For the first time, researchers have identified how cabbage
looper caterpillars in the field develop resistance to the
bacterium Bacillus thuringiensis (Bt), which naturally occurs in
the soil and on plants and has been developed into the most
successful and widely used biological insecticide. When
ingested, the insecticidal toxins in Bt kill insects by
destroying their guts. Insects in the field develop resistance
to it, however, via a genetic mechanism that alters a toxin
receptor in the insect's gut, two Cornell researchers have
discovered. The receptor belongs to a class of digestive enzymes
called aminopeptidase N (APN), two of which undergo changes when
cabbage loopers develop resistance to Bt on crops. Under normal
circumstances, the Bt toxin Cry1Ac, which is a caterpillar-
specific toxin, binds to an enzyme called APN 1 along the wall
of the insect's gut, where the toxin destroys the gut lining.
But when cabbage loopers develop resistance, APN 1 significantly
decreases while another aminopeptidase, APN