The threat of invasive plants across trophic levels

Caterpillars, abundant herbivores in forest ecosystems, are an essential food resource for myriad predators, especially songbirds. Throughout their shared history, caterpillars and plants have been locked in an evolutionary arms race. However, native caterpillars and exotic invasive plants have not had the opportunity to coevolve. Consequently, native caterpillars may struggle to colonize these novel hosts, resulting in depauperate caterpillar communities. If so, insectivores should allocate less foraging effort on non-native plants. Nevertheless, it is unclear whether caterpillar communities are depauperate on non-native hosts and if insectivores preferentially forage on native rather than non-native plants when both are available. In 2022, we quantified caterpillar abundance and compared rates of attack from birds and arthropods on model caterpillars fixed to native and non-native plant taxa: Amur honeysuckle (Lonicera maackii), blackhaw (Viburnum prunifolium), border privet (Ligustrum obtusifolium), ash (Fraxinus spp.), and spicebush (Lindera benzoin). Caterpillar abundance was far lower on honeysuckle than on blackhaw or spicebush; however, abundance did not differ between privet and ash. Concerning predation, attack rates were similar across plant taxa during the spring and summer; however, during fall, attack rates were higher on native hosts overall. This difference was driven by greater bird predation on native plants. Non-native plants appear to be inferior hosts for caterpillars, potentially reducing the food supply for birds in eastern North America. Furthermore, birds may be reluctant to forage on these novel plants. Consequently, exotic plants threaten not only insect herbivores but also bird populations.