Monarch butterfly populations have collapsed by over 80% due to habitat loss and pesticide use. At the scale of billions of individuals, their welfare during migration and at overwintering sites is significant.
Whether and to what degree monarch butterflies experience suffering remains scientifically uncertain, but at the scale of billions of individuals experiencing population crash, even modest probability of morally relevant experience implies significant total welfare concern. Insecticide exposure causes acute toxicity and death at individual level, and population-level loss of overwintering aggregations — where millions cluster in tight formations — removes a phenomenon of extraordinary ecological and potentially experiential richness. The welfare of individual monarchs caught in storms, extreme weather events at overwintering sites, or exposed to pesticides may involve aversive states. Habitat restoration through milkweed planting and reduced herbicide use, combined with overwintering site protection, represents the highest-impact interventions for both population and individual-level welfare.