Vehicle pollution is associated with elevated insect damage to street trees

1. 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. 2. 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. 3. 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. 4. 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 planting trees that are less susceptible to
insect herbivory along heavily traveled roadways.