BIOASSAY TECHNIQUE FOR ENTOMOPHTHORA-SPHAEROSPERMA ON THE SPRUCE BUDWORM, CHORISTONEURA-FUMIFERANA

A technique for conducting bioassays of Entomophthora sphaerosperma on sixth-instar larvae of the spruce budworm, Choristoneura fumiferana, was developed. Four assays were conducted by showering conidia on 10 larvae for each of 10 to 20 doses per assay. Dose was estimated by averaging estimates of the concentration of spores falling on water agar dishes before and after insect exposure. Maximum-likelihood probit analysis indicated significant regressions between log dose and probit mortality for all four assays. LC50 values ranged from 11.21 to 18.77 spores/mm2 with a weighted mean of 16.13 spores/mm2. Slope estimates ranged from 0.92 to 1.87 with a weighted mean of 1.13. These low slope values may have been indicative of a highly variable test insect population, but also suggested a nontoxic infection process by the pathogen. © 1979.