Monarch Butterfly Decline: Welfare and Conservation of a Migratory Icon

Monarch butterflies undertake one of the most remarkable migrations in the animal kingdom, but their populations have declined by up to 90% over the past 30 years due to milkweed loss, pesticide use and climate disruption of their migratory cues.

Key Facts

Monarch butterflies migrate up to 4,500 km between Canada and Mexico annually — one of the longest insect migrations known
They depend entirely on milkweed plants for reproduction — females lay eggs exclusively on milkweed
Milkweed abundance has declined by over 50% in North American agricultural landscapes since the 1990s due to herbicide use
The overwintering population in Mexican mountain forests has declined from approximately 1 billion in the 1990s to around 200 million today
Monarchs are listed as Endangered on the IUCN Red List
Glyphosate-resistant crop varieties have allowed elimination of milkweed from agricultural areas where it previously grew widely
Climate change disrupts the temperature cues that trigger migration and affects nectar flower availability along migratory routes
Logging of overwintering forest in Mexico exposes butterflies to cold temperatures that cause mass mortality

Welfare Considerations

The welfare significance of monarch decline for individual butterflies is debated given uncertainty about insect sentience, but recent research suggests insects have nociceptors and show pain avoidance behaviour. The welfare of individual monarchs is most clearly affected at overwintering sites when logging destroys protective forest cover, exposing millions of individuals to lethal cold. Pesticide exposure along the migratory route causes sublethal effects including disorientation and reduced flight capacity. The disruption of highly evolved migratory behaviour by climate change forces monarchs into novel conditions for which they have no evolutionary preparation. The scale of population decline represents an extraordinary loss of a complex biological phenomenon.

What You Can Do

Plant native milkweed species in your garden or local green spaces to support monarch reproduction
Avoid herbicide use around milkweed plants and advocate for milkweed preservation in agricultural landscapes
Support certification schemes that promote pesticide reduction in agricultural areas crossed by monarch migration routes
Donate to organisations planting milkweed corridors and protecting overwintering forest in Mexico
Participate in citizen science monarch monitoring programmes such as Journey North
Advocate for policy support for pollinator-friendly farming practices that benefit monarchs and other migratory species