Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) has caused poor yields to a range of agricultural crops, particularly to cotton. Pesticides have been used to control this pest with serious undesirable side effects, including the rapid development of high levels of resistance. Since 1996, genetically modified cotton (Bt cotton) has been planted to control H. armigera in Australia. However, it is reported that surviving larvae of all sizes can be found in fields from time to time in all growing regions. Research has shown that the survival of these larvae on Bt cotton is not necessarily due to physiological resistance, and behavioural resistance has been inferred. Extensive work on various aspects of behaviour of H. armigera in Australian has been conducted; however, to date those experiments were carried out using a Bt-susceptible H. armigera strain. Experiments in this thesis were undertaken with both physiologically Bt-resistant and Bt-susceptible lines of H. armigera. One might expect larvae that are susceptible to Bt to show differences in behaviour in comparison to Bt-resistant larvae. Oviposition choice experiements consistently showed that both Bt-resistant and Bt-susceptible moths did not choose plants or plant parts that were less toxic in terms of Bt toxin on which to lay eggs. There was one exception in that Bt-susceptible moths were more likely to lay eggs on squares of Bt cotton plants than those of non-Bt cotton. As expected the mortality of Bt-susceptible H. armigera larvae was significantly higher on structures of Bt cotton plants than on those structures of conventional cotton, and survival was greater on flowers than on other structures of Bt cotton. Bt-susceptible neonates of H. armigera, which were significantly heavier, could starve longer and recover better than Bt-resistant neonates. Although H. armigera neonates did not shift their behaviour towards Bt toxin on artificial diet before their first feeding event, Bt-susceptible neonates showed a tendency to remain on non-Bt diet and move off Bt diet. These behaviours may have allowed them to survive in a Bt environment, and led to a higher percentage of survival and pupation in situations where a choice was offered. The behaviour of Bt-susceptible larvae, which differed from Bt-resistant larvae, could help them avoid Bt toxin by moving off Bt substrates (drop-off behaviour) or staying on Bt substrates but eating their conspecific eggs and so survive the first instar stage (cannibalism behaviour). There was a significant difference in the numbers of Bt-susceptible larvae dropped off the two lines of cotton, more Bt-susceptible larvae dropped off Bt cotton than non-Bt cotton plants over time. Significantly more Bt-susceptible larvae remained on squares and flowers, and this may offer an opportunity for them to survive on Bt-cotton plants. The survival of Bt-susceptible H. armigera larvae significantly improves on Bt cotton plant when they cannibalize eggs before feeding on the plant. Cannibalism may play a significant part in the survival of Bt-susceptible H. armigera larvae on Bt cotton plants. Egg cannibalism could explain the relatively small, but surprising number of Bt-susceptible larvae surviving in Bt cotton fields. In conclusion, subtle differences in the behaviour of Bt-susceptible female moths and first instar H. armigera larvae may allow the larvae to survive on Bt cotton plants. Some of Bt-susceptible larvae laid on squares or flowers that contain less Bt toxin and are generally more nutrient rich have higher chances to survive. In addition, egg cannibalism contributes to the higher survival of Bt-susceptible larvae on other structures of Bt cotton plants. Larvae can move off plant parts with high Bt levels and may encounter less toxic plant parts. Bt-susceptible larvae can survive without food for 48 h, suggesting they have time to re-establish on less Bt toxic plant parts after moving away from higher toxic areas.