Geographical variation of photoperiodic wing form determination and genetic background of reproductive diapause in arrhenotokous populations of Thrips nigropilosus Uzel (Thysanoptera: Thripidae) in Japan

To determine the level of inter-population variation in wing form and the induction of reproductive diapause as affected by photoperiod, females of five populations of arrhenotokous Thrips nigropilosus Uzel were reared under four different photoperiodic conditions at 18°C. The experimental populations originated from Nakijin-son, Motobu-cho, and Uruma-shi in Okinawa Island, Amami-shi in Amami-Oshima Island, and Wakayama-shi on Honshu Island. In all five populations, only macropterae were observed when the photoperiod was 13 h light/11 h dark. Brachypterae were observed as the light period was reduced to 11.5 or 12 h in the Wakayama population, and to 10 or 11.5 h in the Nakijin, Motobu, and Uruma populations. Brachypterous females were observed in the Amami population only when the light period was 10 h. With the same photoperiod, only brachypterae were observed in the Wakayama and Uruma populations. Short-day conditions also induced reproductive diapause in the Wakayama population but not in the Amami and Okinawa Island populations. Under 10 h light/14 h dark, almost all hybrids obtained from crossing Wakayama with Amami populations entered reproductive diapause. Some populations in Okinawa Island may have been originated in northern localities.