CYT1AA PROTEIN OF BACILLUS-THURINGIENSIS IS TOXIC TO THE COTTONWOOD LEAF BEETLE, CHRYSOMELA-SCRIPTA, AND SUPPRESSES HIGH-LEVELS OF RESISTANCE TO CRY3AA

The insecticidal activity of Bacillus thuringiensis is due primarily t o Cry and Cyt proteins. Cry proteins are typically toxic to lepidopter ous, coleopterous, or dipterous insects, whereas the known toxicity of Cyt proteins is limited to dipterans. We report here that a Cyt prote in, Cyt1Aa, is also highly toxic to the cottonwood leaf beetle, Chryso mela scripta, with a median lethal concentration of 2.5 ng/mm(2) of le af surface for second-instar larvae. Additionally, we show that Cyt1Aa suppresses resistance to Cry3Aa greater than 5,000-fold in C. scripta , a level only partially overcome by Cry1Ba due to cross-resistance. S tudies of the histopathology of C. scripta larvae treated with Cyt1Aa revealed disruption and sloughing of midgut epithelial cells, indicati ng that its mechanism of action against C. scripta is similar to that observed in mosquito and blackfly larvae. These novel properties sugge st that Cyt proteins may have an even broader spectrum of activity aga inst insects and, owing to their different mechanism of action in comp arison to Cry proteins, might be useful in managing resistance to Cry3 and possibly other Cry toxins used in microbial insecticides and tran sgenic plants.