ヒシ群落のある溜池におけるジュンサイハムシとその他の昆虫の季節変動

抄録

The flight activity of Galerucella nipponensis, an herbivorous insect of the aquatic plant, Trapa japonica, was investigated using sticky traps in a pond from 1999 to 2001 in Okayama, Japan. Traps were placed above the water surface in a pond, and G.nipponensis adults were caught throughout the period. This suggested that the adults dispersed actively irrespective of season. Captured G.nipponensis adults had a bias toward females. Other insects were more abundant in early summer and autumn, but less so in midsummer. It has been reported that the density of G.nipponensis became extremely low due to predation by Gerris nepalensis in midsummer (Ikeda & Nakasuji, 2002). A positive correlation between the density of G.nipponensis on T.japonica and the abundance of the other insects caught was obtained in the early season, but such a relationship was not observed later in the season. The leaves of T.japonica extend horizontally on the water surface early in the season but grow vertically in the overgrown rosettes late in the season. G.nepalensis is capable of attacking the beetles in the former but not in the latter. It is possible that the abundance of alternative prey of G.nepalensis indirectly influenced seasonal changes in the density of G.nipponensis.