Ovarian dormancy in newly eclosed females of Danaus plexippus was induced and maintained by a temperature of 12°C. Reproductively dormant females removed from overwintering clusters at their formation showed rapid ovarian development when placed in 28°C. Ovarian dormancy in D. plexippus does not show characteristics of diapause and is better described as oligopause.
Non-breeding cluster populations of Danaus plexippus (Danaidae) were monitored at five sites in the Sydney area of New South Wales for 5 years. Meteorological records showed that cluster formation was always preceded by periods of cool overcast weather, during which maximum temperatures remained below 18�C for at least 2 days. Field experiments were carried out to test the hypothesis that low post-eclosion temperatures cause reproductive dormancy in Australian D. plexippus. Butterflies were reared in the laboratory and on eclosion exposed to selected temperatures and photophases for 2-4 days before being tagged and released. Recapture data showed that high post-eclosion temperatures (19 or 28�C) inhibited long-distance flight and resulted in ovarian development, but low temperatures (15:6�C, 7:17 h) inhibited ovarian development and permitted long-distance flight. Intermediate temperatures (e.g. 15�C or 12�3�C continuously) gave mixed results. Photoperiod during or before the period of exposure to low temperature did not affect reproduction. The induction, maintenance and termination of reproductive dormancy in Australian D. plexippus are discussed.
Data were obtained on population and general biology of three non-reproductive overwintering colonies ofthe monarch butterfly, Danaus plexippus (Danaidae) during April-August in 1980,1981 and 1982 at a site near Camden in New South Wales. A mark, release and recapture study showed that the 1981 colony was largely stable and the 1982 colony was migratory. In 1980 and 1981 populations comprised less than 500 individuals but in 1982 a peak population of 3500 butterflies was recorded. Males were sexually active throughout clustering. Most females were mated but showed no ovarian development. Periods of flight and feeding activity were common. The condition of most butterflies did not deteriorate markedly during clustering. Comparisons between winter non-reproductive monarch populations in North America and Australia are discussed.
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