Effects of the ripening stage and the occurrence of split-hull paddy of rice plant on development of nymphs of the rice leaf bug, Trigonotylus caelestialium (Kirkaldy) (Heteroptera : Miridae).

To examine the effects of ripening stage and the occurrence of split-hull paddies of rice plants on the development of Trigonotylus caelestialium nymphs, the nymphs were reared on two rice cultivars, Wasejiman and Koshihikari, that exhibit a high and low rate of split-hull paddies, respectively. The nymphs were released on a caged car with stem and leaf at each of the four rice stages, i.e., at heading and 10, 20 or 30 days after heading, and 10 days after release, the numbers of surviving nymphs, split-hull paddies and injured rice grains were counted. Numbers of split-hull paddies were 0 at the stage of heading and few at 10 days after heading. Survival rates of nymphs were high at the stage of heading and low at 10 days after heading. At 20 and 30 days after heading, Wasejiman exhibited larger numbers of split-hull paddies and supported higher survival rates of T caelestialium nymphs than Koshihikari, although these values varied yearly. There is significant positive correlation between the number of injured rice grains and survival rates of nymphs at these ripening stages, indicating that nymphal development certainly depends on feeding on the rice grains. Most injured grains were found in split-hull paddies, and the survival rates of nymphs were significantly correlated with the number of split-hull paddies. It is considered that nymphs primarily feed on rice grains from the split part of paddies. It is suggested that the occurrence time and the number of split-hull paddies cause fluctuation in the density of T caelestialium nymphs in paddy fields.