Monarch butterfly populations have declined dramatically. While butterfly welfare science is nascent, the mass suffering and death involved in population collapse warrants conservation attention from a welfare perspective.
Monarch butterfly welfare raises questions at the frontier of invertebrate welfare science. Butterflies have nervous systems with 1 million neurons — vastly simpler than vertebrates but potentially sufficient for basic sensory processing. The welfare relevance of population decline lies partly in the aggregate suffering of individual insects experiencing pesticide poisoning, starvation, dehydration, and parasitism, and partly in the loss of the intrinsic value of the spectacular migration phenomenon itself. Conservation of monarch populations is welfare-relevant to the extent that the 100 million individual insects in healthy populations experience positive states during their lives and migration. Habitat restoration and pesticide reduction benefit both monarchs and many other insect species facing similar pressures.