Nonnative pests often cause cascading ecological impacts with multiple detrimental socioeconomic consequences. However, it's unclear how plant diversity may influence insect and disease invasions. High species diversity in host communities may help pest invasions by providing a wider variety of ecological opportunities. Diversity can also dilute invasion success, because low host dominance may make it more difficult for pests to establish. Most studies have focused on fine-scale, experimental, or individual pests, while broad-scale studies, especially in natural ecosystems, are extremely rare. USDA Forest Service scientists at the agency's Southern Research Station led a study that combined a dataset of 130,210 forest plots with a county-level pest occurrence dataset. They found that tree-pest diversity relationships are hump-shaped. Their results were published in the Proceedings of the National Academy of Science. Pest diversity increases with tree diversity at low tree diversity levels, due to facilitation or amplification, and is reduced at higher tree diversity, due to dilution. Thus, tree diversity could regulate forest pest invasion through both facilitation and dilution that operate simultaneously, but their relative strengths vary with overall diversity. These findings show that the role of native species diversity in regulating nonnative pest invasions is more complex than previously understood.