Larvae of one Helicoverpa punctigera and three Helicoverpa annigera strains were stressed by feeding early 3rd instars on artificial diet containing 0.015 mg DiPel 2X (32,000 IU/mg) per ml of diet. The percentage of larvae reaching testing size (that is early 4th instars weighing 30-40 mg) by 10 days from the early 3rd instar (weighing< 5 mg) was 65-75% & 90% for H. armigera & H. punctigera respectively, compared to 99% for the control diet. In addition, those larvae that did survive were about twice as slow to develop from the early 3rd instar to testing size (2.9 & 5.7 days for control and Bt diet, respectively). When larvae reached testing size, they were placed on fresh untreated diet and bioassayed with either fenvalerate, endosulfan or profenofos representing the three major classes of conventional insecticides used against Helicoverpa spp. in Australia