Changing herbivore and natural enemy community structures with habitat management in cabbage (Brassica oleracea)

Row-middle cover crops and reduced tillage techniques in agroecosystems can change the suitability of a field for insects, by leaving plant residues in row aisles. The impact of these residues on multiple trophic levels was studied for two years in an experimental cabbage field in Michigan in summer 2010 and 2011. We hypothesized that greater structural complexity due to cover crop residue negatively impacts the herbivore community and positively impacts the natural enemy community. Weed management intensity and time of oat cover crop kill were examined in a complete factorial design with a total of 6 treatments. Oat cover crop in the row middles was either killed early, intermediate, or late season. Weed management consisted of either pre- and post- emergence herbicides and hand weeding or a low intensity program without post-emergence broadleaf herbicide. Treatments with the cover crop killed late season and lower weed management intensity had higher structural complexity. Insects were sampled weekly, both visually on plants and on yellow sticky traps. Predators were hand collected from the field once in 2010 and six times in 2011. These predators were tested for Pieris rapae and Plutella xylostella DNA in their guts. Higher structural complexity decreased the abundance of some key herbivores and increased the abundance and diversity of some natural enemies, particularly predators of two key pests of cabbage. Further studies on the mechanisms responsible for these trends in the insect community are needed to understand multiple trophic level interactions within cabbage fields with cover crop residues.