Well, the critical time for the monarch population is surviving the overwintering period here in Angangueo.
Cold temperatures are their biggest enemy.
Remarkably they do survive, although not all of them. However, a cold winter in 1992 killed much of the population. Last year there was also a heavy snowfall that killed about 10 percent of the monarchs. The other critical time for the monarchs is after they mate and head north to lay eggs. They depend on milkweed, exclusively, on which to lay their eggs.