Climate Change Threatens Monarch Migration
New research warns that climate change could severely disrupt the mass migration of monarch butterflies. Shifting climate patterns threaten to fracture suitable milkweed habitat in Mexico, which serves as a critical migration corridor. Scientists caution this could push some butterfly populations to the brink and have cascading effects on regional ecosystems.
The annual monarch migration is a multi-generational journey stretching up to 3,000 miles from Canada and the U.S. to overwintering sites in Mexico. It can take three to four generations of butterflies to complete the trip north in the spring and summer. Eastern monarch populations, which overwinter in Mexico, have plummeted by as much as 84% since the mid-1990s. The Western monarch population, which winters along the California coast, has seen an even more catastrophic collapse of nearly 99%. The 2023-2024 winter survey in Mexico recorded the second-smallest area occupied by the butterflies since monitoring began in 1993. Beyond just the availability of milkweed, rising temperatures can also increase the toxicity of the plant itself. As milkweed plants sense warming, they produce higher levels of a toxic compound called cardenolide which can be fatal to the monarch caterpillars that feed on them. Warmer autumns can delay the start of the southern migration by up to six weeks, causing many butterflies to perish in the cold before reaching Mexico. Conversely, warmer and drier conditions impact the health of the oyamel fir forests in Mexico where the butterflies overwinter, and can also lead to an earlier bloom of nectar-producing plants, creating a food shortage for the monarchs later in the season. The widespread use of glyphosate-based herbicides on genetically modified corn and soybean crops has been a major contributor to milkweed loss, particularly in the Midwest. This region is a critical breeding area, and an estimated 850 million milkweed stems were lost between 1999 and 2014 due to herbicide use alone. In December 2024, the U.S. Fish and Wildlife Service proposed listing the monarch butterfly as a threatened species under the Endangered Species Act. This follows Canada's 2023 decision to list the monarch as endangered, granting it legal protection. Conservation efforts are underway to restore monarch habitats. Since 2015, the Monarch Butterfly and Pollinators Conservation Fund has awarded over $31 million to 165 projects, resulting in the restoration of 430,000 acres and the propagation of 1.2 million native milkweed seedlings.
