The cotton bollworm, Helicoverpa armigera, is an insect pest of cotton. The adult cotton bollworm is a moth. The adult female moth lays eggs on cotton plants. The eggs hatch into larvae. The larvae feed on cotton plants by boring holes into the base of flower buds and developing fruits, causing extensive damage and reduction in yield. [Figure 2.1] shows a mature cotton fruit (cotton boll) from an uninfested plant. [Figure 2.2] shows a cotton bollworm inside a developing cotton boll. A gene, cry, from the bacterium Bacillus thuringiensis (Bt), can be inserted into the cotton genome to produce Bt cotton. • The protein coded by cry is toxic to some insects. • This toxicity gives cotton plants some resistance to cotton bollworm. Bt cotton was first approved for commercial use in China in 1997. By 2001, Bt cotton had been planted extensively in northern China. A long-term study on the effect of planting Bt cotton on the occurrence of H. armigera was carried out at an experimental field station in northern China from 1998 to 2007. Two varieties of Bt cotton, v1 and v2, and two varieties of non-Bt cotton, v3 and v4, were used. A number of fields, covering over 2000 hectares, were divided into standard sized plots. Bt cotton or non-Bt cotton were planted in randomised plots. [Figure 2.3] shows one possible arrangement of the plots in one field. • Sampling was carried out every 4 days during the breeding season of H. armigera. • Five fields were selected at random and a total of 100 plants in each plot were examined. • The total number of eggs and larvae on each plant was counted.