Background: Spruce budworm, Choristoneura fumiferana (Clem.), causes cumulative defoliation and hence annual growth loss of the balsam fir, Abies balsamea (L.) Mill, host tree. Annual growth increments of mixed balsam fir stands were measured by stem analysis over a 9 year period (1994-2002), when Bacillus thuringiensis ssp. kurstaki (Btk) was applied to control spruce budworm defoliation. With this approach, it was possible to quantify the change in stand volume growth after aerial spray applications of Btk.
Results: Differences between the periodic volume increment of protected and unprotected plots were statistically significant, while differences between protected and budworm-free plots were not significant. After 9 years, the difference in periodic increment between protected and unprotected plots was 20 m(3) ha(-1), and the difference in periodic mortality was 20.5 m(3) ha(-1).
Conclusion: An economic assessment of Btk treatments indicates that biopesticide aerial spraying operations are justified, as they prevented substantial balsam fir mortality and growth losses over the 9 year study.