Alien interference: disruption of infochemical networks by invasive insect herbivores

Insect herbivores trigger various biochemical changes in plants, and as a consequence, affect other organisms that are associated with these plants. Such plant‐mediated indirect effects often involve herbivore‐induced plant volatiles (HIPVs) that can be used as cues for foraging herbivores and their natural enemies, and are also known to affect pollinator attraction. In tightly co‐evolved systems, the different trophic levels are expected to display adaptive response to changes in HIPVs caused by native herbivores. But what if a new herbivore invades such a system? Current literature suggests that exotic herbivores have the potential to affect HIPV production, and that plant responses to novel herbivores are likely to depend on phylogenetic relatedness between the invader and the native species. Here we review the different ways exotic herbivores can disrupt chemically mediated interactions between plants and the key users of HIPVs: herbivores, pollinators, and members of the third (i.e. predators and parasitoids) and fourth (i.e. hyperparasitoids) trophic levels. Current theory on insect invasions needs to consider that disruptive effects of invaders on infochemical networks can have a short‐term impact on the population dynamics of native insects and plants, as well as exerting potentially negative consequences for the functioning of native ecosystems. The threats posed by invasive insect herbivores on native ecosystems are diverse, complex, with effects rippling through the food chain and impacting entire communities. Here we advance the idea that exotic herbivores can affect chemically mediated interactions between plants and native insects (e.g. attraction of pollinators via floral scents, attraction of natural enemies via herbivore‐induced volatiles). We propose a framework for studying such chemical interferences and review the ways invaders can affect the key users of plant volatiles.