Monarch butterfly populations have declined catastrophically; while insect welfare science is nascent, the scale of their decline and potential nociceptive capacity warrants welfare consideration.
Monarch welfare sits at the frontier of welfare science. While insect sentience is uncertain, monarchs show complex behaviours suggesting sophisticated neural processing: navigating thousands of kilometres using multiple sensory cues, making apparent "decisions" at environmental choice points, and showing learning. Precautionary welfare consideration — minimising likely negative experiences at scale — is increasingly applied to insects. The primary welfare-relevant concern is the scale of mortality: billions of monarchs die from pesticide exposure annually. Pesticide toxicity causes muscular dysfunction, coordination loss, and prolonged incapacitation before death — potential welfare harms affecting an enormous number of individuals.