The North American monarch butterflies (Danaus plexippus plexippus), a species famous for its multigenerational, cross-continental migration patterns, have recently been classified as endangered by the International Union for Conservation of Nature (IUCN), the world’s leading scientific authority on the status of species. This decision comes after decades of steady decline in monarch populations driven by losses of the plants they need as caterpillars, combined with the impact of climate change.
“It’s been so sad to watch their numbers decline so much, so anything that might help them makes me happy, and I think that this designation might help them,” said Karen Oberhauser, a conservation biologist at the University of Wisconsin-Madison who contributed to the assessment. “Although it’s sad that they need that help, that they’ve reached the point where this designation is warranted.”
According to Dr. Oberhauser and her colleagues, the number of Western Monarchs, which live west of the Rocky Mountains, has plummeted by 99.9 percent between 1980 and 2021. Although they rebounded slightly in 2022, they still remain in great danger. Eastern monarchs – which make up most of the monarch butterfly population in North America – also dropped by an estimated 84 percent from 1996 to 2014.
Monarch butterflies depend on milkweed, the only plant that they can consume. After departing from their overwintering grounds in the forests of central Mexico, females deposit eggs on milkweed plants from Texas to Canada, during a multigenerational journey. While habitat destruction in the Mexican forests was an early threat to the butterflies, the fact that American farmers turned to crops genetically modified to withstand glyphosate, a widely used herbicide, created significant problems for the monarchs.
“Glyphosate was suddenly sprayed over vast acreage of farm in the Midwest,” said Anna Walker, an entomologist at the New Mexico BioPark Society who led the assessment. “That took out a lot of the milkweed plants that the monarch caterpillars rely on.”
Besides these problems, climate change has also significantly contributed to the steep decline of monarch populations, by disrupting their ancient cycles through increases in storms, droughts, and other similar events, which can be catastrophic for already vulnerable populations. “We’re starting to see this kind of mismatch between when insects are ready to start the spring and when plants are ready,” Walker explained. “There are a ton of unknowns.”
According to the experts, a viable solution for saving monarchs from extinction is by planting milkweed native to each specific region in the U.S. and Canada. For instance, swamp milkweed is a good, easy-to-grow variety native to most of the western regions of the U.S., which could help migrating butterflies along their journey. Unless such measures are implemented rapidly, we are at risk of losing one of the most fascinating species of insects on our planet.
More details regarding this new assessment of the status of monarch butterflies can be found on IUCN’s website.