Mode of action of entomopathogenic bacteria

• They enter mostly through the mouth and digestive tract. Also, enter an insect using parasitoids and predators.
• Most commercial Bt products contain the insecticidal crystal protein (endotoxin) and spores, but some contain only the toxin component.
• Most of the bacterium including Bt, endotoxin is nontoxic to warm-blooded animals.
• Bacterial spore is sprayed over the foliage and the insect ingests the spores.
• Bacterial insecticides must be eaten by target insects to be effective, they are not contact poisons.
• When Bt is ingested by a susceptible insect, the protein toxin is activated by alkaline conditions and enzyme activity in the insect’s gut.
• If the activated toxin attaches to specific receptor sites, it paralyzes and destroys the cells of the gut wall, allowing the gut contents to enter the insect’s body cavity.
• Ingested Bt spores release the endotoxins which the insect to stop feeding after a few hours and the insect dies within a day or two.
• Bacterial-based biopesticides are innovatively applied.
• It is one of the few so-called insecticides that are acceptable for the management of insects in organic farming systems.
• Bacterium toxins are inserted into several crops using genetic engineering technology.

Limitations:

• Ultraviolet light in sunlight kills the bacterial spores.
• Like synthetic pesticides, the target pests can develop resistance to Bt.
• House flies and Drosophila developed resistance to the exotoxin.
• Tobacco budworm Heliothis viruses have developed resistance to the endotoxin.

Symptoms and pathology

• Multiply and produce toxins in the midgut lumen and the insect loses its appetite, becomes diarrheic, discharges watery faces, and may vomit.
• Larval insects killed by bacteria, rapidly darken in color and are often very soft.
• Internal tissues and organs are rapidly broken down to a viscid consistency, accompanied by a putrid odor.
• The integument remains intact and bacteria are abundant in an insect shortly after death. The cadaver shrivels, dries, and hardens.