Sublethal effects of Bacillus thuringiensis on the spruce budworm, Choristoneura fumiferana

Abstract

AbstractExposing larvae of the spruce budworm, Choristoneura fumiferana (Clemens), to sublethal (≤ 50% lethal dose) levels of Bacillus thuringiensis subsp. kurstaki at various stages of their development significantly increased development time to the pupal stage and reduced pupal size and number of eggs laid per female, but did not affect the proportion of embryonated eggs. The changes in larval development time, pupal weight and fecundity depended on the larval stage that was treated. Exposure of fourth instars delayed larval development and reduced only male pupal weights with no effects on fecundity. Exposure of sixth instars delayed larval development to a lesser extent than exposure of fourth instars but had a pronounced effect on weight of both male and female pupae. The effect on pupal weight was sex dependent, as males tended to be more affected than females. The reduction in male pupal weight did not appear to influence fecundity, because the effect of exposure was explained by the change in female pupal weight. Effects on larval growth and pupal weight were proportional to the dose that was ingested during exposure, and were observed at doses as low as one‐tenth of the LD50. Ingestion of an LD50 caused a 29 or 45% delay in development of, respectively, female or male larvae when exposed as fourth instars and a 30% reduction in female pupal weight when larvae were exposed as sixth instars.