To release captive monarch adults in northern states of the US, temperatures need to be at least 60 F (if cloudy, or 50 F if sunny). There should be a probability of similar temperatures for a few weeks ahead, with no times below freezing. Release in early October will usually be fine. But if your butterflies eclose too late in the fall, you need to find someone who is driving south, or hold them in healthy conditions until spring when danger of frost is past.
Adults can briefly tolerate temperatures a few degrees below freezing, but anything longer or lower will kill them.
Monarchs overwinter in the mountains of central Mexico from November through March, where the weather is cool and moist, seldom below freezing. When the weather is cold or stormy, they rest for days in dense clusters in the branches of Oyamel fur trees. During respites of warmer weather, they may arouse and fly out to drink from puddles or sip nectar from flowers. It's quite likely that even when it's too cold to fly, they suck droplets of moisture that have condensed on their own bodies.
So, maintaining them under high humidity is key. They should be misted once a day, and kept away from drying drafts, especially heating ducts that vent warm air. Find a cool place with low light, probably in a corner of your basement. Place a thermometer nearby, and make sure this location doesn't freeze. Beware of frost on the wall. But high humidity often promotes growth of mold, so let your butterflies dry out at least once a week.
You could keep the butterflies at room temperature. But this would require feeding more often, with a much higher probability that they would die from dehydration. With the butterflies more active at room temperature, it's likely their wings or legs would wear out before spring from constant fluttering.
Your butterflies won't need to feed often because they are cold-blooded, meaning their metabolism is very slow when it's cool. They can live off their fat deposits for months. Nevertheless, we don't know the state of their fat deposits, so it's best to provide them with a chance to feed once a week. That's also a good time to dry them to combat mold. Slowly warm them up over several hours while providing bright light, then offer them honey water (one part honey to 9 parts water). Offer them a q-tip with honey water, then transfer them as they feed to an artificial flower. That's a plastic scrubbie in a dish, sprinkled with the honey water. They learn to feed on the artificial flower, so feeding them takes less of your time and ensures they get all they want. Leave them with the scrubbie for a few hours, then move them back to the basement. If you have more than a few butterflies, it's best to give them individual marks on a wing with a sharpie marker, so you know that all are feeding.
Before releasing any butterflies in spring, they should be checked for the OE parasite. Do not release any that are infected. It's thought that infection with OE makes monarchs too weak to complete the migration--so overwintering in Mexico is how OE is reduced the in the wild population.
Update: My granddaughter had about 10 butterflies that emerged too late to migrate, so she tried the methods outlined above to keep them for the winter. They lasted for a month or so, but all eventually died. I wasn't there to supervise, so I don't know the reason.
Whenever you have a problem raising butterflies, to solve it you first come up with hypotheses for the cause of the problem, then look for evidence for each hypothesis. In this case the hypotheses would be, in order of probability:
They became dehydrated, because she didn't mist them often enough, or the basement humidity was too low.
- The butterflies were of the reproductive kind, not migratory. Reproductive monarchs do not have large fat deposits and typically die after only a few weeks of life. Starting in late August, the migratory monarchs start to eclose. But perhaps because of something about how she raised them, they were instead reproductive monarchs. (It's thought that during the caterpillar stage, declining temperatures, declining day length, and declining food quality trigger the development of migratory monarchs.) I believe you would have to dissect monarchs to distinguish reproductive from migratory ones.
- They starved, because they started with insufficient fat deposits, or fed too infrequently, or were too active because of frequent disturbance.
- Basement environment was inadequate (humidity too high or low, temperature too high or low, light levels wrong, etc.)
- Roosting situation was inadequate. They were kept in a 1" square mesh cube. Maybe they need a larger space, and/or an evergreen branch to cling to.