S. I. Tanaka1, Y. Natuhara2, C. Imai2 & H. Numata1.
1Dept. of Bio- and Geosciences, Grad. Sch. of Sci., Osaka City Univ., Osaka 558-8585, Japan, E-mail:; 2Osaka City Inst. of Publ. Health and Environ. Sci., Osaka 543-0026, Japan.

In Heteroptera, only a small number of species overwinter at the nymphal stages, and their seasonal adaptations remain poorly understood. A scutellerid bug, Poecilocoris lewisi is distributed in central Japan, China and Taiwan, and overwinters in the final (5th) instar. In Osaka (34.5N), final instar nymphs appear in mid-summer and then most of them overwinter. However, a small portion of insects emerge as adults in the same summer. Under long-day laboratory conditions at 25C, adults emerged without any developmental retardation, whereas under short-day conditions final instar nymphs entered diapause. The critical daylength for the induction of nymphal diapause was about 14.5h, which corresponds to the daylength including twilight in mid-August in Osaka. On the other hand, under long-day conditions, female adults entered reproductive diapause. Under quasi-natural conditions, female adults emerging from mid-May to mid-June started oviposition only after summer solstice. When nymphs were collected from the field in May and then reared under a long-day photoperiod (LD 16:8) at 25C, female adults did not lay eggs until 37 days after emergence. However, females transferred to LD 13:11 on the day of adult emergence started oviposition 20 days after the transfer. Furthermore, when insects were reared under LD 16:8 from eggs and transferred to LD 13:11 on the day of adult emergence, they started oviposition 11 days after emergence. The critical daylength for the induction of adult diapause was between 14 and 15h. This photoperiodic response results in a shorter preoviposition period in adults emerging from non-diapause nymphs in mid-August than in adults after overwintering as nymphs. Seeds of the dogwood Cornus controversa, the principal food of this species, ripen in July in Osaka. It is suggested that adult diapause of P. lewisi prevents reproduction until the season when ripen seeds of host plant are available.

Index terms: Heteroptera, nymphal diapause, adult diapause, photoperiod, host plant.

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