MULTIYEAR EVALUATION OF THE EFFECTS OF NOSEMA-LOCUSTAE (MICROSPORIDIA, NOSEMATIDAE) ON RANGELAND GRASSHOPPER (ORTHOPTERA, ACRIDIDAE) POPULATION-DENSITY AND NATURAL BIOLOGICAL-CONTROLS

A 3-yr evaluation of the biological control agent Nosema locustae Cann ing on grasshoppers was conducted on the National Wildlife Refuge at M axwell, NM. Six 0.1-ha plots were treated with the microsporidian in 1 988; grasshopper densities were monitored in treatment and control plo ts through 1990. In 1988, there was a significant time-by-treatment in teraction; grasshopper densities were significantly reduced for severa l weeks, with as much as 50% reduction by the end the season. Infectio ns were highest 6 to 8 wk after treatment. Infection rates differed si gnificantly across developmental stages and subfamilies. Melanoplinae were most heavily infected and suffered the greatest rate of mortality . Three additional orthopteran species were added to the host range of N. locustae. In the treated plots, in 1988, bird densities were signi ficantly higher, and the parasitic wasp Scelio opacus (Provancher) was significantly more frequent in egg pods. Spores of N. locustae were p resent in unhatched eggs and early instar nymphs, but no spores were o bserved on egg pods or foam plugs. In June of 1989, density of the tre ated population was similar to that of the controls, but from mid-july through mid-August the population density was significantly lower in the treated plots. Based on infection and demographic data, this secon d-year population suppression seems to have been the result of N. locu stae. In 1990, treated and control populations did not differ signific antly, and both were reduced below the standard treatment threshold of 9.6 grasshoppers per square meter.