Study On Ecological Adaptability Of Bt (Cry1Ac) Lines Of Spodoptera Exigua

Beet armyworm Spodoptera exigua (Hubner) is one of word-wild polyphagous pest insects and break out intermittently. It has become a major pest in transgenic cotton fields in China. Most transgenic Bt cotton varieties planted in China predominately express Cry 1 Ac insecticidal protein currently and long-term cultivation of Bt cotton will lead to development of resistance of beet armyworm to it. In this paper, Bt (Cry1Ac) resistant beet armyworm strain was used to study effect of Bt resistance on the adaptability to abiotic and biotic factors such as temperature, humidity, host plant and the cold tolerance of beet armyworm which could provide a theoretical basis for probing the evolution of resistance and the comprehensive management of beet armyworm in transgenic Bt crops field. Preliminary results are as follows:1 Effect of temperature on growth and reproduction of resistant strain of beet armywormDevelopmental durations of larvae and pupae of resistant strain decreased significantly with increasing temperature and 24～27℃ was the most suitable for growth and development of beet armyworm. The durations of larvae (23.3d) and pupae (23.3d) of resistant strain were significantly shorter than those of susceptible strain (25.2d; 13.6d) at 21℃. Compared with susceptible strain (11.2d; 8.2d), the durations of larval stage of resistant strain (11.2d; 8.2d) delayed significantly at 27℃ and high temperature 33℃. In the temperature range of 21～24℃, larvae survival, pupation and adult emergence rates of resistant strain were higher than those of susceptible strain. However, in the temperature range of 30～33℃, the pupation and adult emergence rates of resistant strain were lower than those of susceptible strain. At various temperatures, the number of eggs laid by resistant strain females was higher than that of susceptible strain, but at 24℃, the egg hatchability of resistant strain (37.5%) was significantly lower than those of susceptible strain (47.3%), at 33℃, the egg hatchability of resistant strain was zero.2 Effect of humidity on growth and reproduction of resistant strain of beet armyworm70% humidity environment was optimal for the growth and development of beet armyworm. Within 50% to 90% humidity range, resistant strain could complete generation development, but when the relative humidity (RH) was 90%, susceptible strain could not complete a generation. At 70% RH, the durations of larval (12.Od) and pre-pupa (1.5d) of resistant strain were significantly longer than those of susceptible strain (11.2d; 1.3d). At 50% RH, compared with susceptible strain, the period of pre-pupa of resistant strain was delayed (Bt10:1.7d; CK:1.2d), the rate of larval survival decreased (Bt10:90.3%; CK:94.4%) and the pupal weight reduced (Bt10:115.56mg; CK:133.53mg) significantly. Under the same humidity conditions, there were no significant differences between resistant and susceptible strains in pupa duration, adult longevity, pupation rate, adult emergence rate, pre-oviposition duration, oviposition duration, the number of eggs laid by a female and egg hatchability.3 Effect of host plant on growth and fecundity of resistant strain of beet armywormThe resistant strain fed on green vegetables had the shortest larval duration, the highest survival rate of larvae and the heaviest pupal weight, which was 13.0d,74.7% and 90.35 mg respectively, but the paired adults did not spawn. The number of eggs laid by a female of resistant strain (172.2 eggs) feeding on Bt cotton was more than conventional cotton (165.0 eggs), the egg hatchability was also higher than conventional cotton (33B:650%; 33:41.2%). Bt-resistant strain of beet armyworm can not complete the generation development after feeding on celery. After feeding on green vegetables, the rates of pupation (35.3%) and adult emergence (38.5%) of resistant strain were lower than susceptible strain (94.1%; 80.2%). Compared with susceptible strain, the periods of larval (Bt10:14.3d; CK:19.8d) and pupa (Bt10:5.9d; CK:6.2d) of resistant strain were shorter, adult longevity (Bt10:8.Id; CK:6.0d) delayed and egg hatchability decreased significantly feeding on Bt cotton.4 Cold tolerance of resistant strain of beet armywormDifferent developmental stages of beet armyworm had significant impacts on the supercooling points, freezing points and body water content. The supercooling points of pupae of resistant strain were the lowest (-18.18℃), followed by adult,3rd and 5th instar larvae. The trends of body water content of beet armyworm was consistent with the supercooling points. The supercooling points of pupae (-18.18℃) and 5th instar larvae (-9.35℃) of resistant strain were lower than those of susceptible strain (-16.56℃;-8.01℃). Compared with susceptible strain (77.25%; 82.15%), the body water content of 3rd and 5th instar larvae of resistant strain (76.63%; 80.26%) decreased significantly.