Bacillus thuringiensis (Bt) is a variety of crystallogenic Bacillus that is used as a low-toxic microbial insecticide, with a focus on gastric toxicity. The bacteria can produce two major toxins, namely endotoxin (concomitant cell crystal) and exotoxin, which can stop the pests from feeding. Finally, the pests are starved and die. The effect of exotoxin is slow and obvious during molting and metamorphosis. The effect of this drug is slow, and the effect can only be seen in about 2 days after the pest is fed, and the effective period is about 1 day, so it should be used 2 ~ 3 days earlier than conventional chemical agents, and the effect is better in the early age of the pest. It is safe for fish and bee, but highly toxic to silkworm.