A pioneering pest: the winter moth (Operophtera brumata) is expanding its outbreak range into low-arctic shrub tundra

Climate warming allows generalist boreal consumers to expand into arctic ecosystems. We present experimental and observational field data showing that a generalist boreal insect pest – the winter moth (Operophtera brumata Linnaeus, 1758) – is expanding its outbreak range out of the northern-boreal mountain birch forest in northeast Fennoscandia and into the adjacent low-artic shrub tundra. This is the first documented example of an outbreaking boreal insect pest expanding into a tundra ecosystem. The expansion has coincided with a long-term advancing trend in the expected hatching date of moth eggs in spring for the study region. We show that the winter moth can complete development on low-arctic willows and that the density of winter moth larvae in willow thickets is unrelated to the amount of mountain birch (the main host plant in northern-boreal forest) in the thickets. However, we also demonstrate that larval densities on willows show a regional-scale spatial decline when moving awa...