Monarch butterflies undertake one of the most remarkable migrations of any animal, travelling thousands of kilometres between summer breeding grounds and Mexican overwintering sites. Milkweed loss, pesticide use, and logging at overwintering sites threaten this population-level phenomenon.
While the welfare of individual insects is uncertain given questions about insect sentience, the population-level loss of the monarch migration represents an ecological and aesthetic loss of immense scale. Individual monarchs exposed to pesticide-contaminated milkweed show developmental abnormalities, reduced flight performance, and navigation impairment. From a precautionary welfare perspective, the scale of exposure — hundreds of millions of individuals annually — and the neurological effects of neonicotinoids warrant concern. Protecting milkweed abundance and quality on migration routes addresses both population and individual-level welfare simultaneously.