Binding and toxicity of bacillus thuringiensis protein Cry1C to susceptible and resistant diamondback moth (Lepidoptera : Plutellidae)

We studied mechanisms of resistance to Bacillus thuringiensis insecticidal crystal protein Cry1C in the diamondback moth, Plutella xylostella (L.). Bi nding assays with midgut brush border membrane vesicles prepared from whole larvae showed no significant difference between resistant and susceptible strains in binding of radioactively-labeled Cry1C. These results indicate t hat reduced binding of Cry1C to midgut membrane target sites did not cause resistance to Cry1C. Thus, the mechanism of resistance to Cry1C differs fro m that observed in several previously reported cases of resistance to Cry1A toxins in diamondlack moth. We tested Cry1C toxin and Cry1C crystalline pr otoxin against resistant and susceptible larvae using leaf disk bioassays. After adjusting for the size difference between Cry1C toxin and protoxin, w e found that with resistant larvae, toxin was significantly more toxic than protoxin. In contrast, with susceptible larvae, no significant difference in toxicity occurred between Cry1C toxin and protoxin. The resistance ratio s for Cry1C were 19 for toxin and 18 for protoxin. These results suggest th at reduced conversion of Cry1C protoxin to toxin is a minor mechanism of re sistance to Cry1C. Because neither reduced binding nor reduced conversion o f protoxin to toxin appear to be major mechanisms, one or more other mechan isms are important in diamondlack moth resistance to Cry1C.