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 away from the birch forest and into the shrub tundra. Continued monitoring is needed to establish if the outbreaks will spread further into the tundra. The expansion of outbreaking boreal pests into the tundra could alter conventional expectations of increasing vegetation productivity and shrubification in tundra ecosystems.

Protocol for Experimental assessment of winter moth development on willows

The experiment was conducted in 2019 at the location Krampenes (70°06′N, 30°11′E) on the coast approximately 15 km east of the main birch forest line on the Varanger peninsula. The landscape in the area is dominated by thickets of Salix glauca, S. lanata and S. phylicifolia, but there is substantial intermixing of shrubby birch in these thickets. The experiment was conducted by enclosing a sample of 50 willow branches and 50 birch branches with mesh bags with sizes ranging between 100*40 cm and 70*30 cm. This caused the size of the enclosed branches to vary between approximately 50 and 70 cm between bags, but care was taken to not introduce consistent differences between willow and birch in this respect. The bags were mounted on May 7-8, which is well in advance of the hatching of moth eggs in spring. Bags were distributed haphazardly in the landscape, with a minimum distance of 10 m between bags mounted on the same plant species. Due to a lack of foliage in the early spring, the species identity of willows could not be reliably established when the bags were mounted. Hence, the bags provided a more or less random sample of the three dominant willow species in proportion to their occurrence in the landscape. All of the experimental bags were dismounted over a 3-day period from 12-14. July, which corresponds to the late larval to early pupal stage for the winter moth. At this time, the branches were cut down the branch and collecting all larvae, prepupae and pupae in the bag and on the branch. Individuals were stored in individual Eppendorf tubes and frozen. After fieldwork, the individuals from each bag were counted and individually weighed to the closest 0.1 Mg. The instar of each larva was also determined visually by an experienced researcher (OPLV).

Experimental mesh bags:

Information on the mesh bags used in the experiment

meshbag_branchvariables_2019.txt

Winter moth individuals found in experimental mesh bags:

Information on the winter moth individuals found in the experimental mesh bags.

meshbag_larvavariables_2019.txt