Skip to main content

An epic migration, on the verge of collapse.  

In the 1990s, nearly 700 million monarchs made the epic flight each fall from the northern plains of the U.S. and Canada to sites in the oyamel fir forests north of Mexico City, and more than one million monarchs overwintered in forested groves on the California Coast.  Now, researchers and citizen scientists estimate that only a fraction of the population remains, monarchs have declined by more than 80% since the 1990s from central Mexico, and by more than 99% since the 1980s in coastal California.


A map of North America shows the path of the eastern and western monarch migrations. The eastern migration journeys from southern Canada, east of the Rocky Mountains, to the oyamel fir forests of Mexico, and the western monarch migration journeys from the Pacific Northwest and inter-mountain west to the coast of California.


Loss of milkweed breeding habitat due to the widespread use of herbicide-resistant crops.

Pesticide use, which kills non-target insects and degrades habitat.

Climate change is affecting monarch populations in a number of ways.

Logging and development have shrunk monarch overwintering sites.

Monarchs require suitable habitat that provides host plants for breeding and flowering plants to provide nectar for adults. In the case of monarchs, the loss of milkweed means the loss of breeding habitat. Loss of milkweed from prime migration routes is primarily due to the dramatic increase in the use of the herbicide resistant crops Commonly known as Roundup™ Ready Crops, these corn and soy crops are genetically modified to be resistant to glyphosate, a broad-spectrum herbicide that kills everything other than the resistant crop, including milkweed. Harsher winters in monarch overwintering sites have caused larger than usual die-offs. Erratic weather may also delay the emergence of milkweed in spring and change the bloom time of flowering plants that provide resources to migrating monarchs. Legal and illegal logging in the oyamel fir forests of Mexico where eastern monarchs overwinter has removed important winter cover for the species and impacted microclimates that protect the butterflies from extreme cold and precipitation. In California, many sites where western monarchs overwinter have been lost due to development.

Conservation Efforts

In the three decades since first standing up for monarchs, the Xerces Society has gained extensive experience and accumulated expertise in all aspects of monarch conservation. Our work now extends to protecting and managing the habitats that support all stages of the monarch’s lifecycle throughout the transcontinental range of the butterfly’s migration. See our recent blog posts about monarch conservation for the latest updates on our work, as well as the work of our partners.