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 Study
In our initial proposal, we intended to test cardboard mulch, however, due to not having organically approved material for this project, we eliminated it. We tested black plastic mulch and metalized/reflective aluminum on black plastic mulch to compare against a bare soil control. We 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 subsequent development of the second generation?’ to evaluate the effectiveness of black and reflective mulches in interrupting swede midge reproduction by preventing late instar larvae access to the soil for pupation.
The experiment was set up in a completely randomized design, with four replicates per treatment and three treatments. Each experimental unit consisted of the following: 12” x 12” x 12” pop-up insect meshed cage (RestCloud, China), with a black plastic 12-inch diameter pie pan, 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 to nine-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 cauliflower 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 until the twelfth day.
We hypothesized that the mulch treatments would suppress the emergence of swede midges, thereby limiting plant damage and reducing subsequent generations. Data collection involved monitoring the emergence of midges on sticky cards. Analysis of adult emergence was performed using R (4.4.2., R Core Team, R Foundation for Statistical Computing, Vienna, Austria). Initially, we attempted a Poisson regression but determined the variance was greater than the mean. We then completed a negative binomial regression analysis due to overdispersion in count data. The dependent variable in our negative binomial regression was the number of adults who emerged per sticky card, with mulch type serving as the independent variable.