Bacillus thuringiensis (B.t.) + sublethal doses of the organophosphates fenitrothion or Orthene® were aerially applied to spruce budworm infested white spruce and balsam fir trees. B.t. + fenitrothion applied at 10 BIU + 17.5 g active ingredient in 4.71./ha (4 BIU + 0.25 oz in 0.5 gal/acre) and depositing at 1.5 to 3.0 BIU + 2.1 to 4.9 g/ha (0.03 to 0.07 oz/acre) did not protect the trees in the year of application owing to too low application rates but apparently gave good protection 1 year later without further treatment. A B.t. + Orthene combination applied at 20 BIU + 42 g/ha (8 BIU + 0.6 oz/acre) and depositing at the rate of 6.9 BIU + 12.6 g/ha (2.29 BIU + 0.18 oz/acre) provided good protection for both tree species in the year of spray.
Moth emergence was substantially reduced by 20 to 30 BIU B.t. + Orthene combination treatment. Thuricide fenitrothion, Dipel fenitrothion, and 30 BIU Orthene treatments reduced oviposition rate by at least 62%, 47%, and 47%, respectively. B,t. + a low dose of Orthene apparently had no major destructive effects on larval, pupal, or egg parasites whereas B.t. + a low dose of fenitrothion seriously reduced egg parasitism.
The data indicate the possibility of using B.t. in combination with low doses of chemical insecticides against the spruce budworm as an alternative to high doses of chemical insecticides alone.