Plants growing in vegetationally diverse habitats or near taxonomically distinct neighbors often experience less herbivory than plants in more simple habitats. When plants experience more herbivory in these situations it is called associational susceptibility and is most common when herbivores spill from their preferred plant host onto neighboring plants. Cankerworms are common pests of urban trees that have been shown in forests to disperse from preferred to less preferred hosts. I found that two common characteristics of urban habitats, low vegetational diversity and exotic plants, affect cankerworm herbivory of non-host understory plants. In an urban landscape I measured cankerworm herbivory on native dogwood trees growing in the open and below cankerworm host and non-host trees. Herbivory of native dogwoods was ten times greater below cankerworm hosts than on trees below non-hosts or in the open. At an arboretum I measured herbivory of native and exotic plants growing below cankerworm hosts in simple landscape plantings and in natural forests. Associational susceptibility of native dogwoods and Rhododendron spp. disappeared when they were growing in complex natural forests even though cankerworm hosts were more abundant. Cankerworms consistently preferred native plant species more than exotic congeners in laboratory experiments. As such, exotic plants experienced very little herbivory regardless of habitat. Herbivorous pests are often more abundant on urban plants than plants in natural habitats. My research shows that, although some plants experience more herbivory when growing near cankerworm hosts, increasing urban habitat complexity could reduce pest damage overall.
