Interactions between Cry1Ac, Cry2Ab, and Cry1Fa Bacillus thuringiensis toxins in the cotton pests Helicoverpa armigera (Hübner) and Earias insulana (Boisduval)

One of the most effective strategies recommended to delay insect resistance to Bt-plants is concurrent expression of several toxins in the same plant. A new generation of Bt-cotton, including Bollgard II and WideStrike, has been developed to simultaneously express two different Cry toxins, Cry1Ac and Cry2Ab, and Cry1Ac and Cry1Fa, respectively. The aim of this study was to determine the individual and combined toxic effect of Cry1Ac, Cry2Ab, and Cry1Fa in the cotton pests Helicoverpa armigera and Earias insulana, as well as the nature of the interactions between these toxins, as determined by mean lethal concentration (LC50) values and larval growth inhibition studies. Singly, all three assayed toxins were more toxic against E. insulana than against H. armigera larvae. Toxin Cry1Ac was significantly more toxic than the other two on H. armigera, while toxin Cry1Fa was the least toxic and caused no significant mortality. When combined, Cry1Ac and Cry1Fa showed an additive interaction in all proportions analyzed for both pest species, whereas Cry1Ac and Cry2Ab interacted synergistically in mixtures comprising 1:1 or 1:4 of each toxin against H. armigera. In E. insulana, there was no synergism between Cry1Ac and Cry2Ab but both these toxins showed a high insecticidal activity when administered individually and in mixtures. This study suggest that each particular toxin or toxin combination expressed in transgenic Bt-cotton should be carefully selected depending on the most important pest species present in each geographical area.