Can. Ent. 116: 131-137 (1984) Diet incorporation assays detected no difference in susceptibility of instars IV, V, and VI of spruce budworm exposed to Bacillus thuringiensis, either in terms of dose, LC50, or time, LT50. The hypothesis that the dose-response for larvae of each instar fitted a common regression line was statistically acceptable. Differences in the pattern of changes in larval weight of instars IV, V, VI larvae in response to dose were not detected.Small plots were sprayed with 5.5, 10.9, 21.9 BIU/ha at times corresponding to peak instar 111, early IV, peak IV, peak V, and peak VI. Instar 111 larvae were controlled less effectively than were instar IV or V larvae which were controlled equally well. Instar VI larvae were also less effectively controlled than instar IV and V larvae probably because a significant proportion had reached the non-feeding prepupal stage before the full effect of B.t. was obtained.
ResumeAucune diffkrence de vulnCrabilitC en fonction de la dose, CL50, et du temps, TL50, n'a Ct C dtcelCe chez des lanes de la tordeuse des bourgeons de 1'Cpinette du 4. au 6. stades soumises a un rCgime alimentaire contenant Bacillus thuringiensis. A l'analyse groupCe des rksultats obtenus pour chaque stade, l'hypothese d'une droite commune de rkgression Ctait statistiquement acceptable. On n'a pas d6celC de diffkrence dans I'tvolution pondkrale des larves en fonction de la dose.Sur des parcelles traitkes avec 5.5, 10.9 et 21.9 U.I./ha a divers moments correspondant au maximum du 3e stade, au dCbut du 4. stade, au maximum du 4, au maximum du 5' et au maximum du 6., les larves du 3e stade ont Ct C moins bien rCprimCes que celles des 4 et 5' stades. Mtme constat pour les larves du 6. stade, peut-ttre pace qu'une partie notable d'entre elles ont cessC de se nourir avant de mCtamorphoser aux chrysalides, avant que B.t. ne fasse completement effet.