Invaders in the concrete jungle: Effects of urban environments on elm defoliators

Urban areas are expanding rapidly, with the majority of the global and US population inhabiting them. Urban forests are critically important for providing ecosystem services to the growing urban populace, but their health is threatened by invasive insects. Insect density and damage are highly variable in different sites across urban landscapes, such that trees in some insect “hot spots” experience outbreaks and are severely damaged while others are relatively unaffected. To protect urban forests against damage from invasive insects and ensure subsequent ecosystem service delivery, we must first understand the factors that promote herbivore abundance and damage to their hosts across urban landscapes. This study explores how a variety of environmental factors that vary across urban habitats influence abundance of invasive insects. Specifically, we evaluate how vegetational complexity, distance to buildings, impervious surface, host availability, and abundance of co-occuring herbivores impact three pests of elm: the Elm Leaf Beetle (Xanthogaleruca luteola), the Elm Flea Weevil (Orchestes steppensis), and the Elm Leafminer (Fenusa ulmi).

We found that vegetational complexity, impervious surface, and abundance of co-occuring herbivores are associated with pest abundance, but direction and strength of associations are dependent upon insect life history. Building distance and host availability were not associated with pest abundance. Results of this study can be used to inform future urban tree planting and pest management efforts in an era where globalization and climate change make the urban forest particularly vulnerable to attack.