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.

1. (a)(i)State two ways in which the design of the field study contributes to the validity of the results. 1 2[2]
2. (a)(ii)Suggest two variables which cannot be standardised in this field study. 1 2[2]
3. (b)(i)Table 2.1 shows the results of this study. Table 2.1 year density of eggs /mean number of eggs per 100 plants density of larvae /mean number of larvae per 100 plants Bt cotton (v1 + v2) non-Bt cotton (v3 + v4) Bt cotton (v1 + v2) non-Bt cotton (v3 + v4) 1998 810 920 18 500 1999 995 930 18 340 2000 620 595 72 237 2001 730 782 12 240 2002 200 180 15 173 2003 52 50 24 61 2004 6 5 15 54 2005 120 120 27 75 2006 422 420 33 144 2007 195 190 8 8 The researchers decided to calculate the standard error for each of the mean values shown in Table 2.1. State why they decided to calculate standard error.[1]
4. (b)(ii)The t-test could be used to find out if the difference in the mean number of larvae on Bt cotton and non-Bt cotton in each year is significant. Explain why the t-test could be used to analyse the data in Table 2.1.[2]
5. (b)(iii)A trend is a change over time. Describe three trends shown by the data in Table 2.1.[3]
6. (c)(i)H. armigera is a pest on a variety of other crop plants in Asia such as peanuts, soya and rice. Another long-term field study was carried out to investigate the effect of growing Bt cotton on the density of the larvae of H. armigera on other crop plants growing in the same region of northern China. Table 2.2 shows the results of this study. Table 2.2 year mean population density per year of larvae on other crop plants/ number per hectare before the commercial use of Bt cotton 1992 to 1996 5865 after the commercial use of Bt cotton 1997 to 2001 2011 2002 to 2006 761 State one variable that should be standardised in this field study.[1]
7. (c)(ii)Suggest one conclusion about the effect of Bt cotton on H. armigera that could account for the results in Table 2.2.[1]

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