ESTIMATION OF MONITORING RANGE OF SEX-PHEROMONE TRAP FOR THE RICE STEM BORER MOTH, CHILO-SUPPRESSALIS (WALKER) (LEPIDOPTERA, PYRALIDAE) BY MALE HEAD WIDTH VARIATION IN RELATION TO 2 HOST PLANTS, RICE AND WATEROATS

The monitoring range of a sex pheromone trap for the rice stem borer m oth, Chilo suppressalis, was evaluated by the bimodal distributions of male moth head width. The distribution of male moth head width in bot h flight seasons was a combination of the two normal distributions. Th e larger one was shown to be from male moths that fed on water oats (Z izania latifolia) during the larval stage and the smaller one was that of males that fed on rice plants (Oryza sativa). Male moths emerging from water oats were caught by pheromone traps set at six locations ne ar a water oats population (< 250 m) at a higher rate in the second fl ight season than in the first. The reverse relationship was observed a t five other locations (> 250 m). These results suggest that the monit oring range of a sex pheromone trap for this species is narrower in th e second flight season than the first one, probably because of competi tion between wild virgin females and synthetic sex pheromone.