Field efficacy of Bacillus thuringiensis galleriae strain SDS-502 for the management of alfalfa weevil and its impact on Bathyplectes spp. parasitization rate

Abstract

Alfalfa weevil, Hypera postica Gyllenhal, is an important pest in forage alfalfa worldwide, and especially so on the Northern Plains of North America. Neither the weevil-specific fungus, Erynia phytonomi, nor the weevil's parasitoids are able to routinely suppress outbreaks as they do in the eastern U.S. A new Bacillus thuringiensis var. galleriae, having a Cry8Da coleopteran-active toxin, has been recently commercialized. We examined the efficacy of this B. thuringiensis product against the H. postica in replicated field trials in north central Montana. Because it has been suggested that efficiency of the parasitoids, Bathyplectes curculionis and Oomyzus incertus, was inversely proportional to host numbers (i.e., parasitoid efficiency increased when host population is low), we also sought to determine if a partial reduction of larval H. postica populations with a B. thuringiensis would yield to greater parasitoid efficiency, manifested as higher percent parasitism among the surviving larvae. The B. thuringiensis gave 27-40% reduction in weevil numbers at the low label rate, 55-59% for the high label rate. Mean parasitism at the two research locations varied from 5-26% and 17-36% respectively, but application of the B. thuringiensis had no significant effect on parasitism levels, i.e. parasitism was not greater in treated than in carrier control plots.