Immunohistochemical detection of binding of CryIA crystal proteins of Bacillus thuringiensis in highly resistant strains of Plutella xylostella (L.) from Hawaii.

We detected binding of insecticidal crystal proteins from Bacillus thuringiensis in one susceptible strain and six resistant strains of diamondback moth, Plutella xylostella, from Hawaii. Immunohistochemical tests with tissue sections from larval midguts showed specific binding of CryIA(a), CryIA(b), and CryIA(c) to brush border membranes. CryIE, which is not toxic to P. xylostella, did not bind to midgut tissues. Larvae from one of the resistant strains ingested extremely high concentrations of a commercial formulation containing the three CryIA proteins without suffering midgut cell damage or mortality. This same resistant strain had previously been found to have greatly reduced binding of radioactively-labeled CryIA(c) to vesicles prepared from brush border membranes. The finding that binding as detected in immunohistochemical tests was not sufficient for toxicity suggests that low levels of binding can occur without harmful effects; the tests did not reflect in vivo binding, or post-binding factors contribute to resistance. Comparison with resistant strains from Florida and the Philippines, which did not bind CryIA(b) in previously reported immunohistochemical tests, suggests that more than one mechanism of resistance to B. thuringiensis may occur within a single species.