DAMAGE POTENTIAL OF HESPEROTETTIX-VIRIDIS (ORTHOPTERA, ACRIDIDAE) ON A RANGELAND WEED, GUTIERREZIA-SAROTHRAE

The vulnerability of broom snakeweed, Gutierrezia sarothrae (Pursh) Br itton & Rusby, to herbivory by the grasshopper, Hesperotettix viridis (Thomas), was quantified by placing various densities of grasshoppers on individual caged plants. Field experiments were conducted in the su mmers of 1989 and 1990 at 2 shortgrass prairie locations in eastern Ne w Mexico. Zn general, grasshoppers removed most photosynthetic tissue from small plants within 7 d at an initial density of 16 grasshoppers per plant, and in approximate to 14 d with an initial density of 8 per plant. Grasshopper densities of 2 per plant rarely consumed all photo synthetic tissue, irrespective of plant size, during caged experiments . Although not consistent over sites and years, grasshopper feeding re duced plant volume and the number of photosynthetic stems from small a nd medium plants at densities of 8 and 16 grasshoppers per plant. Gras shopper defoliation caused 8, 23, 34, and 47% G. sarothrae mortality a t densities of 2, 4, 8, and 16 grasshoppers per plant, respectively. G rasshopper feeding pressure was significantly correlated with observed defoliation and suggests feeding by >4 H. viridis per plant results i n a significant decrease in aboveground photosynthetic biomass.