EFFECT OF EXPOSURE TIME TO BACILLUS THURINGIENSIS ON MORTALITY AND RECOVERY OF THE SPRUCE BUDWORM (LEPIDOPTERA: TORTRICIDAE)

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Abstract
Spruce budworm larvae were subjected to bioassay with Bacillus thuringiensis (Bt) to study the effects of exposure time and assay period on mortality, as well as on the possibility of recovery after a feeding-inhibitory dose. At all concentrations cumulative mortality increased until about day 10 of the assay. Extension of the exposure period from 1 day to continuous (equivalent to a 6 day exposure), resulted in a 500-fold reduction in LC50 (increase in potency) and an equivalent reduction in LT50. Some larvae recover if allowed to feed on diet without Bt even after exposure for 4 days to a very high concentration of Bt. The growth rate of recovered larvae is normal, but the time required for normal growth to resume varies with dose and exposure duration. Equations expressing the effect of exposure duration and dose on mortality and recovery period are presented. The implications of these findings for control of spruce budworm are discussed.