TOXICITY OF BACILLUS-THURINGIENSIS SPORE AND CRYSTAL PROTEIN TO RESISTANT DIAMONDBACK MOTH (PLUTELLA-XYLOSTELLA)

A colony of Plutella xylostella from crucifer fields in Florida was us ed in mortality bioassays with HD-1 spore, CryIA(a), CryIA(b), CryIA(c ), CryIB, CryIC, CryID, CryIE, or CryIIA. The data revealed high level s of field-evolved resistance to HD-1 spore and all CryIA protoxins an d no resistance to CryIB, CryIC, or CryID. CryIE and CryIIA were essen tially not toxic. When HD-1 spore was combined 1:1 with protoxin and f ed to susceptible larvae, spore synergized the activity of CryIA and C ryIC 5- to 8-fold and 1.7-fold, respectively, and did not synergize th e mortality of CryIIA. When fed to Florida larvae, spore failed to syn ergize the activity of all three CryIA protoxins, synergized the activ ity of CryIC 5.3-fold, and did not synergize the mortality for CryIIA. Binding studies with CryIA(b), CryIB, and CryIC were performed to det ermine possible mechanisms of resistance. The two techniques used were (i) binding of biotinylated toxin to tissue sections of larval midgut s and (ii) binding of biotinylated toxin to brush border membrane vesi cles prepared from whole larvae. Both showed dramatically reduced bind ing of CryIA(b) in resistant larvae compared with that in susceptible larvae but no differences in binding of CryIB or CryIC.