| The effect of lead and cadmium (the Pb2+concentration in feeds were0.3,1.2,4.8,19.2and76.8mg/kg, while the Cd2+concentration in feeds were0.2,0.8,3.2,12.8and51.2mg/kg) to the experimental population of three successive generations of beet armyworm was studied using life table in the laboratory. Accumulation and excretion activities of the two metals were also detected by ICP-AES. At the same time, the Pb2+and Cd2+content and MT content in larval midguts and the correlation between them were studied in order to expressing detoxification mechanism of beet armyworm to lead and cadmium. The concrete results obtained were as follows.(1) The Pb2+and Cd2+contents were significantly different when S. exigua was reared under different Pb2+and Cd2+concentrations in the same generation in a Significant dose-dependent manner. The concentration of Pb2+and Cd2+in different life stages of beet armyworm increased with prolonging of stress time. The concentrations of Cd2+in different developmental stages of beet armyworm follow the order:larvae> pupa> adult.While the order was larvae>adult> pupa when fed on Pb2+. With the time extension, their concentrations of Pb2+and Cd2+decreased. The concentrations of the two ions decreased quickly in mated females.Five days after eclosion, Cd2+concentration in mated females was61.02%of the day of emergence, compared with70.81percent in the control group.(2) Beet armyworm could excrete heavy metals by means of faeces, prepupa exuviate and puparium. Furthermore, Larvae’s excretion capacity of Cd2+was stronger than that of Pb2+. Beet armyworm could resist the toxicity of heavy metals by increasing the level of MT expression in midguts. Under Pb2+(Cd2+) stress, it was very conspicuous direct correlation between express quantitation of MT and the content of Pb2+(Cd2+). High level accumulation of heavy metals could significantly induce expression of midgut MT, and the inducibility of Cd2+was higher than that of Pb2+.(3) At the beginning of metal stress, high-Pb2+(Cd2+) inhibited the larvae from growing up, while low-Pb2+(Cd2+) could promote the growth of larvae. As the generation number increased, the effect to larvae gradually enhanced:Development duration of S. exigua was basically prolonged when generation added. We found that different concentrations of Cd2+had no regular effect on adult longevity, while high-Pb2+could significantly shorten the adult lifespan. Low concentration of Pb2+and Cd2+had no significant effect on three successive generations of beet armyworm larvae mortality, pupation and emergence rate. But the mortality, pupation and emergence rates of the second and third generation dropped significantly at the highest concentration group (Pb2+76.8mg/kg and Cd2+51.2mg/kg), with the regards of the third generation, the larvae mortality rose to65.18%and62.79%respectively. Pb2+and Cd2+can also affect the sex ratio of the beet armyworm. At the beginning of heavy metal stress, the female ratio declined under low-Pb2+(Cd2+), but when treated by the highest concentration of Cd2+and three high concentrations (4.8,19.2and76.8mg/kg) of Pb2+, the female ratio of three consecutive generations of beet armyworm had a significant increase, thereinto, for the treatment of Pb2+76.8mg/kg, the female ratio of beet armyworm was11.58percentage points higher than that of the control group. With the continuation of generations, the female ratio increased in all treated groups. Low-Pb2+(Cd2+) had no bad influence on the pre-spawning, spawning duration, fecundity and hatching rate of three successive generations of beet armyworm, they had even played a positive role. For example, in the first generation, the fecundity was94.2larger than the control group when Pb2+concentration were0.3mg/kg in feed. But high-Pb2+(Cd2+) can significantly inhibit the reproduction of the beet armyworm, showed by the preoviposition extended, fecundity and hatchability declined, and the inhibitory effect was more obvious with the continuation of the generation.(4) According to the survival rates of different developmental stages, female constituent ratios and the spawning numbers, we constructed out the laboratory population life table. As the chart shows, whether stressed by Pb2+or Cd2+, the population trend index (I) of beet armyworm were as follows:low doses promote the growth of the population, high doses inhibit the growth, meanwhile, with the increase of stress time, the index of population trend (I) declined rapidly. |
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