ATTEMPTS TO INCREASE THE PREVALENCE AND SEVERITY OF INFECTION OF GRASSHOPPERS WITH THE ENTOMOPATHOGEN NOSEMA LOCUSTAE CANNING (MICROSPORIDA: NOSEMATIDAE) BY REPEATED FIELD APPLICATION

Abstract

AbstractInitial promising results with the microsporidium Nosema locustae Canning as a means of controlling grasshoppers have been followed by some cases of limited field performance. However, at sublethal doses this entomopathogen can reduce grasshopper feeding, reproduction and development, suggesting that the damage caused to range, forages and crops might be reduced if the prevalence of infection in the field could be increased over previously achieved levels. In a field experiment, we treated plots totalling 780 ha with bran bait containing N. locustae at either 2.5 or 5.0 × 109 spores per ha in each of two consecutive years, and grasshopper populations and the prevalence and degree of infection were monitored every two weeks during the summers, for three years. Little or no background infection occurred in the untreated plots. Infections of grasshoppers with N. locustae in the treated plots during the first year of application, typically increased from less than 5% by the 4th week to 10–15% by the 8th week after application. The higher application rates resulted in only slightly higher percentages infected. Most of the infections were classified by microscopic examination as trace to moderate, although heavy infections were observed in the second year. Up to 35% of some Melanoplus species collected from the treated plots were found to contain N. locustae spores, but intensive monitoring indicated population reductions that were inconsistent among sites and generally less than 50%. Other species, notably Aeropedellus clavatus, Ageneotettix deorum, Bruneria brunnea and Camnula pellucida, failed to acquire significant infection by N. locustae. The presence of these apparently less susceptible species did not account for the poor overall performance of the treatments. Two annual applications of N. locustae did not greatly increase the rate or severity of infections in grasshopper populations, although at one of the three sites, trace infections were found in the treated plots six years later.Although Nosema may eventually have value as a component of an integrated grasshopper pest management system, the low virulence cannot be readily overcome by repeated applications.