Baseline toxicity of Cry1A toxins to the spotted bollworm, Earias vittellaF.

The baseline toxicity of Cry1A toxins on seven field populations of Earias vittella (Fab.) was determined through log dose probit analysis. In almost all populations tested the spotted bollworm larvae were found to be extreme ly susceptible to all the three Cry1A toxins. Results indicated that the th ree Cry toxins tested against neonates were very effective in the nanogram range. The LC(50)s for Cry1Aa ranged from 0.332 to 1.608 ng/sq.cm. of treat ed leaf. Cry1Ab exhibited an LC50 in the range of 0.167 to 0.751 while LC50 to Cry1Ac ranged from 0.280 to 1.215 ng/sq. cm. The data from all assays w ere pooled together and subjected to probit analysis to obtain a composite log dose response. The LC50 deduced from the data were 0.59, 0.47 and 0.88 ng/sq cm, for Cry1Aa, Cry1Ab and Cry1Ac, respectively. This data would be u seful to consider the respective LC(50)s as the baseline susceptibility ind ex for resistance monitoring through the conventional log dose probit assay s. The LC90 values deduced from the composite data were 446, 291 and 627 ng /sq cm, for Cry1Aa, Cry1Ab and Cry1Ac respectively. These values represent the diagnostic doses for routine monitoring of resistance to the respective toxins through discriminating dose assays. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.