T. Jonathan Davies

Aim Tree pests have major impacts in both natural and agricultural systems. Despite their ecological and economic importance, it is likely that pests go unreported in many regions. Using a global dataset of tree pests and multiple metrics of pest diversity standardized for host availability, we describe the biogeography of under‐reporting and reveal potential for future pest invasions. Location Global. Time period Contemporary. Major taxa studied Pathogens, insect pests and native tree hosts. Methods We assembled the global distributions of 3,828 tree pests and 2,689 host tree species across 226 countries, and calculated two metrics of pest diversity that account for host availability: pest saturation (i.e., the proportion of the known pests of native host trees present with countries) and pest deficit (the number of known pests of native trees that have not yet been reported in countries). We used Bayesian regression models to identify how sampling, socio‐economic variables, climate and ecological drivers affect estimates of pest saturation and deficit. Results We show that most countries are reported to have fewer than 50% of the pests for which native hosts are available—corresponding to hundreds of additional pest species per country. Pest saturation was lowest in Africa and central Asia, while pest deficit was highest in eastern Europe. Accounting for research output, pest saturation was higher in warmer and wetter countries, while pest deficit was highest in countries with greater host phylogenetic diversity. Low saturation and high deficit even in well‐documented countries suggest a considerable potential for pest range expansion if barriers to dispersal are lowered. Main conclusions Our findings indicate that, although all countries could potentially host additional pests, countries with low research output and high host diversity should be prioritized for future pest discovery and surveillance to ensure timely detection and implementation of pest mitigation strategies.