EFFECT OF TEMPERATURE ON MORTALITY AND RECOVERY OF SPRUCE BUDWORM (LEPIDOPTERA: TORTRICIDAE) EXPOSED TO BACILLUS THURINGIENSIS BERLINER

Abstract

Spruce budworm larvae were bioassayed against Bacillus thuringiensis Berliner to study the effect of temperature on the expression of toxicity. Temperatures between 16 and 28°C did not affect the ultimate level of toxicity (LC₅₀). However, LT₅₀’s increased from 2–8 days at 28°C to 11–20 days at 16°C, depending on concentration of the pathogen. When larvae were force-fed with a single dose, temperature had a similar effect on the time course of mortality without affecting the level of mortality. Feeding inhibition of force-fed larvae commenced immediately after dosing. Larvae that did not recover died without further feeding, even at lower temperatures when death occurred 2–3 weeks after dosing. Recovering larvae resumed feeding after 2 (28°C) to 6 (13°C) days. Recovered larvae took longer to develop and produced lighter pupae than untreated larvae. Our data suggest that temperature-dependent feeding and recovery did not contribute to quicker death at higher temperatures. Expression of the toxin itself appears to depend on temperature, possibly through the influence of temperature on metabolic rate of affected gut cells. Implications of these findings for the efficacy of spruce budworm control operations are discussed.