Other bacteria may turn the host body red or yellow. Some naturally occurring bacteria can cause epizootics, especially if the pest population is under stress from lack of food, overcrowding, or cold weather. These epizootics are not as common as those caused by other naturally occurring pathogens. Commercial formulations of Bacillus thuringiensis , however, are widely used. Greenhouses, tree and field crops, waterways and thousands of acres of forests are sprayed annually with commercial Bt products.
Young larvae are usually most susceptible.
Caterpillar growth may be retarded even if less than a lethal dose is eaten. Determining when most of the pest population is at a susceptible stage is key to optimizing the use of this microbial insecticide. Not all caterpillar pests are equally susceptible to Bt. Beet armyworm has proven difficult to control, and some moth species, including some populations of diamondback moth, a major worldwide pest of cole crops, have evolved resistance to the Bt variety kurstaki toxins. Corn earworm, squash vine borer larvae, and codling moth larvae are susceptible, but field control is difficult because they rapidly bore into and are protected by plant tissue.