A Method for Forecasting Potential Losses from Grasshopper Feeding on Northern Mixed Prairie Forages

Abstract

Information on the potential loss of forage expected from grasshopper feeding in a given season would benefit ranchers and land managers if the loss could be estimated in the spring before peak forage production. A method was developed for forecasting such losses on the northern mixed prairie when most species of grasshoppers are in the 3rd and 4th instars. The method is based on the assumptions that forage losses (forage consumed + destroyed) are directly proportional to grasshopper size and density and that during the period between the 3rd instar and death, which usually does not exceed 46 days, density decreases linearly to 0. Using feeding ratios (weight of forage destroyed/weight of adults), losses/day were calculated for each of 26 grasshopper species and for 3 groups of species separated by weight. Forecasted loss estimates for any population can be estimated if the density and species are determined, or a loss of 43 mg/grasshopper feeding day could be used as a general average if species cannot be identified. Forecasted losses are related to grasshopper density and to observed losses at 1 site in Montana [USA] during a 3-yr period.