Integration of Bait Formulations of Nosema locustae and Carbaryl for Control of Rangeland Grasshoppers12

Abstract

Large blocks of rangeland were treated by aircraft with bait formulations of Nosema locustae Canning and carbaryl on wheat bran. Treatments consisted of 2 controls, straight N. locustae, straight carbaryl, and 2:1 and 1:2 mixtures, all applied at a total of 1.68 kg of bait/ha. The average longevity of grasshoppers in the control plots was inversely proportional to the population density. All treatments that contained carbaryl reduced grasshopper densities within 48 h and thereby drastically reduced average longevity. In addition, subsequent longevity among grasshoppers that survived the carbaryl treatments was significantly shorter than in untreated plots with similar densities. This reduction in longevity appeared to be associated with enhancement of the predator-prey ratio by the selective elimination of grasshoppers. The straight N. locustae treatment caused a slight but significant reduction in grasshopper longevity, but no effects on longevity could be confidently attributed to the N. locustae component in plots treated with bait mixtures. The viability of the N. locustae may have deteriorated during unanticipated storage of formulated bait for 11 days. The potential forage loss caused by 9.57 grasshoppers/m² (the current USDA/APHIS action threshold) was estimated at $8.84/ha ($3.58/acre). The bait treatments saved forage worth $0.74–$6.30/ha ($0.30–$2.55/acre).