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Monarch butterflies obtained at monthly intervals during the annual adult cycle were held in summer conditions for 5, 10, and 20 days. Examination of these revealed in both sexes an adult reproductive diapause characterized by depressed growth of those reproductive organs sensitive to juvenile hormone. Diapause began for both sexes in late August or early September; female diapause ended in December while male diapause ended in November. Diapause intensity, maximal in September-October, appeared to be greatest in females. In summer conditions, reproductive tract growth in postdiapause females, but not males, was comparable to that observed in prediapause animals. Previously mated postdiapause, but not diapause, females showed a greater response to summer conditions than did virgins. Storing prediapause monarchs in winter conditions for up to 5 months did not induce diapause, indicating that diapause may be induced in a preadult stage. Diapause monarchs of both sexes responded to juvenile hormone with pronounced growth of the pertinent reproductive organs, suggesting that decreased production of juvenile hormone may be a characteristic of adult diapause. The possible advantages of diapause to the survival of the adult population are discussed.
William S. Herman (Sun,) studied this question.