A Method for Estimating Economic Injury Levels for Control of Rangeland Grasshoppers with Malathion and Carbaryl

Abstract

A theoretical "average" rangeland grasshopper weighs 81.6 mg (dry weight) in the adult stage and consumes 9, 22, and 53 mg of forage/day in the 4th instar, 5th instar, and adult stages, respectively. Criteria for a computer program are presented whereby grazing pressure from grasshopper infestations can be predicted as a function of initial density and normal daily rate of survival. The benefits of a contemplated control measure may then be estimated through appropriate adjustment of the survival rate. By assigning dollar values to the worth of forage and cost of treatment, the lowest infestation that will justify control measures can be determined. The technique is demonstrated for 2 effective but dissimilar insecticides, malathion and carbaryl. By using actual treatment costs for 1981 control programs and by assuming that an AUM (364 kg of forage) saved from destruction by grasshoppers has a marginal value product of $14, it was calculated that grazing by grasshoppers must approach 0.25 AUM/ha before treatment becomes economical. If treatments are not applied before carrying capacity has been depleted by grasshoppers, then the forage that is saved cannot be harvested. Thus, early treatments with both chemicals are much more economical than late treatments.