The most widely used bacterium in insect control is

The most widely used bacterium in insect control is Bacillus thuringiensis.
Bacillus thuringiensis (Bt) is a unique bacterium in that it shares a common place with several chemical compounds which are used commercially to control insects important to agriculture and public health.
Although other bacteria, including B. popilliae and B. sphaericus, are used as microbial insecticides, their spectrum of insecticidal activity is quite limited compared to Bt.