Baseline susceptibility of the diamondback moth, Plutella xylostella (Linnaeus) to Bacillus thuringiensis Cry1A toxins in India

Toxicity of Bacillus thuringiensis Cry1A δ-endotoxins to the 6-day old larvae of 14 different field populations of the diamondback moth, Plutella xylostella (Linnaeus) collected from seven states in the country was determined by leaf dip bioassay method. Cry1Ab was two times more toxic than Cry1Ac. The median lethal concentrations, LC50 72 h, varied from 0.002 to 0.386 μg/ml for Cry1Ab and from 0.011 to 0.324 μg/ml for Cry1Ac. The winter population of P. xylostella showed significantly lower (4-fold) susceptibility to Cry1Ab than the summer population at Hisar. The discriminating concentration of 15.8 μg/ml for Cry1Ab over a 72 h assay based upon LC99 for the most resistant strain is proposed for the routine monitoring of resistance.