Monarch butterflies must decide in advance when it is time to migrate south. Temperature is not the best indicator of the season. There can be cool periods in summer and heat waves in autumn. Many insects, including monarchs use the ratio of daylight to dark to determine the season. In autumn, a short day length will trigger a developing monarch larva to develop into an adult that migrates south instead of mating and producing another generation in the north.
Monarchs of eastern North America that overwinter in Mexico, determine when to start their migration northward from a combination of temperature and increasing day length. The date the monarchs start north is temperature dependent. Cold weather will delay the migration. Warm weather will cause the migration to start early. Milkweeds will germinate later in cold weather and earlier in hot weather. By using a combination of photoperiod and temperature, the Monarchs have a better probability of finding abundant, newly emerged milkweed on their northward migration.