LACK OF CROSS-RESISTANCE TO OTHER BACILLUS-THURINGIENSIS CRYSTAL PROTEINS IN A POPULATION OF PLUTELLA-XYLOSTELLA HIGHLY RESISTANT TO CRYIA(B)

Competition experiments were performed with brush border membrane vesicles of diamondback moth larvae using I-125-labelled CryIA(b) and unlabelled CryIA(a), CryIA(b) and CryIA(c). The results suggested a model with a single binding site for CryIA(b). Heterologous competition showed that CryIA(c) competed as effectively as CryIA(b) for the CryIA(b) binding site, whereas CryIA(a) competed less effectively. Tonicity tests were performed on third instar larvae with trypsin-activated insecticidal crystal proteins (ICPs) and a commercial formulation of Bacillus thuringiensis (Bt) (Dipel). A laboratory colony was found to be susceptible to all four ICPs tested and to Dipel. CryIA(b), CryIA(c) and CryIB were approximately equally toxic, and CryIA(a) was less tonic than the other ICPs by one order of magnitude. In contrast, a recently collected insect population front the Philippines was 236 times less susceptible to CryIA(b) than the control colony. However, for the other ICPs no significant differences were found between the two colonies. Dipel was slightly move toxic to the Philippines colony than to the control colony. The lack of cross-resistance suggests that CryIA(a) and CryIA(c) possess other binding sites than that recognized by CryIA(b). It is proposed that this specific resistance could correspond to a biotype present in the Philippines and is not the result of selection with Bt insecticides.