On Caterpillars (eg. European Corn Borer): Bt proteins expressed in ECB Bt corn (Cry 1Ab, Cry1F, Cry1A.105 and Cry2Ab2) are only lethal to a specific group of insects that are caterpillars in their juvenile stage (Lepidoptera).
European Corn Borer larva (Marlin Rice)
On Beetle Larvae (eg. Corn Rootworm): Bt proteins in CRW Bt corn (Cry3Bb, Cry 34Ab1/Cry 35Ab1 or mCry3A) are only lethal to a specific group of insects that are beetles as adults. The larvae are targeted for control.
Corn Rootworm larvae (Marlin Rice)
These groups of insects have receptors in their gut linings specific to the Bt Cry protein structures that are targeted to control them and alkaline gut environments, both of which are necessary to activate the toxin.
As the larvae feed, the insect's own digestive enzymes activate the toxic form of the protein. The toxin binds to specific receptors on the cells in the lining of the larvae's gut, and ruptures these cells. Within hours, the larvae stop feeding and their stomach lining ruptures. Bacteria from the gut invade the body cavity and the larva becomes infected and dies within 24 to 48 hours.
Animals and humans have acidic digestive systems and lack the receptor cells that the protein requires for binding, so the protein poses no risk to these groups and is digested like any other protein.