This study compares caterpillar communities (Lepidoptera and Hymenoptera sawflies) in agricultural hedgerows dominated by aggressive non-native plants (novel hedgerows) and hedgerows comprised largely of indigenous plants. It differs from controlled common garden studies because it examines the impact of evolutionarily novel plants that have replaced native plants in unmanaged ecosystems and thus provides a more accurate measure of what is happening to interactions between plants and insects in invaded natural areas. Four hedgerows of each treatment were searched methodically along 100 m transects during June and July 2011 for a seasonal total of 500 min each. Seasonal totals of species richness, caterpillar abundance, and caterpillar biomass were recorded and compared using linear mixed models. We also estimated a Lepidoptera productivity index for each hedgerow based on the plant species composition and biomass as well as literature host records for each species present. Although we found similar richness of plant species, and more plant biomass in novel hedgerows, novel hedgerows had 68% fewer caterpillar species, 91% fewer caterpillars, and 96% less caterpillar biomass than native hedgerows. Moreover, novel hedgerows were predicted to have 5.4 times less Lepidoptera productivity based on their plant composition. Interactions between caterpillars and hedgerow plants were also significantly impacted by introduced plants, with novel hedgerows supporting 84% fewer interactions between plants and caterpillars and 57% less interaction diversity than native hedgerows. This is a considerably larger impact of novel plants on a critical component of terrestrial food webs than has been measured in common garden experiments at the same location and demonstrates that plant–herbivore interactions are compromised when novel species replace resident species, even if plant species richness does not change.