Monarch Butterflies: A Fragile Migration at Risk

Since they were first discovered in backyards across America, Monarch butterflies have become as American as apple pie. Generations of schoolchildren have raised Monarchs in classrooms, watching striped caterpillars transform into large orange‑and‑black butterflies. These creatures are legendary for their migration: each year they travel nearly 4,000 kilometers from Mexico to Canada — an extraordinary feat for an insect weighing less than one gram.

How Did the Problem Begin?

The Monarch holds a unique place in human imagination. These butterflies are ambassadors of nature and symbols of summer in gardens.

Yet once‑familiar sights are now disappearing. Monarch populations have declined drastically due to widespread pesticide use and global climate change. The Center for Biological Diversity has worked tirelessly to secure protection for them under the U.S. Endangered Species Act.

In the forests where Monarchs overwinter, their numbers were once so vast that the sound of their wings was described as a rushing stream or summer rain. Early newspaper accounts spoke of tree branches breaking under their weight.

Population Decline

In 2017, the annual census of Monarchs overwintering in Mexico’s mountain forests — where 99% of the world’s Monarchs migrate each winter — showed a 27% drop from the previous year and more than 80% since the mid‑1990s. Extreme winter storms contributed to this decline. Monarchs need large populations to withstand harsh weather, pesticides, and climate shifts.

By 2020, the number of overwintering Monarchs had fallen another 53% compared to the previous year. Numbers are now well below the threshold at which scientists predict migration could collapse.

The heart of the Monarch’s range is the Midwestern Corn Belt, where most Monarchs are born on milkweed plants growing in farm fields. Due to widespread spraying of corn and soy with Roundup, milkweed has disappeared from much of this core habitat. Climate change also threatens their overwintering grounds in Mexico’s mountain forests.

Conservation Campaigns

The first major conservation efforts began in August 2014, when the Center and allies petitioned to list Monarchs as “threatened” under the Endangered Species Act. In December 2014, the U.S. Fish and Wildlife Service announced that protection might be warranted, triggering a formal review. By law, the review should have been completed within 12 months. When no decision was issued by March 2016, the Center for Biological Diversity and the Center for Food Safety filed suit, resulting in a legal settlement requiring the agency to act.

Unfortunately, in December 2020 the Service concluded that protection was “warranted but precluded.” Scientists agreed Monarchs needed protection, but safeguards were postponed indefinitely, leaving the species vulnerable.

A Call to Action

As Monarch populations fell below critical thresholds, in March 2020 the Center and over 100 groups urged Congress to increase funding to $100 million annually to protect Monarchs and their habitats.

The decline of Monarchs is a warning of broader environmental change. Their shrinking numbers, alongside declines in other butterflies and bees, threaten human well‑being, since food security depends on the ecological services pollinators provide. History offers tragic lessons about the unexpected collapse of abundant species. Complacency and false assumptions about resilience can have dire consequences when timely action is not taken.

The migration of the Monarch butterfly is at risk of being lost unless swift protective measures are taken. The Center continues to fight to save Monarchs — and all native pollinators — from pesticides such as glyphosate and dicamba through its environmental protection programs.