Influence of forest structural complexity on associational resistance to invasive insect pests in U.S. forests

The ability of nonhost plants to decrease the incidence of insect pests, a form of biotic resistance termed associational resistance, has been explored as a potential tool for mitigating the establishment and spread of invasive insects in forest ecosystems. Associational resistance can emerge through multiple diversity-driven pathways, with evidence indicating that the composition and structural arrangement of nonhost tree communities having an underexplored role. The plant apparency theory proposes that a plant’s vulnerability to an insect herbivore depends on its likelihood of being detected within its community. Invasive insects may locate host trees more easily when they are more physically apparent than neighboring nonhost trees in heterogeneous environments. Despite promising evidence that structural complexity mediates resistance to invasive insects, existing tests of host apparency-driven resistance have been conducted in localized systems or with limited sets of tree species, restricting our ability to generalize these mechanisms across diverse forest types, broad insect assemblages, and varying climatic gradients. To address this gap, we conducted a macroecological analysis of the relationship between structural complexity and invasive insect richness. Leveraging spatially and temporally extensive datasets from the Forest Inventory and Analysis database, we used structural equation modeling to identify characteristics of vegetation structure associated with invasive insect richness across the continental U.S. We found that increased structural complexity in forests is linked to reduced invasive insect richness, though the strength of this resistance effect depends on insect dietary breadth, and forest type and temperature. These findings suggest that strengthening structural complexity in forest ecosystems could be an effective management strategy for reducing invasive insect establishment.