There are some butterflies that emerge during the very beginning of the migration that won't migrate. We aren't quite sure why they don't do so, but some will stay in the area and be reproductive. However, the caterpillars that result from these pairings won't survive the winter. Chip has written several articles about degree days (the amount of time needed to develop from egg to reproductive adult) in the Monarch Watch monthly email Updates. (You can find the index here
) Often, he will mention partial generations. For example, he might say the degree days in Michigan will give the monarchs enough time to go through 2 1/2 generations. That half generation is the group of caterpillars/pupae that aren't able to develop into adult butterflies. The butterflies that do emerge very late often won't reproduce; they won't receive the light and temperature cues to do so.
Monarchs are not freeze tolerant, so they won't be able to stick around and tough out the winter. It is highly unlikely they can adapt to cold temperatures. The monarchs that survived the extreme cold, wet weather at the overwintering spots in 2002 and 2004 did so because they were in the middle of masses of monarchs, not because they were genetically a bit different and were able to tolerate the cold, wet weather. In other words, the butterflies at the edges of the clusters were no different from those deep inside the mass of monarchs. (Except, perhaps, that they were unlucky in their position in the cluster!)
So, to sum it all up: monarchs cannot overwinter in places that consistently get freezing temperatures. The very late butterflies won't mate - it's not warm enough and they don't have the light cues - and often the last generation is incomplete. Sometimes these butterflies do last a bit longer because of the cool temperatures; they are not as active as those in the summer and do not expend as much energy. (This is actually one of the reasons the butterflies live so long at the overwintering sites. The temperatures in Mexico usually don't dip below freezing, but they are just barely warm enough for the butterflies to fly (generally 55 degrees F or warmer), so they mostly rest in clusters.)
Whew! I hope this explains things a bit!