In Australia, transgenic cotton plants expressing the cry1Ac gene from Bacillus thuringiensis
Berliner variety kurstaki are less toxic to Þrst-instar Helicoverpa armigera (Hu¨ bner) after
the plant is producing fruit. We developed two bioassay methods (leaf mush, leaf disk) to test if the
physiological state of the plants explained changes in toxicity and a third method (diet incorporation)
was developed to quantify the toxicity of Bt leaves when mixed in chickpea diet. Cry1Ac protein was
less toxic to H. armigera larvaewhenthe protein was mixed with leaves from fruiting versus presquare
conventional cotton. Differences in LC50 varied from 2.4- to 726-fold, depending on the source of
toxin and conventional plant material. These results suggest that plant-toxin interactions in fruiting
cotton are reducing the toxicity of the Cry1Ac protein. The possible role of tannins in these changes
is discussed.