| Beet armyworm, Spodoptera exigua Hubner, is a worldwide distributed polyphagous pest which has gotten high resistance to many kinds of chemical pesticide. Therefore, it's very necessary for people to find new control methods, especially in biological control. Beauveria bassiana Vuillemin (Ascomycota:Hypocreales) is one of the most widely distributed and extensively studied entomopathogenic fungus. Because of its high activity against S. exigua and essentially no toxicity or infectivity toward vertebrates, it is a promising pesticide used in cotrolling beet armyworm. For the sake of better utilization of B. bassiana against S. exigua in the field, we evaluated the effect of factors such as temperature and humidity on its biological characteristics and virulence. At the same time, we assayed the dynamics of numbers of the hemocytes, total contents of free amino acids in haemolym and phenoloxidase (PO) activity in S. exigua larvae after being affected by B. bassiana. The main results obtained were summarized as follows:The biological characteristics of the strain was studied. The germination rate and production of the strain on PDA media under different temperature, humidity, pH and light application time were determined. The suitable range of temperature for the germination of the spores was20-30℃, the best temperature being25℃. At temperature of25℃, The germination rate made an ascent from30.3%to96.8%as RH rose from50%to95%. The strain could grow on PDA with pH4.0-7.0, while it got a rapid growth under pH6.0. the production of the spores under different pH also had similar dynamics. There were no significant difference in the growth of strain between in darkness and in illumination condition from a longer time period, however, darkness could improve germination of the spores better than illumination in the initial two days.The effect of temperature, humidity, dietary stress and larval instar on the virulence of Beauveria bassiana to Spodoptera exigua was studied in laboratory. At relative humidity (RH) of (95±5)%, Mortality of the larvae was significant lower at temperature of18℃and30℃than at other temperatures. For the2nd instars larvae the highest mortality (95.56%) was obtained at27℃in the10th days after treatment. At the temperatures of18,21,24,27, and30℃, the LT50values to2nd instars larvae were6.07,5.69,4.92,4.79and6.68days, respectively. The mortality of the2nd instars larvae after10th days of treatment was enhanced from70.00%to97.77%when relative humidity increased from (55±5)%to (95±5)%. LT50values to2nd instars larvae at humidity (RH) of (55±5)%,(65±5)%,(75±5)%,(85±5)%and (95±5)%,were7.01,5.80,5.61,5.50,4.83days. Under temperature of25℃and RH (95±5)%, the LT50values to long-time starved larvae (4.92d) were smaller than that to normal fed instars larvae (6.17d) significantly. Under temperature of25℃and RH (95±5)%, the LT50values to2nd instars larvae (4.90d) were smaller than those to3nd instars larvae (5.72d) significantly. Therefore, there were significant differences in virulence of B. bassiana to S. exigua at different temperatures, relative humidity and dietary stress of larvae. Temperature range of24-27℃, relative humidity of more than90%, enough dietary stress and much younger larvae were the most suitable condition for B. bassiana infecting S. exigua.We assayed the dynamics of numbers of the hemocytes, total contents of free amino acids in haemolym and Phenoloxidase (PO) activity in S. exigua larvae after being affected by B. bassiana. The results showed that The number of the hemocytes in the normal S. exigua larvae were stabilization, while there were changes when larvae were affected by B. bassiana. Compared with the healthy larvae, the total number of hemocytes after affection within1d had not significant changes. After being affected1d to2d, number of total hemocytes in affected larvae increased quickly after a small-scale decrease, when that of healthy larvae got a small decrease. On the third day the total hemocytes of affected larvae reached the peak of number, afterwards the number decreased gradually. the countes of Plasmatocyte and Granulocyte of the affected larvae also had similar dynamics. On the first day ater affection, the total contents of free amino acids in hemolym of the affected S. exigua larvae had no significant change until the next day the total contents decreased to122.4±24.3mg/100mL. From then on, however, the contents increased gradually. PO activities of affected larvae increased markedly once the affection taken place on the first and reached the peak on the second day, with the PO being1.30times the activity of healthy larvae. however, PO activities of affected larvae had a drop gradually during the next four days, with PO being0.59times the activity of healthy larvae on the sixth day. Those of the normal S. exigua larvae were stabilization during the period of test. |
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