Field-evolved resistance to Bacillus thuringiensis toxin CryIC in diamondback moth (Lepidoptera: Plutellidae)

Previous results have shown that diamondback moth, Plutella xylostella (L.), populations resistant to toxins from Bacillus thuringiensis subsp. kurstaki were susceptible to toxin CryIC. Use of commercial formulations of B. thuringiensis subsp. aizawai that contain CryIC has increased recently. Analysis of two commercial formulations by high pressure liquid chromatography showed that CryIC accounted for 26% of the CryI protein in the B. thuringiensis subsp. aizawai formulation, but did not occur in the B. thuringiensis subsp. kurstaki formulation. CryIAb was the most abundant CryI protein in the commercial formulations of B. thuringiensis subsp. aizawai and kurstaki. We found resistance to CryIC in a field population of diamondback moth from Hawaii that had been treated with B. thuringiensis subsp. aizawai. Leaf residue bioassays showed that, at 5 d after treatment with CryIC, LC₅₀s for colonies derived from this population in 1993 and 1995 were ~20 times greater than the LC₅₀ for a susceptible laboratory colony. For a nearby population that had not been treated with B. thuringiensis subsp. aizawai, responses to CryIC did not differ significantly from those of the susceptible laboratory colony. Resistance to Cry1Ab was lower in a Cry1C- resistant colony than in a Cry1C-susceptible colony that had been selected with B. thuringiensis subsp. kurstaki. These results suggest that the gene(s) conferring resistance to Cry1C segregate independently from the gene(s) conferring resistance to Cry1Ab. In contrast with previous results with colonies derived in 1989, resistance to B. thuringiensis subsp. kurstaki in a colony derived in 1993 from the same field population did not decline when exposure to B. thuringiensis stopped. Thus, stability of resistance is not necessarily a fixed character, even for a specific population and pesticide. Despite substantial resistance to CryIC and B. thuringiensis subsp. kurstaki, resistance to a spore-crystal formulation of B. thuringiensis subsp. aizawai was only 2- to 4-fold.