Effects of photoperiod, temperature, and host plant age on induction of reproductive diapause and development time in Danaus plexippus

1. Diapause induction in monarch butterflies Danaus plexippus was studied using adults captured from the wild in Minnesota and Wisconsin, and individuals reared under outdoor and controlled conditions. 2. Oocyte presence in females and ejaculatory duct mass in males were used to indicate reproductive status. 3. Some wild individuals were in diapause in mid‐August, and all males and females were in diapause by late August and early September respectively. 4. Individuals reared under decreasing day lengths and fluctuating temperatures were more likely to be in diapause than were individuals reared under long or short day lengths or constant temperatures. 5. Individuals fed potted old Asclepias curassavica plants were more likely to be in diapause than were those fed potted young host plants; when cuttings of Asclepias syriaca plants from the field or greenhouse pots were used, there was no effect of host plant age. 6. Extremely high temperatures increased the number of day‐degrees required for development from egg to adult, while decreasing day lengths and older host plants tended to decrease the number of day‐degrees required for development. 7. There appears to be a continuum of reproductive development in monarchs, with gradual declines in mean ejaculatory duct mass and oocyte production during the late summer. 8. None of the experimental treatments led to 100% diapause, and diapause was more likely to occur in monarchs subjected to more than one diapause‐inducing cue.