Inheritance of resistance to Bt canola in a field‐derived population of Plutella xylostella

Crops expressing Bacillus thuringiensis ( Bt ) insecticidal Cry proteins are grown on millions of hectares. Recommendations to delay resistance are based on a high expression/refugia strategy that aims to kill resistant heterozygotes and enable some susceptible insects to survive. Leaf‐dip bioassays on F1 crosses of Malaysian populations of diamondback moth ( Plutella xylostella (L)) showed that Cry1Ac resistance was not fully recessive. The survival of ca 50% of heterozygotes on Bt canola ( Brassica napus L) leaves expressing low concentrations of Cry1Ac agreed with a non‐fully‐recessive model for resistance. Extrapolations based on log dose‐logit mortality regressions for heterozygotes using leaf‐dip bioassays showed that a relatively high level of expression, of ca 2000 ng Cry1Ac mg −1 total leaf protein, would be required to give 90% mortality to heterozygotes. If high enough levels of expression of Bt toxin to kill heterozygotes cannot be achieved and maintained under field conditions, the effectiveness of the high‐dose/refugia strategy would be reduced. Copyright © 2003 Society of Chemical Industry