草地におけるウリハムシモドキの密度変動

I. 草生状態と発生密度の関係

抄録

Experiments have been carried on to elucidate the relation of the population density of the false melon beetle, Atrachya menetriesi FALDERMANN, to the grass coverage and the legume-grass ratio in the grassland. Assessment of larval, pupal, and adult densities was made by direct count in the field. For the assessment of egg density soil samples were taken in early spring after the eggs became diapause-free and absorbed water from the soil. The egg were separated by a combined method of sieving and floatation. The results are summari-zed as folows:-
When 1 /600 a. sample unit was adopted in a mixed sward of the ladino clover and the orchard grass, number of individuals per unit area was apparently low, in every developmental stages, on units with very high coverage or with very low rate of legume. Using a sampling cylinder of 3.5 cm in diameter, it was confirmed that the eggs were deposited mainly on the parts covered with the ladino clover alone. In a field cage planted with the ladino clover, many of the released adults concentrated in oviposition period upon the plot with lower grass coverage. In conclusion it was pointed out that not only local distribution of the beetle but also the total population level in a grassland may be largely determined by the grass coverage and the legume-grass ratio through the process of habitat selection especially of the adult.