My enemy's enemy is still my enemy: The spillback of an endemic herbivorous insect from a weed onto endemic host plants

Invasive species can significantly impact native food webs through indirect effects. Examples of apparent competition typically involve a non-native consumer disproportionately affecting one host population in the presence of another. Our research finds evidence for apparent competition between two herbs, one invasive and one endemic, and mediated by an endemic insect herbivore. The European pasture herb, ragwort (Jacobaea vulgaris, formerly Senecio jacobaea), is an invasive weed in several parts of the world, including New Zealand. New Zealand is also home to 19 native species of Senecio - thirteen of which are endemic - which support an endemic insect fauna. Some of these insects have since expanded their host range to include the invasive ragwort. We examined the relationships between one of those herbivores, the New Zealand magpie moth (Nyctemera annulata, Lepidoptera: Arctiidae), ragwort and one endemic (S. wairauensis) and two native host plants (S. minimus and S. quadridentaus). In our laboratory assays, magpie moth larvae found ragwort as attractive as native host plants and larvae fed only ragwort developed at comparable rates to those fed only native hosts. Our landscape surveys revealed a strong association between ragwort infestations, magpie moth abundance and increased levels of herbivore damage of the endemic S. wairauensis. Likewise, S. wairauensis was more likely to be present in transects where ragwort was rare. There is convincing anecdotal evidence that the native magpie moth became far more abundant because of an invasive weed and our study suggests that this detrimentally affects native plants through population spillback.