Grasshoppers efficiently process C4 grass leaf tissues: implications for patterns of host-plant utilization

Leaf-chewing insects are commonly believed to be unable to crush the nutrient-rich bundle sheath cells (BSC) of C 4 grasses. This physical constraint on digestion is thought to reduce the nutritional quality of these grasses substantially. However, recent evidence suggests that BSC are digested by grasshoppers. To directly assess the ability of grasshoppers to digest C 4 grass BSC, leaf particles of Bouteloua curtipendula (Poaceae) were examined from the digestive tracts of two grasshopper species: Camnula pellucida (Scudder) (primarily a grass feeder) and Melanoplus sanguinipes (Fabricius) (a forb and grass generalist) (Orthoptera: Acrididae). Transmission electron microscopy was used to make the first observations of BSC crushing by herbivorous insects. Camnula pellucida and M. sanguinipes crushed over 58% and 24%, respectively, of the BSC in ingested leaf tissues. In addition, chloroplast and cell membranes were commonly disrupted in uncrushed BSC, permitting soluble nutrients to be extracted, even when BSC walls remain intact. The greater efficiency with which C. pellucida crushes BSC is consistent with the idea that grass-feeding species are better adapted for handling grass leaf tissues than are generalist species. By demonstrating the effectiveness with which the BSC of B. curtipendula can be crushed and extracted by both species of grasshoppers, this study suggests one reason why C 4 grasses are not generally avoided by grasshoppers: at least some C 4 grasses can be more easily digested than has been hypothesized.