As North Carolinians spend time outdoors enjoying nature, they might spot some beautiful monarch butterflies heading back from a winter vacation in Mexico. Each fall, monarchs in the United States and Canada embark on a long journey to overwintering destinations: Populations in eastern North America travel to the Sierra Madre Mountains of Mexico, while those in western North America travel to California.
The oyamel fir forests of Mexico and the Pacific Coast of California offer ideal microclimates for monarchs; they aren’t too hot or too cold, and they provide enough humidity so that the monarchs don’t dry out. At these sites, monarchs congregate on tree trunks and branches for the winter, mating and laying eggs in February. When winter is over, the new generation of monarchs travel back to their northern homes, with some travelling as far as 3,000 miles! Luckily for North Carolinians, many of these monarchs travel right through our state to return home.
The monarch butterfly is unique in that it is the only butterfly known to make a two-way migration. Monarchs can produce up to four generations each summer, with adults normally living from two to five weeks. The last generation of the year consists of the butterflies that make the long journey south. While most monarchs live for a few weeks, the last generation is different — adults can live eight or nine months! This longevity is due to a process called “reproductive diapause”, a delay of reproduction until February or March. By delaying reproduction, the overwintering monarchs conserve energy, allowing them to live much longer than other generations.
Even more fascinating, the migrating generation of monarchs has never been to Mexico before. No single monarch makes a complete round trip. So, how do they know their way to the mountain areas in Mexico? Many scientists believe that monarchs use a combination of directional aids, which can include the magnetic pull of Earth and the position of the sun.
Talk about picky eaters! Milkweed plants are the only plants monarch caterpillars eat. The plant itself offers protection. Milkweed has toxic chemical compounds that cause monarchs to become distasteful to predators after eating it. Monarchs warn predators of their toxicity with bright orange and black coloration, which screams “don’t eat me!” After the young of the overwintering generation matures, they head back north. Still, this generation won’t see the habitat of their parents because it takes three to four generations to reach the northern U.S. and Canada!
The monarch migration is a captivating event, but monarch populations are declining. In 2019, 300 million monarchs travelled north from Mexico, but only 141.5 million are making the journey this year. Scientists from World Wildlife Fund Mexico estimate population sizes by measuring the area of the trees that appear orange from the clustering of butterflies. In 2019-2020, the eastern population filled approximately seven acres in their overwintering site in Mexico, a 53% decrease from last year. The western population also remains critically low. Last year’s population estimate was the lowest level ever recorded, and the number is nearly the same this year. With only 29,418 butterflies, the population estimate is less than 1% of that observed in 1980. The decline in the monarch population has largely resulted from a loss of breeding habitat in the United States, caused by herbicide use and land development. This December, the U.S. Fish and Wildlife Service is set to issue a decision on whether to protect monarchs under the Endangered Species Act.
Now that you know all about Monarchs, how can you help with their population decline? One of the best ways is to practice responsible gardening. Any backyard can become butterfly habitat by incorporating native, flowering plants! While adults feed on nectar of a variety of plants, caterpillars only feed on milkweed, so native milkweed is especially important. You can also limit the use of herbicides by using other control methods such as flytraps, pheromone traps, or simply hand-pulling weeds. With citizen science, you can also help researchers learn more about monarchs. You can help map the migration by reporting sightings or participate in tagging projects to help scientists track them.
In North Carolina, we can expect to see these beautiful butterflies travelling north from May to July. Keep an eye out and you just might see a swarm of monarchs, called a kaleidoscope, flying through our state! At summer’s end, you can find monarchs returning to Mexico from September to early October and go directly through Asheville. As you marvel at their beauty, consider giving these travelers a tasty snack by planting native milkweed and other nectar plants!
Blog written by Courtney Smith, N.C. Forest Service Forest Health Intern