| The microcystins, a series of endotoxins, isolated from Microcystis aeruginosa in water bloom in lakes, showed bioactivities against mammals, fish, birds and invertebrates. A few researches conducted abroad investigated on insecticidal activities of microcystins on some insects and demonstrated that the naturally occurring compounds were toxic to some species of insect. However, few researches on mechanisms of microcystins in insect have reported. In China all researches on microcystins focus on effects of microcystins on environment and health and their control rather than their usage in agriculture. The work in the thesis will fill the gap in the study of insecticidal activities of the toxins in China and help to understand the mode of action of microcystin in insects.The toxic strain of Microcystis aeruginosa CCAP 1450/4 was chosen to be cultured in the laboratory and microcystin crude extract was produced from the cyanobacteria, meanwhile, purified microcystin-LR was obtained. Two important crop pest insects, Helicoverpa armigera(HUbner) and Pieris rapae linne, were selected as experimental insects to investigate insecticidal activities of microcystin-LR and crude extract, including antifeeding and repellent activities. The mechanisms of microcystin-LR in the two insects were studied as well. The work will provide potential insecticidal substances from which hopefully some new insecticides or new control measure can be developed, furthermore, it is helpful to reveal the insecticidal mechanism of microcystin. The main results are summarized as follows.1 Insecticidal activities of microcystin-LR against cotton bollworm, H. armigera, were investigated. It showed that microcystin-LR were toxic to the insect when microcystin-LR was injected into the coelom of 4th instar larvae of H, armigera, LD50 value at 24 h and 48 h were 12.89 mg/kg and 7.24 mg/kg respectively, which were comparable in efficacy to the insecticide malathion. However, in topic application experiment toxicities of microcystin-LR were remarkable less than those of malathion against the cotton bollworm larvae, even when applied topically to the larvae dose up to 64.0mg/kg microcystin-LR provided no lethal effect, therefore, it demonstrated low contact toxicity of microcystin-LR.2 Oral toxicities of microcystin crude extract against H. armigera larvae were studied. In the experiment the cotton bollworm larvae were fed with the artificial diet mixed with microcystin crude extract and deltamethrin and ? fenpropathrin were used for efficacy comparison. The work showed LD50 values of the extract against 2nd instar land 3rd arvae at 72 h were 23.75 fig/ml and 83.0μg/ml. Therefore, it demonstrated the extract was highly oral toxic to H. armigera larvae and younger larvae were more sensitive to microcystin than older larvae. In comparison with the insecticides the oral toxicities of the extract were 3-9 times less than those of deltamethrin and fenpropathrin. When 4th instar larvae were fed with the diet the extract provided sublethal effects which affected the growth and development of the insect, including prolonged larval and pupal stages, low pupation rate, deformed pupae et al.. It suggested that microcystin crude extract could be used directly to control some pest insects.3 Toxicities of microcystin-LR against P. rapae larvae were obtained. When injiected into 4th instar P. rapae, microcystin-LR gave 24 and 48 hour LD50's of 3.53mg/kg and 2.35mg/kg, respectively. It showed relatively high toxicities against the insect although toxicities of microcystin-LR were lower than those of the insecticide, carbofuran. Hence the toxicity of microcystin-LR was reduced 10 folds when administered by the oral route, this...
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