Geographic variation in susceptibility of the spruce budworm,Choristoneura fumiferana(Lepidoptera: Tortricidae), toBacillus thuringiensisssp. kurstaki is limited

Abstract

Geographic variation in susceptibility to the pathogen Bacillus thuringiensis Berliner ssp. kurstaki was assessed among five populations of the spruce budworm, Choristoneura fumiferana (Clemens), across Canada. Larvae or pupae were collected from Prince Edward Island, northern New Brunswick, central Ontario, northeastern Alberta, and northeastern British Columbia. The response of field populations to the 1980 industrial standard of the HD-1 strain of the pathogen was compared with the concurrent response of a laboratory colony as an internal standard. Equal slopes of the probit regression lines indicated homogeneity of variation in susceptibility among populations. LC50values ranged from 2.6 international units (IU) per sixth-instar larva for the most susceptible population to 8.3 IU per larva for the least susceptible. Although significant, the variation in population susceptibility was within the range of variation that is commonly observed among repeated bioassays against cohorts of a single population. We conclude that variation in susceptibility to B. thuringiensis among spruce budworm populations is low. The data provide a basis for monitoring changes in susceptibility of spruce budworm populations in response to increased use of this pathogen in forest-protection programs.