Feeding by a native grasshopper reduces broom snakeweed density and biomass

Broom snakeweed [Gutierrezia sarothrae (Pursh) Britton and Rusby] destruction by 1, 3, or 5 snakeweed grasshoppers [Hesperotettix viridis (Thomas)] per plant was quantified and compared with forage gain the year of and the year after herbivory. Grasshoppers were caged (6.25m2 cages) in 1991 and 1992 over dense stands of broom snakeweed growing in association with shortgrass rangeland near Corona and Folsom, New Mexico. A significant negative relationship between grasshopper feeding pressure in each cage and broom snakeweed biomass was found. The dry weight 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% reduction in biomass. Mortality varied between sites and years; however, 3 grasshoppers per plant killed about 69% of the broom snakeweed and reduced 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 standing 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 sites. Feeding by low and medium densities of grasshoppers did not increase grass biomass in most situations. The increase in grass biomass only after grasshoppers removed most of the broom snakeweed is similar to the response observed from other methods of broom snakeweed removal such as band thinning, chemical control, and burning. Preferred host plants such as broom or threadleaf snakeweed must be present for "specialist" snakeweed grasshoppers to occur. However, if snakeweed grasshoppers are present, care should be taken to ensure their survival.