DOI: 10.5061/dryad.sxksn0336
title: Data from: A pioneering pest: the winter moth (Operophtera brumata) is expanding its outbreak range into low-arctic shrub tundra

description: 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.
description: 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
 <i>Salix glauca</i>, S<i>. lanata</i> and
 <i>S. phylicifolia</i>, 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).
description: 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