In order for an adult monarch to fly, temperatures need to be above 55 degrees Fahrenheit. While monarch eggs and caterpillars can survive some exposure to cold, these cooler temperatures slow their development time and prolonged exposure may cause sub-lethal effects (e.g. monarchs may take longer to develop, leaving them more vulnerable to disease and predation).
Since there is not a distinct boundary separating breeding and migratory generations, there will be caterpillars that are developing late into the season as temperatures decrease and milkweed ages. Not all of them will survive. In many cases, the milkweed plants (caterpillar food source) may be more vulnerable to freezing temperatures than the caterpillars themselves. Similarly, killing frosts can eliminate remaining nectar plant species that are in bloom, which feed fall migratory adult monarchs. These late season monarchs have a lower chance of surviving the long-distance migration due to a number of additional stressors that they face, including temperatures too cold for them to fly and lack of available nectar to fuel their journey. For monarchs, other pollinators, and thousands of other organisms, these cycles and stressors are part of the natural cycle of life and the seasons.