Selection and characterization of the Bacillus thuringiensis strains toxic to Spodoptera eridania (Cramer), Spodoptera cosmioides (Walker) and Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae)

Among the pests of cultivated plants, the Spodoptera species complex is one of the most important. This genus is composed of polyphagous insects that attack, among other crops, cotton, corn, soy and castor bean. In Brazil Spodoptera frugiperda has been recognized as a primary pest in cotton, and Spodoptera eridania (Cramer) and Spodoptera cosmioides (Walker) species are important pests in cotton and soy. This study was performed in 2008 and was focused on selecting and characterizing Bacillus thuringiensis strains highly pathogenic to S. cosmioides, S. eridania and S. frugiperda and identifying Cry proteins toxic to these species. One hundred strains that exhibited toxicity to Lepidoptera were evaluated through bioassay and the most toxic ones were characterized through morphologic, biochemical and molecular analyses and LC50 values were determined. Since the cry1Aa, cry1Ab, cry1Ac and cry2Aa genes were present in most of the toxic strains, recombinant Bt producing toxins encoded by representative genes from these families, were individually tested. The Cry proteins showed different levels of toxicity to the three Spodoptera species, with Cry1Aa and Cry1Ab the most toxic to S. cosmioides, Cry2Aa the most toxic to S. eridania and Cry1Aa, Cry1Ab and Cry2Aa most toxic to S. frugiperda. Cry1Ac, a component of some transgenic cotton varieties such as Bollgard I, extensively used in Brazil, presented low toxicity to the three species studied.