Plastic mulches reduce abundance of some arthropods but are not detrimental to pollinators in primocane raspberries

Cultural practices modify agroecosystems to prevent or reduce pest outbreaks, but they may be detrimental to beneficial arthropods and non-target effects are not commonly evaluated. Plastic mulches are an effective management practice for spotted-wing drosophila (Drosophila suzukii Matsumura) since they increase UV radiance in the plant canopy to reduce larval infestation of fruit. In this two-year study, we evaluated how black, white, and metallic plastic mulches impact populations of arthropods, including predators of D. suzukii and raspberry pollinators in Wisconsin. The three plastic mulches did not affect arthropod richness and did not affect the abundance of about 65% of arthropod groups identified. In the raspberry canopy, the black mulch decreased the abundance of Orthoptera and Thysanoptera and increased Orius spp.; the white mulch increased Diptera, Thysanoptera, and Orius spp.; and the metallic mulch decreased Coleoptera, micro-Hymenoptera, Orthoptera, and Thysanoptera. On the ground, all mulches decreased the abundance of Gryllidae, the black and metallic increased Formicidae, and the white increased Staphylinidae. We reported ten insect groups visiting raspberry flowers. Bombus impatiens Cresson was the most common, accounting for 82% of flower visits. The plastic mulches did not reduce flower visitation, though the white plastic mulch increased the abundance of B. impatiens compared to the other mulch treatments. Plastic mulches can reduce populations of pests such as D. suzukii and overall have no severe non-target effects on other arthropods in Wisconsin raspberry.

Methods
We assessed populations of arthropods using two types of traps set up when D. suzukii adults were present in early July until early October 2021 and 2022. In the canopy, we trapped arthropods using 15.25 cm2 clear sticky cards (Alpha Scents, West Linn, OR) placed where fruit was present in the center of each plot in all five blocks and replaced weekly.  To account for changes in canopy height as the plants grew, the sticky cards were adjusted vertically as needed.  On the ground, we used pitfall traps made from 532 mL 9.9 cm diameter plastic cups placed into holes in the ground so the top of the cups were flush with the soil or mulch surface. For the plastic mulch treatments, a 20 cm2  piece of mulch was hot glued (© Stanley Black and Decker, Inc., worldwide) on the top of each cup, and the section covering the trap opening was cut out so that the mulch formed an apron around the trap. The traps were placed into the holes so that the mulch aprons covered the in-field mulch to prevent insects from falling between the in-field mulch edge and the cup. Four wires were attached to the lip of each cup to suspend a 10 cm diameter clear plastic cup lid 3 cm above the top of the trap to block rainwater and non-arthropod bycatch. Pitfall traps were filled with 150 mL of drowning solution (30% propylene glycol diluted with water and one squirt of unscented dish soap) for three days each week in all five blocks. 

Arthropods trapped on the sticky cards or in the pitfall traps were identified to different taxonomic levels, with non-Insecta classified as Araneae, mites, Opiliones, Collembola, or Diplopoda, insects identified to order, and predators and known natural enemies of D. suzukii (Lee et al., 2019) identified further.