Insecticide spray containing Bacillus thuringiensis bacteria kills bagworms. Not all Bacillus thuringiensis strains kill bagworms, so the spray's labeling must specifically state that it does so to kill them effectively.
The kurstaki strain of Bacillus thuringiensis effectively kills bagworms. Most insecticide sprays containing Bacillus thuringiensis require a mixture of 1/2 to 4 teaspoon pesticide to 1 gallon water. This mixture is most effective when sprayed onto plants during the bagworm’s feeding stage. Ready-to-use insecticides with carbaryl are sprayed on the upper and lower sides and infested areas of plant. Some insecticides are intended for use only for spot treatment and may cause damage if the entire plant is sprayed.