北海道奥尻島の鼠類

Abstract

This investigation has been carried out in order to explain the environmental factors controlling the onset of diapause in final instar of thh rice stem borer being chiefly reared on artificial diets under aseptic condition. It was demonstrated that the larva, particularly in its middle stage, is susceptible to photoperiod. If the rice stem borer belonging to the Saigoku strain is exposed during the larval feeding period to a short day length which may range from about 8 to 14hr, it enters diapause in the final instar, while a long day from 14.5 to 16hr prevents it. In the Shonai strain however, critical day length is observed at 14.5〜15hr. It is certain that this difference is due to the adaptation to seasonal fluctuation of day length within the area of distribution. Complete darkness (probably continuous illumination as well) plays neither role of induction nor prevention of diapause. The temperature of larval stage also plays some role concerning diapause. Under the complete darkness, high temperature such as 3C℃ prevents diapause, whereas low temperature such as 20℃ induces it, but the temperature effects are masked considerably in day lengths which act as antagonistic agencies. Intermediate temperature (25℃) has no influence upon diapause. So far as this investigation goes, the effects of photoperiod and temperature on egg stage are so slight that the induction or prevention of diapause may be finally decided by the environmental condition of larval stage. In experiment where the larvae were reared on the rice plant instead of synthetic media, it seemed that the diapause was not associated with phases in the growth cycle of the host plant. From the results above mentioned, it was ascertained that photoperiod alone can cause the induction of diapause in the rice stem borer larvae of the second brood in Japan.