Trees provide critical ecosystem services to urban residents, such as local cooling, air purification, and runoff reduction (Bolund & Hunhammar, 1999). Most people now live in cities, and urban populations continue to grow globally (United Nations, Department of Economic and Social Affairs, Population Division, 2019). Thus, urban ecosystem services are increasingly important. Urban trees also serve as foundational species that support biodiversity. In particular, trees provide food for insect herbivores (e.g. caterpillars) and the vertebrates that rely on them, such as birds and lizards.Herbivores can damage or kill trees (Crawley, 1983) by feeding on leaves and reducing photosynthesis (Zangerl et al., 2002) and growth (Zvereva et al., 2012), along with related ecosystem services, such as local cooling by trees. When herbivory is accompanied by widespread abiotic stressors in cities, (e.g. heat and/drought), its

Vehicle pollution is a pervasive aspect of anthropogenic change across rural and urban habitats. The most common emissions are carbon- or nitrogen-based pollutants that may impact diverse interactions between plants and insect herbivores. However, the effects of vehicle pollution on plant-insect interactions are poorly understood.Here, we combine a city-wide experiment across the Sacramento Metropolitan Area and a laboratory experiment to determine how vehicle emissions affect insect herbivory and leaf nutritional quality.We demonstrate that leaf damage to a native oak species (Quercus lobata) commonly planted across the western US is substantially elevated on trees exposed to vehicle emissions. In the laboratory, caterpillars preferred leaves from highway-adjacent trees and performed better on leaves from those same trees.Synthesis and applications. Together, our studies demonstrate that the heterogeneity in vehicle emissions across cities may explain highly variable patterns of insect herbivory on street trees. Our results also indicate that trees next to highways are particularly vulnerable to multiple stressors, including insect damage. To combat these effects, urban foresters may consider installing trees that are less susceptible to insect herbivory along heavily traveled roadways.