Development of sustainable, native grass-based bioenergy production systems in the prairie region of Minnesota: Insect response to plant community type and management

Background/Question/Methods

Farmland restored from monocultures to diverse, native perennials may provide an alternative biomass energy source, farm income, and ecosystem services such as supporting insects that serve as natural predators and pollinators.  In this on-farm research, we surveyed insect communities in 2012 in a low-diversity restored prairie managed for biomass production with a split-plot design of four fertilizer (zero, composted cattle manure, half rate NPK, and full rate NPK) and two harvest (harvest and no-harvest) treatments.  Each treatment was replicated in four (20x60 ft) plots.  For broader analysis of potential insect-related benefits of native perennial plants, we compared insects from the harvested and unharvested zero-fertilizer plots with insects from three additional nearby plant community types (four plots of each):  high-diversity restored prairie, native prairie, and conventional soybeans.  Within each plant community and fertilizer x harvest treatment we trapped ground-dwelling insects in pitfall traps once in August, and sampled canopy-associated insects with sweep nets in both early August and in mid-September.  All arthropods collected were identified to order.  Insects were typically identified to family and assigned to functional groups.  Abundance data were analyzed with quasipoisson regression (R, GLM) and family richness with Margalef’s index.

Results/Conclusions

Within the experimental low-diversity restoration managed for biomass production, harvest was the only significant predictor of insect abundance.  Neither harvest nor fertilizer impacted family richness.  Mean insect abundance in sweep net samples was 36% greater in harvested plots than unharvested plots (p=0.002); abundance was not affected by harvest in pitfall trap samples.  Parasitoid and herbivore abundance, but not predator abundance, was greater in harvested treatments.  Across plant community types, insect abundance (both overall and within functional groups) and family richness were lowest in soybeans.  Insect abundance was greater in both the low-diversity restoration (harvested, zero fertilizer plots, mean=527±74 (se)) and the high-diversity restoration (436±37) than in native prairie (291±37; p=0.07; p=0.04).  Both restored stands supported at least as many pollinators, predators, and parasitoids as native prairie.  Greatest family richness was observed in the low-diversity restoration but it was not statistically different from that in the high-diversity restoration and native prairie.  These results suggest that low-diversity restored prairie managed with harvest could be a sustainable source of biomass for bioenergy production, especially as an alternative to conventional soybeans, because it hosts a robust insect community that can provide a number of ecosystem services.