Helicoverpa armigera shows a much higher tolerance than Heliothis virescens towards the Cry lAc toxin present in INGARD and BOLLGARD II cotton. Unlike H. virescens, the major pest of cotton in the USA, some H. armigera are able to survive on INGARD cotton as the plants mature. This survival increases the risk of resistance development.

The impact of proteases in the insect gut was identified as a possible explanation of the higher tolerance of H. armigera. This project contributed to testing the hypothesis by identifying the initial cleavage sites in the Cry1Ac toxin exposed to the gut juices of H. armigera. The two major peptide fragments from the digest were separated and the most likely initial cleavage site was identified from the sequence of the fragments and computer-assisted modelling of the three dimensional structure of the toxin. These analyses showed that the initial cleavage was made by chymotrypsin whereas trypsin is the main protease in the H. armigera gut.

With the identification of the amino acid that is the target for the first cleavage in the Cry1Ac toxin, it is now possible to determine if protease activity is the basis for the higher tolerance in H. armigera. It will then be possible to modify the toxin gene slightly to make the toxin more stable in the H. armigera gut. Use of an appropriately modified gene in cotton would make it more effective than INGARD and BOLLGARD II in reducing the risk of resistance. This thesis is available from the CRDC Library.