Effect of bioenergy crop type and harvest frequency on beneficial insects

Switchgrass (Panicum virgatum) and a mix of native warm season grasses with forbs (NWSG-mix) have received attention as cellulosic biofuel sources. While numerous studies support greater biodiversity within those biofuel crops than corn, it is unclear how switchgrass and NWSG-mix affect beneficial insects such as pollinators (PO) and predators/parasites/parasitoids (PPP). Also, few empirical studies have explored the effects of biofuel crop harvest on insects. We investigated the responses of PO and PPP insects to those two biofuel crops and harvest frequency in Mississippi, USA, during 2011–2013. We established 16 fields on agricultural lands and randomly assigned each one of 4 treatments: switchgrass with single harvest; switchgrass with multiple harvests; NWSG-mix with single harvest; NWSG-mix with multiple harvests. We set up 3 sampling stations per field and conducted insect sampling 1–3 times monthly during summer to fall using colored pan traps. We tested treatment effects with general linear mixed models and redundancy analyses. While treatment effects varied by season and year, biofuel crop type influenced abundance and insect family diversity but harvest frequency did not. During summer, total abundance and guild-level (PO and PPP) abundance were high in switchgrass in 2011. During fall, total abundance and abundance of PPP were high in NWSG-mix in 2011 and 2012 and abundance of PO in 2011 and 2013. Family richness was also greater in NWSG-mix across seasons in 2011 and 2012 and during fall in 2013. More families tended to be associated with NWSG-mix than switchgrass. Our results suggest that 1) biofuel crop type is a more important factor influencing beneficial insects than harvest frequency and 2) compared to NWSG-mix, switchgrass may provide more resources for insects during summer or during the first year after establishment; however, NWSG-mix could enhance insect diversity and abundance of pollinators for a longer period of time.