FEEDING BY A NATIVE GRASSHOPPER REDUCES BROOM SNAKEWEED DENSITY AND BIOMASS

Broom snakeweed [Gutierrezia sarothrae (Pursh) Britton and Rusby] dest ruction by 1, 3, or 5 snakeweed grasshoppers [Hesperotettix viridis (T homas)] per plant was quantified and compared with forage gain the yea r of and the year after herbivory, Grasshoppers were caged (6.25m(2) c ages) in 1991 and 1992 over dense stands of broom snakeweed growing in association with shortgrass rangeland near Corona and Folsom, New Mex ico, A significant negative relationship between grasshopper feeding p ressure in each cage and broom snakeweed biomass was found, The dry we ight of broom snakeweed herbage removed per grasshopper per day was 45 mg at Folsom and 85 mg at Corona, Feeding by grasshoppers stocked at 5 per plant killed 91% of the broom snakeweed resulting in a 75% reduc tion in biomass, Mortality varied between sites and years; however, 3 grasshoppers per plant killed about 69% of the broom snakeweed and red uced biomass by 61%. One grasshopper per plant killed 53% of the broom snakeweed and reduced biomass an average of 39%. In 1991, removal of most broom snakeweed by the high density of grasshoppers increased sta nding crop of grasses 23% at the end of the treatment year and 44% one year after treatment compared with grasshopper-free cages at the 2 si tes, Feeding by low and medium densities of grasshoppers did not incre ase grass biomass in most situations, The increase in grass biomass on ly after grasshoppers removed most of the broom snakeweed is similar t o the response observed from other methods of broom snakeweed removal such as hand thinning, chemical control, and burning, Preferred host p lants such as broom or threadleaf snakeweed must be present for ''spec ialist'' snakeweed grasshoppers to occur. However, if snakeweed grassh oppers are present, care should be taken to ensure their survival.