抄録
In the 1991-strain of Aquarius paludum in Kochi, both critical photoperiods for wing-form and diapause shifted to longer values by 1 h when specimens were grown under gradually decreasing photoperiods. However, the 1999-strain didn't respond at all to decreasing (14.5L to 13.5L) or increasing (12.5L to 13.5L) photoperiods. Photoperiodic response for wing-form determination by the 2002-strain was witnessed only in a small range between 60% and 95% as the proportion of long-winged form, and the critical photoperiod (14L) was a little bit longer than the critical value of 13.75L for the 1991-strain. The critical photoperiod for diapause induction in the 2002-strain was estimated to be 13.5L-10.5D under a constant photoperiod, which was one-half hour longer than that of the 13L-11D shown by the 1991-strain. In August and September in Kochi, the critical day lengths for wing-form determination and diapause induction of the 1999–2002-strains remain to be 14L-10D and 13.5L-10.5D, respectively, even under decreasing day-length. Therefore, the two critical values were “shorter” than those of “15L-9D” and “14L-10D” of the 1991-strain. The annual average temperature (17.1±0.5°C) and annual average high and low temperatures (35.6±1.3, −3.1±0.9) in 1991–2000 were higher than those (16.4±0.5, 34.9±1.1, −4.4±1.4) a decade ago (1981–1990). In Kochi, the global warming in the last two decades may have caused A. paludum to select “shorter” critical photoperiods for the currently dominant strain (1999–2002-strain) in terms of wing-form determination and diapause induction under decreasing day-length in August to October (14.8 h to 11.5 h) in compared to the former strain (1991-strain) dominant a decade ago. This “shorter” critical photoperiod leads to a longer active fall season for reproduction and larval growth of A. paludum.
