Summary In 1990–2003, during a complete 10‐year outbreak cycle, the synchrony of the birch defoliating outbreaks of the geometrids Epirrita autumnata and Operophtera brumata was studied quantitatively in the northern part of the Fennoscandian mountain chain (the Scandes). Data were supplemented with similar data from 1964 to 1966 and historical information. A 30‐year series of field data from one locality in southern Scandes made possible interregional comparisons. In 1991, outbreaks started in north‐eastern Fennoscandia and moved westward like a wave and reached the outer coast of north‐western Norway in about 2000. This wave is a new observation. In the same years, a previously documented outbreak wave moved southward along the Scandes. Outbreak periods have usually occurred around the middle of each decade. Seemingly unrelated population peaks at the decadal shift 2000 were reported from islands at the coast of north‐western Norway. They are shown here to have been the final ripples of the east–west wave. At some localities, O. brumata peaked 2 years after E. autumnata. A lag of 1 or 2 years also occurred at the locality in southern Scandes. This interspecific time lag is a new observation. In accordance with the north–south wave, a time‐lag of 1–2 years occurred between the fluctuations of northern and southern E. autumnata and O. brumata populations. The population peak of E. autumnata occurred 1 year earlier at one locality than at a nearby locality. This pattern and particular altitudinal shifts of the O. brumata population density at these localities repeated in two outbreak periods. This indicates that, for example, local climate may modify outbreak synchrony between nearby localities. At the same localities, O. brumata peaked first at one altitude and 1 or 2 years later at another altitude. This vertical lag is a new observation. E. autumnata shows fluctuation traits similar to some other cyclic animals, e.g. the larch budmoth in the European Alps, some European tetraonid birds and the Canadian snow‐shoe hare. These similarities (and dissimilarities) in intra‐ and interspecific synchronies and causes of E. autumnata and O. brumata synchronies, regionally, locally and among the two species are discussed.

Intensities of warble fly larvae, Hypoderma tarandi (L.), were examined in slaughtered reindeer (Rangifer tarandus tarandus L.) from different summer grazing areas of Finnmark County, northern Norway. To test the hypothesis that larval abundance decreases with increase in post-calving migration distance (i.e., distance from calving grounds), herds with differing migration distances were sampled. The prevalence of infection in the total sample of 1168 animals was 99.9%. The study revealed significant differences in larval abundance among herds from different summer grazing areas. Herds with post-calving migrations have significantly lower larval abundances than herds remaining on or near the calving grounds for the whole summer. Between-herds variation in abundance of H. tarandi larvae is assumed to reflect differing densities of the infective stage (adult flies) on the herds' summer ranges. Larval abundance in a herd is in turn negatively correlated with the distance between the main larval shedding areas (i.e., calving grounds) and the areas of greatest transmission (i.e., summer pastures). These results are discussed in relation to transmission of other parasites common to Rangifer and suggest that this host's post-calving migration may be a behavioural adaptation that reduces levels of parasitic infections.

Species range displacements owing to shifts in temporal associations between trophic levels are expected consequences of climate warming. Climate-induced range expansions have been shown for two irruptive forest defoliators, the geometrids Operophtera brumata and Epirrita autumnata, causing more extensive forest damage in sub-Arctic Fennoscandia. Here, we document a rapid northwards expansion of a novel irruptive geometrid, Agriopis aurantiaria, into the same region, with the aim of providing insights into mechanisms underlying the recent geometrid range expansions and subsequent forest damage. Based on regional scale data on occurrences and a quantitative monitoring of population densities along the invasion front, we show that, since the first records of larval specimens in the region in 1997-1998, the species has spread northwards to approximately 701N, and caused severe defoliation locally during [2004][2005][2006]. Through targeted studies of larval phenology of A. aurantiaria and O. brumata, as well as spring phenology of birch, along meso-scale climatic gradients, we show that A. aurantiaria displays a similar dynamics and development as O. brumata, albeit with a consistent phenological lag of 0.75-1 instar. Experiments of the temperature requirements for egg hatching and for budburst in birch showed that this phenological lag is caused by delayed egg hatching in A. aurantiaria relative to O. brumata. A. aurantiaria had a higher development threshold (LDT A.a. 5 4.71 1C, LDT O.b. 5 1.41 1C), and hatched later and in less synchrony with budburst than O. brumata at the lower end of the studied temperature range. We can conclude that recent warmer springs have provided phenological match between A. aurantiaria and sub-Arctic birch which may intensify the cumulative impact of geometrid outbreaks on this forest ecosystem. Higher spring temperatures will increase spring phenological synchrony between A. aurantiaria and its host, which suggests that a further expansion of the outbreak range of A. aurantiaria can be expected.

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Summary1. We show that the population ecology of the 9-to 10-year cyclic, broadleaf-defoliating winter moth (Operophtera brumata) and other early-season geometrids cannot be fully understood on a local scale unless population behaviour is known on a European scale. 2. Qualitative and quantitative data on O. brumata outbreaks were obtained from published sources and previously unpublished material provided by authors of this article. Data cover six decades from the 1950s to the first decade of twenty-first century and most European countries, giving new information fundamental for the understanding of the population ecology of O. brumata. 3. Analyses on epicentral, regional and continental scales show that in each decade, a wave of O. brumata outbreaks travelled across Europe. 4. On average, the waves moved unidirectionally ESE-WNW, that is, toward the Scandes and the Atlantic. When one wave reached the Atlantic coast after 9-10 years, the next one started in East Europe to travel the same c. 3000 km distance. 5. The average wave speed and wavelength was 330 km year À1 and 3135 km, respectively, the high speed being incongruous with sedentary geometrid populations. 6. A mapping of the wave of the 1990s revealed that this wave travelled in a straight E-W direction. It therefore passed the Scandes diagonally first in the north on its way westward. Within the frame of the Scandes, this caused the illusion that the wave moved N-S. In analogy, outbreaks described previously as moving S-N or occurring contemporaneously along the Scandes were probably the result of continental-scale waves meeting the Scandes obliquely from the south or in parallel. 7. In the steppe zone of eastern-most and south-east Europe, outbreaks of the winter moth did not participate in the waves. Here, broadleaved stands are small and widely separated. This makes the zone hostile to short-distance dispersal between O. brumata subpopulations and prevents synchronization within meta-populations. Journal of Animal Ecology 2013Ecology , 82, 84-95 doi: 10.1111Ecology /j.1365Ecology -2656Ecology .2012 8. We hypothesize that hostile boundary models, involving reciprocal host-herbivore-enemy reactions at the transition between the steppe and the broadleaved forest zones, offer the best explanation to the origin of outbreak waves. These results have theoretical and practical implications and indicate that multidisciplinary, continentally coordinated studies are essential for an understanding of the spatiotemporal behaviour of cyclic animal populations.