Determination Of Species And Parasitism Rates Of The Larval Parasitoids Of Meadow Moth, Loxostege Sticticalis (Lepidoptera: Pyralidae) In Different Habitats

The meadow moth, Loxostege stictaialis L. (Lepidoptera: Pyralidae) is one of the major serious insect pests to the economic crops in the northern, northeastern and northwestern China. There have been 23 outbreak years since the establishment of P. R. China,and great yield and economic loss were resulted in during the outbreak periods. The parasitoids played an important role on the population of the host. However only a few researches had involved in tachinids, hymenopteran parasitoids of this pest. Only the describing of species and parasitism rate were simply documented in our country and USSR, which limited the application of them and the understanding the occurance regularity of the host. In order to illustrate the role of the parasitoids on the population of the host, and to develop the effective approach of utilizing them, we carried the systematical research on the tachinids, hymenopteran parasitoids. The main results were summarized as follows:Our investigation of the overwinter generation of 2003 in Hunyuan of Shanxi province, the overwinter generation of 2007 in Siziwangqi of Inner Mongoliav, the first generation of 2004~2006 in Kangbao of Hebei province and the first generation of 2007 in Xing'anmen of Inner Mongolia, showed that there were 10 species of tachinids in these area. Exorista civilis Rondani was as the dominant species. There were 13 species of hymenopteran parasitoids attacking the host lavae, with 7 species of Braconidae, 5 species of Ichneumonidae and 1 species of Chalcidoidea in these area. Perilampus nola Nikol'skaya and Mesochorus sp. was hyperparasitiod. The dominant species in these area were also found..Exorista civilis and Nemorilla maculosa were the two important macroovipar tachinid parasitoids. In order to know the parasitic characteristics of the two tachinids, we conducted a field study in Kangbao, Hebei province. Investigations showed that the parasitism rate of the final host instar was the highest. The number of eggs deposited on a final instar host larvae by the two tachinids varied from 1 to 8, with the single egg individual dominant, and then decreased as the number of eggs increased. Inspection of the tachinid eggs located on different positions and surfaces of the final instar host larvae demonstrated that the head and thorax, and the lateral side was the most favorite position and side attacked by the tachinids, in comparison with the abdomen, the dorsal and abdominal sides of the host larvae. The survival of the tachinid progeny was related to the egg number on the host. The host larvae with one tachinid egg could develop into the tachinid only in proportion of 66.7%.But the host larvae developed only into the tachinids when it had two or more than two eggs even though only one tachinid fly could be survived in a host. Based on these results, the role of the tachinids were discussed. Macroovipar tachinid parasitoids played an important part on the next generation, but limited roles on the current generation.The character of the main parasitoids was also found. Agrypon flexorius was with the preference of fifth instar, and the host parasited by this species could develop into pupae, which was easily recognized. Cotesia sp. was a gregarious parasitiod, and the number of the parasitoids counted from 2 to 12, mainly from 6 to 8 in one host. Perilampus nola Nikol'skaya was with the preference of third and fourth host instar. The parasirsm rate of Mesochorus sp.could reach as high as 8.21%. In order to know the relationship between the host stages and parasitism rate of parasitiods, we conducted the field study in Kangbao in Hebei province. The results suggested that parasitism rate of tachinids of the second instar was the lowest, with 2.7%, and the final host instar was the highest, with 34.95%. There were no difference between the rate of cocoon and fifth instar. No variation of parasitism was found after the host pupated. Inspection of the parasitism rate of hymenopteran parasitoids demonstrated that they could successfully parasitized the second to fifth host instar, with preference for the second instar, with 53.61%. The parasitism rate of the final host instar was the lowest only 12.26%. The species investigated from cocoon was the same as that from host larvae. No variation of parasitism was found after the host pupated. No parasitoid parasitized pupae was found. Based on these results, the role of the parasitoids was illustrated. Tachinid parasitoids played an important role on the next generation, but limited roles on the current generation. However hymenopteran parasitoids played an important role on the current population.We also conducted the study on the factors that affected the parasitism rate. The host density was one of the factors. And the relationship between the parasitism rate and host density was conducted at the different spatial scale. The result suggested that there is no positively relationship found in the small scale, however positively relationship was found in the large scale. Diapause was another factor. The parasitism rate of the first diapausing generation was lower. The diapausing of the second generation had no effect on the parasitism rate. The study on the vegetation structure showed that the higher the damaged degree of plant was, the higher the parasitism rate was. But the opposite relationship was between the plant density and the parasitism rate. Nutrition supplement was favourable to the increasing of the parasitism rate of hymenopteran parasitoids.