Climate Change Threatens Monarchs
Climate change is likely to disrupt the mass migration of monarch butterflies, according to new research. A study finds that suitable milkweed habitat in Mexico is expected to shift further south, which could fracture existing migration routes and threaten the species' population stability.
The eastern monarch population, known for its spectacular journey of up to 3,000 miles to Mexico, has plummeted by over 80% since the 1990s. The western population, which overwinters in coastal California, has seen an even more drastic decline of over 95% in the same period. In the 1980s, their numbers in California were estimated to be around 4.5 million; the 2024-2025 count recorded only 9,119. The monarch migration is a multi-generational relay. The butterflies that fly south to Mexico and California are not the same individuals that return north. It can take three to four successive generations to complete the journey back to the northern United States and Canada. The generation that migrates south, however, is a "super generation" that can live up to seven or eight times longer than the others. Beyond climate change, the widespread loss of milkweed is a primary driver of the monarch's decline. Milkweed is the only plant that monarch caterpillars eat, and its prevalence has been decimated by the use of herbicides in agriculture, particularly on Roundup Ready crops. Pesticides, especially neonicotinoids, also pose a significant threat. These neurotoxic insecticides are systemic, meaning they are absorbed into the plant's tissues, making the entire plant toxic to insects like monarchs. This has made U.S. agriculture 48 times more toxic to most insects. In response to the dramatic population crash, the U.S. Fish and Wildlife Service has proposed listing the monarch butterfly as a threatened species under the Endangered Species Act. This would allow for the designation of critical habitat and the implementation of a federal recovery plan. In Canada, monarchs were listed as endangered under the Species at Risk Act in 2023. Warmer temperatures can also disrupt the monarch's migration cues and reproductive cycle. Unseasonably warm autumn weather has been shown to delay their southward migration by as much as six weeks. Higher temperatures can also trick monarchs into breaking their non-reproductive state, known as diapause, causing them to lay eggs when they should be conserving energy for the long flight south. Conservation efforts are underway to plant native milkweed, restore habitat, and promote sustainable land management practices. Organizations like the Xerces Society and Monarch Watch are leading citizen science initiatives to track population numbers and migration routes, providing crucial data for conservation strategies.
