And now, the details:
Monarch butterflies cannot withstand freezing temperatures and so are not able to overwinter as many other butterflies do. Around October they begin their migration south and west to get to warmer climes in Mexico and parts of Southern California. Once there, they hibernate on specific types of trees: the oyamel fir in Mexico and the eucalyptus tree in California. They use the same trees each and every year. How amazing, when you consider that they aren’t the same butterflies that arrived the year before.
This makes them very vulnerable. The crucial overwintering sites are under threat. Habitat destruction tends to occur because people want to cut down the trees to use the lands for homes, farms and roads.
In the spring the monarchs must head north again because their larval food plants –– the milkweeds –– do not grow in the overwintering locations.
Back they come. An individual monarch butterfly doesn’t live very long, though. The females lay eggs along their return journey then die; the eggs hatch and the new butterfly continues north, and so on. By the time they get here, we may be seeing the third or fourth generation of travelers.
This spring’s monarch count is, once again, down 20-30 percent. There are many causes for this decline, including habitat destruction in their overwintering sites. Here in the United States, we can point to our use of Roundup-resistant corn and soybean crops which allow the fields to be sprayed with this noxious herbicide that kills everything, including milkweed. Monarch Watch estimates that more than 100 million acres of milkweed habitat are now gone and planted to Roundup-resistant crops. We also mow our roadsides which removes yet another habitat for milkweed plants.