| Summary: | Forest protection against eastern spruce budworm, <i>Choristoneura fumiferana</i> (Clemens), relies on aerial applications of <i>Bacillus thuringiensis</i> (<i>Btk</i>). However, <i>Btk</i> prescriptions have been developed for balsam fir (<i>Abies balsamea</i> (L.) Mill.) stands, generating uncertainty as to the relevance of this protection approach on white spruce (<i>Picea glauca</i> (Moench) Voss). The main goal of this study was to evaluate the efficacy of three <i>Btk</i> application treatments (early application, late application, and double application) for protecting white spruce and balsam fir in mixed stands. Our results show that all <i>Btk</i> treatments tested kept defoliation under the 50% threshold on balsam fir (27.7 to 38.1% less defoliation than in controls). In contrast, differences in defoliation among treatments were not significant for white spruce. Larval mortality was significantly lower on white spruce than on balsam fir. The low efficacy of <i>Btk</i> treatments on white spruce may be explained by its shoot phenology (most bud caps were still present during the early application), and its foliar chemistry, which could decrease <i>Btk</i> efficacy. Consequently, many larvae may survive simply because <i>Btk</i> spores do not reach them, or because they may consume sublethal doses. We recommend maintaining the current strategy in mixed stands, as it provides a good protection for balsam fir. Further research is needed to determine the causes of the low efficacy of <i>Btk</i> treatments on white spruce.
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