The conservation community sorely lacks a global indicator of change in insect populations. Given widespread insect declines, addressing this gap is key for conservation and policy targets. We suggest that butterfly monitoring programs can serve as the foundation for an effective global network of insect monitoring. To assess this potential, we bring together an international consortium and calculate a “Global Butterfly Index” using the Living Planet Index approach. Based on 10,386 population trends of 213 univoltine species, we found that overall declines in butterfly populations are predictable based on species traits. Our effort should pave the way for the development of a global network of butterfly population monitoring schemes. Since butterflies are the best monitored insects and have strong emotional value for the public, a global infrastructure for butterfly monitoring can be a flagship for insect conservation, informing policymaking and spurring societal transitions towards sustainable futures.