ツマグロイナゴの個体群動態

抄録

A local outbreak of the grasshopper Mecostethus magister occurred in 1961 on a marshy grassland, 5 ha in area. This species completes its life-cycle in Miscanthus sinensis-community, excepting at the time of the outbreak when many individuals invaded into cultivated land. The population dynamics of this species from 1961 to 1963 was described briefly in the present paper. Number of adults emerged in this grassland was(25〜50)×10^4 in 1961,and 3.5×10^4 in 1962. A key-factor for this decrease was suggested to be flooding. As the grassland was submerged during July of 1961,most of females died having no time to oviposit their egg-pods. About 10 to 20 per cent of eggs were killed by predation, while no egg-parasite has been discovered so far. Nymphal mortality was high from the first to third instar but reduced after the fourth instar. The number of egg-pods laid in 1962 was remarkably larger than that in 1961,because of no flooding and low adult mortality. Soil profile showed that this grassland was gradually drying up due to artificial drainage of surrounding basin, offering good conditions for the grasshopper's propagation.