Although grasshoppers can compete with cattle for forage when abundant, they also make positive contributions to grassland health. As the default approach to rangeland grasshopper outbreaks in the western U.S. remains large scale chemical control programs, research is needed to understand positive and negative effects of grasshopper herbivory. Grasshoppers are often the dominant grassland herbivore relative to livestock, both by herbivore biomass and plant biomass removed, but the effects of grasshoppers on plant composition and grassland sustainability have received limited attention. We conducted a 5 year manipulative experiment at a grass dominated northern mixed prairie site in eastern Montana from 2011 through 2016, examining the impact of differing levels of grasshopper herbivory on primary production, vegetation composition and nitrogen availability. Herbivory treatments consisted of 0, 5, 15 and 40 Melanoplus sanguinipes grasshoppers maintained in 1m2 insect netting cages. Cages were counted biweekly, with grasshoppers added to maintain treatment density levels throughout the growing season. The experiment continued until mid to late September yearly, when cages were removed and vegetation plots clipped in each cage to assess primary production and nitrogen content of grasses. We assessed vegetation composition at 100 points in each cage in the spring, including in the year following cage removal, using a laser point frame. We recorded both foliar and basal hits and used Daubenmire frames to assess forb abundance. Resin capsules were used to assess nitrogen bioavailability for plants.
Grasshopper herbivory led to lower grass biomass at the end of the season in drier years, particularly in the highest density treatment where herbivory pressure was strongest. In high precipitation years, effects of herbivory on primary production were weak even with high grasshopper densities. At the highly grass dominated site, the mixed grass and forb feeding species M. sanguinipes significantly reduced end of season forb biomass relative to the treatment with no grasshoppers. However, overall forb abundance was not affected by herbivory at the end of the 5 year experiment. Basal grass cover was higher in the highest density treatment in some years, but no treatment differences existed in foliar grass cover. The combined results provide little evidence that sustained high densities of grasshoppers negatively affected vegetation composition and production over a 5 year experiment. As a result, this northern mixed prairie system appears resilient to high levels of herbivory from mixed feeding grasshoppers.