Carry over effects of the entomopathogen Bacillus thuringiensis ssp. Kurstaki on Choristoneura fumiferana (Lepidoptera: Tortricidae) progeny under various stressful environmental conditions
• In the present study, we documented the lethal and sublethal effects of the entomopathogen Btk on spruce budworm and its progeny under various environmental conditions. We hypothesized that aerial spray of Bacillus thuringiensis ssp. kurstaki (Btk) could affect the biological performance of the surviving spruce budworm (Choristoneura fumiferana Clem.) populations and their progenies and that Btk sublethal effects could be widened by other types of stress (i.e. temperature conditions and changes in food suitability from year to year).
• The results from a 3-year field experiment indicated that Btk treatments decreased the fitness of the surviving larvae whatever the prevailing temperature and nutritional conditions.
• The detrimental Btk effects on the parental generation carried over to the offspring. The percent of egg hatch and first-instar survival were negatively affected by Btk whatever other stress spruce budworm parents underwent.
• The present study also highlighted the fact that the effects of temperature and nutritional stress suffered by the parents could carry over to the next generation. Balsam fir flowering, which provided larvae with pollen rich in nitrogen, favoured both the parental generation and the fitness of their offspring. Spruce budworm mothers allocated to their progenies large amounts of energy reserves (triglycerides and glycogen) that greatly enhanced the survival of the early stages.
• Egg hatch and the survival of first-instar larval progeny were drastically affected when their parents had reduced larval growth as a result of exposure to cool temperatures that had desynchronized insect and bud phenology.
• Budworm mothers submitted to negative impacts of previous defoliation allocated low amounts of energy reserves to their progeny. This lack of energy associated with unfavourable temperature conditions (i.e. high temperatures in late summer and in early fall and an extended cool period in spring) drastically reduced survival of diapausing second-instar larvae.
• These results highlight the importance of considering the various sources of stress when attempting to evaluate the impact of a control agent on an insect pest population and its progenies.