These butterflies start to arrive along the Gulf coast of the USA at the end of March and through April when they mate, lay eggs on southern milkweeds, and die. Their offspring continue the migrational pattern by flying further north to repeat the same process, arriving in the Great Lakes states in early June. It is this accumulation of butterflies in the northern USA and southern Canada that then migrates south in the Autumn, flying all the way back to Mexico through October, where they will overwinter and reproduce the young that will begin another migrational journey north the following spring. All together, there are 3-4 generations of monarch butterfly each year. A very small number of monarchs overwinter in pine trees at a few sites along the Gulf coast of Florida, near Tampa and possibly west of Tallahassee.
Several factors threaten the overwintering sites in Mexico.
Many forests are being destroyed by loggers, an industry that threatens so many species worldwide. Also, due to the ever-increasing human population, expansion of villages into the overwintering sites is creating problems. Pesticides pose yet another threat to the monarch population. Ecotourism, which sometimes can create money to support species' conservation, is negatively affecting the monarch population because human presence is disruptive to the monarch's fragile ecosystem. Due to the number of threats to the monarch populations, only time will tell if yet another species will be destroyed by the unfortunate practices of an uncaring, exponentially-growing human population.