It will freeze soon in my area, and I'm still seeing monarch caterpillars. Will they survive? What should I do?
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 or senesces. Not all of them will survive. In many cases, the milkweed plants (and 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.
If temperatures are predicted to stay around or above 55 F, you may consider bringing a few late caterpillars indoors to speed up their development in your warmer house before sending them off on their migratory journey (when at least 55 F, and ideally sunny). Please use best practices for rearing monarchs when bringing caterpillars indoors. Late season observations and rearing (survival) should also be reported to monarch citizen science programs.
As milkweed and nectar resources become less available (and temperatures continue to drop), you may be able to bring caterpillars indoors to raise them to adulthood, but the likelihood of those monarchs reaching their overwintering destinations decreases drastically after peak migration. In these cases, you may wish to keep those adults for use in educational programming, or provide them to a local school, educational facility, or youth group for them to observe and learn from. These experiences for youth are highly valuable, and may drive long-term conservation benefits for monarchs.