Hungry moths moving north

By Bjørn Økland, Norwegian Forest and Landscape Institute

The autumnal moth, the most damaging insect pest on birch in Scandinavia, is moving northwards. The last 15-20 years the moth’s outbreak area appears to have expanded into the coldest and most continental parts of Norway’s far north. This expansion is probably driven by a warming climate that has opened up these previously so hostile areas.

The autumnal moth (Epirrita autumnata) is considered the most damaging insect pest on birch trees in Northern Norway and high-elevation forests in Southern Norway. In these areas the moth population undergoes regular mass attacks and may defoliate vast forests areas. Normally, the outbreaks last for around seven years, while it takes eight to ten years for new outbreaks to develop.

The birch trees easily survive a single defoliation episode, but will die if they are completely stripped of leaves several years in a row. Since re-growth of northern birch forests is so slow, the environmental impact of moth attacks can be dramatic, leaving a decade-long legacy of dead birch forests.

The autumnal moth lays its eggs on the naked birch branches in the autumn and the eggs overwinter. The eggs are well adapted to the harsh climate and can survive temperatures as low as 37˚C. In certain areas however, particularly in Finnmark County, extreme temperatures may kill the eggs. Occasionally during moth outbreaks, birch trees may remain green and undamaged along river banks and in low-lying areas where temperatures can drop below the lethal temperature. However, studies of long time series have shown that over the past 15-20 years outbreak of the autumnal moth has been expanding towards the coldest and most continental parts of Northern Norway. A gradually warming climate has probably enabled this expansion and opened up these previously so hostile areas to the moths.

Many different moth species tend to be involved in the outbreaks in the northern birch forests. Colloquially these species are often referred to as “lauvmakk”, or leaf-eating worms. A common denominator for many moth species is that they are expanding northwards or into higher altitudes as these areas have gradually warmed up in recent years. One of the species which regularly occur together with the autumnal moth is the winter moth (Operophtera brumata). The winter moth is notorious for its mass attacks at lower elevations but did not previously overlap with the autumnal moth in the coldest areas. In recent years however this seems to have changed, and the winter moth is now becoming increasingly prevalent in outbreaks in higher altitude forests. Studies using aerial photography show that the most recent winter moth/autumnal moth outbreaks in Troms County have extended all the way to the tree line, and long-time series show a rapid expansion of outbreaks areas north- and eastward in Troms and Finnmark. Another relative of the autumnal moth, the northern winter moth (Operophtera fagata), was previously only known to have outbreaks at low elevations, but has the last couple of years been involved in outbreaks in mountainous birch forests in Skjåk, Southern Norway. It remains to be seen whether the recent expansions in outbreak areas for these moth species are a transient phenomenon or if they represent a permanent shift in response to a warming climate.

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Photo: Vladimir KononenkoNorwegian Forest and Landscape Institute

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