Monarch butterflies undertake one of the most remarkable migrations on Earth, travelling up to 4,800 km from Canada to Mexico, with populations declining catastrophically due to milkweed loss, pesticide use, and climate change.
Whether monarch butterflies have capacity for suffering in a morally significant sense is scientifically uncertain. Their nervous systems are far simpler than vertebrates. However, the scale of their decline represents an enormous loss of biological richness, and the conditions causing their decline — pesticide exposure, habitat loss — likely impair physiological function in surviving individuals. Mass die-offs during migration from extreme weather events cause rapid death for millions of individuals. The ecological role of monarchs as pollinators and prey supports entire food webs. The precautionary approach to insect welfare, combined with the ecological significance of the species, makes monarch conservation an important priority regardless of the welfare question.