Consumption of diffuse knapweed by two species of polyphagous grasshoppers (Orthoptera: Acrididae) in southern Idaho

Consumption of diffuse knapweed ( Centaurea diffusa Lam.) by 2 polyphagous grasshopper species, Melanoplus sanguinipes (F.) and Oedaleonotus enigma (Scudder), was studied using microhistological analysis of grasshopper crop contents. Grasshoppers were confined to cages containing C. diffusa and Sisymbrium altissimum L., a member of the mustard family known to be readily eaten by these 2 grasshopper species. Preference indices for knapweed were lower than for S. altissimum in 4 of 5 trials. An uncaged population of M. sanguinipes on a knapweed-infested site consumed only small amounts of knapweed until late summer when most other plants were senescent. Results suggested that diffuse knapweed s low palatability to generalist herbivores may confer to it a competitive advantage over other rangeland plants.