草地におけるウリハムシモドキの密度変動

II. 牧草の刈取り時期と発生密度の関係

抄録

Investigations were undertaken in order to ascertain the effect of mowing on the populatior density of the false melon beetle, Atrachya menetriesi FALDERMANN, in the grassland. The results are as follows:-
According to observations during several years at a ladino clover sward, the density of the beetle population including the egg stage was kept at higher level in the plot mown in the period of adult activity than in the check, except of an outbreak year in which case the density became lower in the mown plot than in the check because of emmigration of adults from the former due to the food shortage. Small scale field tests showed that in general the adult density was higher in the plots mown in the period of its activity than in the check during a few weeks just after mowing operation. As a result of the tests it was also found that in a mixed sward of the ladino clover and the orchard grass the number of eggs per unit area was much higher in the plots mown before and at the peak of adult activity than in the rest. In a field cage adults showed a marked preference to the part at which the grass was cut off just before releasing adults. Based on the results given above, as a means of preventing establishment of the beetle population at high level in the grassland, it was recommended to avoid harvesting during the earlier half of the period of adult activity. This means might be effective especially in large scale grasslands where the clover is mixed with gramineous plants not preferred by the beetle.