Transgenic cotton, expressing an insecticidal toxin from Bacillus thuringiensis (Bt cotton), has been widely cultivated for a long period in some areas of China. Therefore, there is the potential for the cotton bollworm, Helicoverpa armigera, to develop resistance to Bt cotton toxins. The United States Environmental Protection Agency (US EPA) is rethinking the requirement for some regions to delay developing natural refuge crops where Bt cotton growth has been highly successful. The policy of establishing refuges was intended to delay pest population development of Bt-toxin resistance. Bt cotton is widely grown in Xinjiang Province, which is the largest cotton-producing area in China, but no studies have been conducted on potential refuges in this region of China. In recent years, stable carbon isotope techniques (δ¹³C) have been used to identify the larval host-types (i.e., whether the hosts are C₃ or C₄ plants) of cotton bollworm. These methods have been used to evaluate the Bt refuge function of non-cotton crops.
In this study, intelligent forecast light traps were established to collect adult cotton bollworms from May to September 2013, from the 147^th and 121^st regiments of the Shihezi reclamation area located in northern Xinjiang. The captured moths were placed into bottles and preserved in 95% alcohol until analysis. At each of the sites, 50 males and 30 females moths were selected on the day of capture, their wings combined into two bulked samples respectively to analyze their host type (C₃ or C₄) using δ¹³C analysis. At the same time, 30 female's wings and corresponding spermatophore were separated to one single sample to analyze the mating type and percentage of mating frequency between C₃ and C₄ host. A total of 23 bulked samples from the 147^th regiment and 25 bulked samples from the 121^st regiment were tested. The proportions of C₃ and C₄ hosts were calculated according to a standard curve constructed in 2012. Approximately 50% of cotton bollworms were derived from C₄ hosts, and approximately 50% from C₃ hosts in late May and from early August to September. From June to July, 100% of cotton bollworms were derived from C₄ hosts. To evaluate the mating frequency of cotton bollworms at the two sites, we selected 210 anatomically adult females in the 1^st and 2^nd generations from the 147^th and 121^st regiments. The percentage of effective mating between the two larval host-types (C₃ and C₄) was approximately 10%. Mating frequency ranged from 0.9 to 2.1 per adult female at the two sites, and was higher at the 121^st regiment site than at the 147^th regiment site. Maize is considered to be the most important natural refuge for Bt-susceptible cotton bollworms in northern Xinjiang, since maize is the main C₄ host plant in this area. Thus, maize may provide refuges for susceptible cotton bollworms in the autumn of one year and the spring of the following year. There is a short period in which cotton bollworms derived from C₃ and C₄ hosts simultaneously co-exist, and there is a very small percentage of effective mating between cotton bollworms derived from C₃ and C₄ hosts. This will affect the capacity to dilute the Bt-resistance in cotton bollworms, and may be related to the non-synchronization of C₃ and C₄ crops and male and female cotton bollworms.