Cover crops have neutral effects on predator communities and biological control services in annual cellulosic bioenergy cropping systems

Maize stover is beginning to be used as a cellulosic biofuel feedstock in the Midwestern United States; however, there are concerns that stover removal could result in increased soil erosion and loss of soil organic matter. Use of a winter cover crop following maize harvest has the potential to mitigate these impacts and may have additional benefits by providing continuous living cover in annual crop habitats leading to changes in insect predator communities and increased biocontrol services. However, cover crops may also be harvested in cellulosic biofuel systems, adding a disturbance event that may negatively affect biocontrol. We contrasted insect predator communities and their impacts in four potential bioenergy cropping systems in Michigan and Wisconsin (USA) during the 2013 and 2014 growing seasons. Two annual maize systems were harvested for grain and stover; one maize system included a cereal rye/Austrian winter pea cover crop. Two perennial systems, switchgrass and a mixed prairie grasses and forbs, were harvested in the fall for biomass. Predatory insect abundance and diversity were lower in both annual cropping systems than in the perennial cropping systems and the inclusion of the cover crop did not significantly alter these responses. Similarly, removal of sentinel insect egg prey was also lower in the annual versus perennial cropping systems, with no significant influence of cover crop. We also explored the potential for cover crops to harbor prey populations in the spring that might encourage oviposition by mobile predators and potentially lead to local population sources or sinks of predators depending on the timing and effect of cover crop harvest. We found that existing predator communities in the cover crop treatments effectively suppressed aphid population growth, limiting their attractiveness to mobile predators. While we found no significant positive or negative impacts of this cover crop system on biocontrol services, bioenergy cover cropping systems could be managed to increase multiple ecosystem services by altering cover crop identity, or timing of planting and harvest.