Abstract
The effect of the photoperiod on the induction of diapause in rice stem borer (Chilo suppressalis WALKER) larvae collected from three localities: Hsilo (23.8°N), Tounan (23.7°N) and Pingtong (22.7°N), in Taiwan was studied. The larvae were reared at 25°C under different photoperiods. The period of the mature larval stage (the diapause stage) was prolonged under short day lengths (10L-14D or 8L-16D) which shows that the larvae enter diapause under short day lengths. The critical day lengths for inducing diapause in the Hsilo and Pingtong populations were 9-10h but were around 11h for the Tounan population. Except for the Tounan population, these critical day lengths are much shorter than the natural photoperiod (14.5h max, 11.3h min), indicating that these populations hibernate without diapause. Although some of the Tounan population may enter diapause as a result of the winter day length, the depth of the diapause may be shallow because it was terminated without exposure to low temperatures. Some individuals of all the tested populations did not enter diapause although they were reared under short day lengths such as 8L-16D, showing that there are photoperiod-insensitive individuals in the rice stem borer in Taiwan.
