Optimal conservation of migratory monarch butterflies requires immediate international coordination.

The eastern North American monarch butterfly, known for its spectacular annual migration between Mexico, USA, and Canada, is currently the focus of intense conservation attention to minimize extinction risk and reach a conservation target of approximately 132 million individuals (equivalent to occupying 6 ha of overwintering habitat in Mexico). Given that migratory eastern North American monarchs breed over successive breeding generations across a vast area, reaching this conservation objective must account for the inherent uncertainties of undertaking conservation actions across space and time. We integrated a density-dependent full-annual-cycle matrix population model with stochastic dynamic programming to identify the optimal sequence of conservation actions (restoring habitat in three different breeding regions or protecting habitat on the wintering grounds), spanning three countries, to reach each of three objectives: maximize population viability, meet the population target, and maximize population size. Using an annual budget of $30 million over 5 years, we find that a coordinated approach that would primarily focus on habitat restoration in the central USA would best help achieve each of the three recovery objectives. Importantly, monarchs had a higher chance of reaching each conservation objective following a strategy that was coordinated across all three nations and commenced immediately rather than be delayed. Our results provide quantitative evidence for the necessity of coordinated international efforts to conserve migratory species and a tool for decision-makers in Canada, USA, and Mexico to recover this iconic and highly threatened butterfly.