Field trials conducted in 1990 and 1991 demonstrated that commercial formulations of Bacillus thuringiensis var. kurstaki Berline applied late to fifth and sixth instar spruce budworm Choristoneura fumiferana (Clemens) expressly to avoid affecting the larval parasitoid Apanteles fumiferanae Viereck were as efficacious in controlling spruce budworm populations as a regularly timed spray that was targeted against third and fourth instars. In 1990, with high budworm populations (50 to 70 larvae per 45 cm branch), the corrected budworm population reduction after the early spray (34.6%) was less but not significantly different (p > 0.05) from that in the late treatment (55.3%). Defoliation of balsam fir Abies balsamea (L.) Miller in the early and late spray treatments was also not significantly different [87.5 +/- 8.8% and 70.3 +/- 12.8% (xOBAR +/- s.d.), respectively] from that in the control plots (87.1 +/- 9.2%), but was considered unacceptably high. In 1991, with moderately high budworm populations (16-45 larvae per branch) the corrected budworm population reductions resulting from the early and late treatments (76.7 and 92.8%, respectively) were not significantly different (p > 0.05). The percentage defoliation in the late treatment (28.6 +/- 12.3%) was significantly higher statistically than that in the early treatment (18.7 +/- 7.6%) but was biologically acceptable. Both levels were significantly less than the defoliation observed in the check plots (42.2 +/- 14.8%), which was unacceptably high. These results suggest that commercial formulations of Bt applied undiluted to moderately high budworm populations at 30 billion (10(9)) international units (BIU) ha-1 can be judiciously delayed to conserve the survival of A. fumiferanae and still be efficacious.
