Synergistic activity between Bacillus thuringiensis Cry1Ab and Cry1Ac toxins against maize stem borer (Chilo partellus Swinhoe)

To select a toxin combination for the management of maize stem borer (Chilo partellus) and to understand possible mechanism of synergism among Bacillus thuringiensis Cry1A toxins tested. Three Cry1A toxins were over expressed in Escherichia coli strain JM105 and used for diet overlay insect bioassay against C. partellus neonate larvae, both alone and in combinations. Probit analysis revealed that the three Cry1A toxins tested have synergistic effect against C. partellus larvae. In vitro binding analysis of fluorescein isothiocyanate (FITC)-labelled Cry1A toxins to midgut brush border membrane vesicle (BBMV) shows that increase in toxicity is directly correlated to an increase in binding of toxin mix. A high Cry1Ac to Cry1Ab ratio leads to an increase in efficacy of these toxins towards C. partellus larvae and this increase in toxicity comes from an increase in toxin binding. Use of Cry1Ab and Cry1Ac combination could be an effective approach to control C. partellus. Furthermore, we show it first time that possible reason behind increase in toxicity of synergistic Cry1A proteins is an increase in toxin binding.