Survival, weight gain, and oviposition of resistant and susceptible Plutella xylostella (Lepidoptera: Plutellidae) on broccoli expressing CrylAc toxin of Bacillus thuringiensis

Leaf bioassays and oviposition choice tests were used to investigate the effects of transgenic broccoli expressing Cry1Ac toxin of Bacillus thuringiensis (Berliner) on susceptible and resistant Plutella xylostella (L.) larvae. Survival of susceptible 2nd instars on Cry1Ac-expressing broccoli declined from 99.1 to 19.2% at 24 and 72 h, respectively, and larvae exhibited an average weight loss of 0.2 mg/10 larvae at 24 h. Larvae that evolved resistance to foliar sprays of B. thuringiensis subsp. kurstaki in the field, however, showed no debilitating effects from the Cry1Ac broccoli. Survival of resistant larvae at 24 and 72 h was 98.6 and 90.8%, respectively, and weight gain at 24 h was 1.7 mg/10 larvae, none of which was significantly different from survival or weight gain on control plants. In oviposition choice tests, both susceptible and resistant females were unable to discriminate between Cry1Ac and normal broccoli, laying approximate to 38-41 eggs per plant per 2 females. Comparing mortality of susceptible larvae on 2 lines of transgenic broccoli (J1R and K20) derived from independent transformation events, we found that the majority of the variance (43.2%) in toxin expression was caused by transformation. Depending upon the transformation, plant could be a significant source of variation but toxin expression within plant was always fairly uniform. Our data indicate that resistance to sprays of B. thuringiensis can confer resistance to plants when similar toxins are involved.