45’ and were divided into 12 rows. The rows were 3’ wide with 2’ spacing. Each trial was set up in a split-plot design, with 4 blocks, 3 treatments per block, 3 replicates per treatment for a total of 12 replicates per block and a total of 36 replicates. Each replicate was a cage consisting of a 7’ by 7’ 80 g ProtekNet (Dubois Agroinovation, Quebec, Canada) draped over two 24” aluminum hoops with the edges of the fabric buried into the soil. We were careful to ensure that no soil remained uncovered within the cages on top of mulch treatments.

Field beds for both trials were tilled and shaped, and the mulches were placed on May 5, with organic clover sown in between the rows. We initiated a soil test before planting to determine fertilization needs, and a 13-0-0 organic feathermeal amendment was added when transplanting. The early trial was planted in May, and the late trial was planted in July. The effect of black and reflective plastic mulches compared to bare ground on swede midge emergence, crop damage, and marketable yield of ‘Belstar’ F1 organic broccoli (Johnny’s Seeds) was measured. We seeded ‘Belstar’ broccoli in a greenhouse approximately 5 weeks before the intended transplant date.

We evaluated black and reflective plastic mulches as ground barriers for swede midge emergence and asked the following research questions.

• Do the black and reflective plastic mulches act as physical barriers preventing larvae access to the soil for pupation?
• Do the mulches lessen plant damage caused by swede midge?
• How does the mulch affect marketable crop yield?

All analyses were conducted in R. Damage ratings were analyzed using a cumulative link mixed model (CLMM) ordinal logistic regression with a random intercept for block, damage rating as the outcome variable, and treatment as the predictor. Adult emergence and marketable yield were analyzed using non-parametric tests.

Early Planting. Seedlings were transplanted on May 10 with 18” spacing and replaced as needed for up to 1 week. In these cages, we grouped 3 eight-week-old plants and introduced 10 female and 10 male midges from our lab colony. One week later, 6” by 6” yellow sticky cards were added to each cage within the plant canopy to collect emerging adults. Plants were collected 21 to 30 days later, and adults on cards were counted to assess emergence rates. To evaluate how these mulch treatments influenced crop damage, we used a 5-point swede midge damage rating scale (Fig. 2) developed by researchers at Cornell Cooperative Extension - Cornell Vegetable Program (Hoepting, 2019), starting on 17 June and continuing every 2 weeks thereafter until harvest. Finally, we recorded market fresh weight to assess if black or reflective mulch affected vegetable quality.

Late Planting. Seedlings were transplanted on July 18. We increased plant spacing to 24” and reduced the density to 1 plant per cage. In the first trial, using 3 plants per cage led to overcrowding, which made it difficult to accurately assess damage ratings and may have contributed to lower marketable fresh weight. Instead of infesting three cages per row, we chose to infest two and use the third cage to monitor for natural infestation, resulting in 24 experimental replicates. Five days after infestation, one 3” x 5” yellow sticky card and one 3” x 5” white sticky card (Alphascents.com Canby, Oregon) were placed within the plant canopy to capture emerging swede midge flies. We used smaller cards in this trial because the larger cards used in the first trial were frequently blown down by the wind. The white cards were added to align with the white sticky card inserts commonly used in swede midge Jackson traps.

In addition to the previous research questions, for this trial, we also wanted to assess the effect that the mulches had on soil temperature, and added the following research questions.

• Do mulches reduce or increase soil temperature?
  1. How does this affect market fresh weight?
  2. Does the temperature get hot enough to potentially kill swede midge in the soil?

HOBO Pendant Temp/Light, 64K sensors (Onset Computer Corporation, Cape Cod, Massachusetts) were buried just below the soil surface (~5 cm), inside and outside of the treatment cages, and recorded temperature data hourly throughout the trial period. Outside temperatures were collected from the Saint Paul weather station data. Individual crop damage ratings were collected from the treatment cages and 10 random field plants per row beginning on August 2, and then continuing every 2 weeks until harvest. Market fresh weight was collected from treatment cages on 9/26. There was no marketable yield for vegetables outside of cages due to high swede midge pressure outside of treatment cages.