Influences Of Cry1Ac Insecticidal Protein On Growth And Development, Reproduction, Food Utilization And Flight Capacity Of Beet Armyworm, Spodoptera Exigua (H(?)bner)

The beet armyworm, Spodoptera exigua (Hübner) is an improtant pest of many economic crops, which is widely distributed in the subtropical and temperature zone and with intermittence, omnivory, and long-distance flight capacity. In recent years, the occurrence of beet armyworm increases not only extends the scope, but also the degree in our country, which caused huge economical loss. Bacillus thuringiensis(Bt) is a kind of gram-positive bacterium widely used in insect resistance crops of transgenic Bt gene, which produces a poisonous insecticidal crystal protein(ICP) against lepidoptera pests along with producing spores. However, most commercial cultivation of transgenic Bt crops predominately expressed Cry1Ac insecticidal protein which is not sensitive against beet armyworm. So, this widespread planting of transgenic Bt crops has been accompanied by increasing resistance of beet armyworm which has become a dominant pest in Bt crops field recently, and brought large difficulty to develop integrated pest management program. This paper studied on the response of larval development and adult reproduction of beet armyworm which fed on artificial diet containing different concentrations of Cry1Ac insecticidal protein in early-instar stage, whole larval stage and three successive generations, the efficiency of food utilization on the old larvae, and the influences on flight capacity in the 1-day adults. We hope to reveal the influences of Cry1Ac insecticidal protein on larval development, adult reproduction, food utilization, and flight capacity, and support some scientific evidences for ecological safety evaluation on the commercial cultivation of transgenic Bt crops and integrated pest management program of beet armyworm population in Bt crops field. The major results obtained were as follows.Larval development and adult reproduction of beet armyworm were examined when early-instar larvae fed on diet containing different concentrations of Cry1Ac insecticidal protein of 5, 10, 20, 40, 80μg/g for 6 days and then transferred to non-Cry1Ac toxic diet. The results showed that, this group of toxic concentrations had significant inhibition on growth and development of the young larve or directly resulted in death, but male or female pupal weight were not significantly different from those of control. The highest number of eggs laid per female was observed at 10μg/g treatment. These results indicated that, at the certain range of concentration, Cry1Ac insecticidal protein could have some positive promotion on reproduction.When beet armyworm were fed on the diet containing low concentrations of Cry1Ac insecticidal protein of 3.125, 6.25, 12.5 and 25μg/g for three successive generations, the results showed that toxic effect of Cry1Ac insecticidal protein on beet armyworm larvae was not obvious, but it had significant inhibition on growth and development of the larvae. Larval development was significantly prolonged in the first and second generations, but was close to the control in every Cry1Ac treatment in the third generation. Pupal duration was also significantly prolonged in the first generation, but became shorter in the second generation and had no significant difference in the third generation compared with the control. The day-3 pupal weight was significantly declined and became stable with increasing of generations. The low concentrations of Cry1Ac had no significantly affected pre-oviposition period, and oviposition period was not affected in the first generation, but was significantly shorter in the second and third generations. Eggs laid per female was significantly reduced but became stable in three successive generations, and the influence of Cry1Ac on eggs was shrinking between generations except the 3.125μg/g treatment. It showed that the larvae and offspring had a good adaption to Cry1Ac toxic diet. And the effect of low concentrations of Cry1Ac on longevity of male was more than female. Furthermoe, beet armyworm were fed on the diet containing high concentrations of Cry1Ac insecticidal protein of 5, 10, 20, 40, 80μg/g for one generations, the results also showed that Cry1Ac insecticidal protein had not significant toxic effects on larvae but significantly prolonged the larval development period.The day-3 pupal weight and eggs laid per female were also significantly declined and reduced respectively. Whether the concentrations of Cry1Ac insecticidal protein low or high in experiments, pupation rate, emergence rate, abonormal rate and sex ratio all had no significant difference compared with the control.The efficiency of food utilization on beet armyworm were obtained when 5^th and 4^th instars fed on Cry1Ac toxic diet of 3.125, 6.25, 12.5, 25μg/g and the diet of 25, 50, 100, 200μg/g respectively. The results showed that, the low concentrations had not significantly affected the 6 indexs of utilization of 5^th instars fed for 12 hours, but on the 4^th instars fed for successive 48 hours. The relative growth rate, relative consumed rate, efficiency of conversion of ingested and digested food were all significantly declined compared with the control when 4^th instars fed on the diet containing high concentrations for 1 day. But every index were all declined compared with the first day in every treatment after feeding 2 days, and the relative growth rate, efficiency of approximate digestion which significantly increased were became increasing compared with the control. These results indicated that, the detoxifying enzymes system in the 4^th and 5^th instars could be activated in short time in order to prepare for pre-pupating when they fed on Cry1Ac diet.The influences of Cry1Ac insecticidal protein on flight capacity in the 1-day adults of beet armyworm were obtained when larvae fed on diet containing high concentrations of 25, 50, 100, 200μg/g until to prepupa. The results showed that, this group concentrations of Cry1Ac insecticidal protein had significantly affected the flight distance, flight duration and flight velocity of the 1-day adults. The certain concentrations of Cry1Ac(25&50μg/g) could have positive promotion on the 1-day female adults, although it did not reach the 5% significant level, and the percentage of adults with strong flight capacity were also higher at 25 and 50μg/g, distance exceed 40 km were 33.3% and 43.3%, duration exceed 8 h were 84.8% and 76.7, velocity exceed 4 km/h were 39.4% and 56.7% respectively. But high concentration had significant inhibition on its flight capactiy, the flght distance at 200μg/g was only 21 km which significantly lower compared with the control. However, this group concentrations of Cry1Ac had no promotion on the male adults, and high concentrations also had significant inhibition on its flight capactiy.According to the results of influences on larval development, adult reproduction, food utilization, and flight capacity when larvae fed on diet containing Cry1Ac insecticidal protein in different concentrations and different feeding times, and combined with the population occurrence in the insect resistance crops of transgenic Bt gene, we clearly revealed that, the early-instar could survive, growth and reproduction after eating the plant tissue expressing Cry1Ac insecticidal protein in the later growing stage of transgenic Bt crops which could have some positive promotion on their reproduction. Larvae and offspring could have a good adaption to the plant tissue expressing Cry1Ac insecticidal protein in growth period, although the succeeding population declined, but became stable with increasing of feeding generations gradually. The influences of plant tissue expressing Cry1Ac insecticidal protein on food utilization of old-instar larvae became weaker with increasing of feeding times. At the certain range of concentration, Cry1Ac insecticidal protein could have positive promotion on flight capacity of the 1-day female adults although the oviposition amount declined. So, the above conclusions obtained proved that, with increasing resistance risk of beet armyworm to the transgenic Bt crops which has become a dominant pest in Bt crops field, the occurrence of beet armyworm increases not only extends the scope, but also the degree. For that reason, we should monitor the population dynamics of beet armyworm freequently in the transgenic Bt crops field, make some more scientific strategies of integrated pest management program.It could be beneficial to improve and enhance the population dynamics prediction and forecast level for this pest, and support some more scientific evidences for ecological safety evaluation on the non-target pests of transgenic Bt crops.