The northern regions are warming more rapidly than the global mean. This may cause problems in boreal forests if pest insects expand their ranges north. In Finland, the Nun moth (Lymantria monacha (Linnaeus)) is a potential forest defoliator that has earlier occurred sparsely along the southern coast of the country, but that might become a significant pest as it is in Central Europe. In this study we describe the changes in distribution and abundance of L. monacha in Finland, analyze these in relation to changing climate, and discuss management implications for the situation of a potentially serious pest expanding its range rapidly to new areas.

We used data from two long-term databases, the open access Insect Database (1960–2013), and Nocturna (1993–2013), the national monitoring scheme for night-flying moths. A trend of rising L. monacha abundances in Southern Finland since the 1990s was discernible in both datasets. Furthermore we found that the species has expanded its range from the southern coast northwards to approx. 63 °N, i.e. about 200 km, during two decades.

To compare the development of the L. monacha population with climatic variables we calculated three temperature parameters, EminT – the minimum temperature during the egg stage of L. monacha in winter, LT – the average temperature for the larval stage, and PAT – the average temperature for the pupal and adult stage. Model selection methods using information criteria ranked highest models where L. monacha abundance was related to EminT and PAT. This indicates that the recent success of L. monacha in Finland may be related to higher winter survival of eggs or improved dispersal and reproduction success of adult moths. The experimentally confirmed median freezing temperature of L. monacha eggs is −29.5 °C. Minimum winter temperatures on the southwestern coast of Finland have not dropped below this lethal limit since 1987. This corresponds temporally well the L. monacha upswing starting in the 1990s. Furthermore, it was notable that the temperature during the larval period (May – June) did not increase during the last decades, which suggests that high early summer temperatures have not been necessary for the northward expansion of L. monacha.

L. monacha is a major pest in coniferous forests in Central Europe, and may become a threat throughout its expanded range. We outline a multilevel monitoring programme that has proven efficient in L. monacha outbreak areas, and advocate risk reduction through forest conversion to mixed and ecologically stable stands.