Alien plants versus alien herbivores: does it matter who is non-native in a novel trophic interaction?

• Novel insect herbivore–plant interactions result from plant and herbivore introductions.
• Evolutionary naïveté may benefit either invader.
• Plant introductions may have large direct ecosystem effects.
• The effects of herbivore introductions may be indirect, via host plants.
• Facilitative interactions exist between plant and herbivore invaders.

Introductions of both plants and herbivorous insects have had tremendous impacts on the world's ecosystems. Novel herbivorous insect–plant interactions are important consequences of introductions of either plants or herbivorous insects. We contrast novel herbivorous insect–plant interactions that arise due to plant versus insect introductions with the aim of understanding whether the causes and consequences of the interaction depend on which party is non-native. The biotic context of the herbivore–plant interaction, in terms of mutualists, predators, and competitors can limit the prevalence of that interaction and varies between native and introduced ranges. Introduced plants can have a large, direct impact on their environment, whereas the impact of introduced herbivorous insects is often mediated through the plants that they consume.