Climate-induced increase of moth multivoltinism in boreal regions

Aim An increase in multivoltinism in ectothermic animals has been proposed by several authors as a possible outcome of climate warming, especially in high latitudes. We tested this prediction with large-scale empirical monitoring data for boreal moth communities. Location Finland, northern Europe. Methods Our data set comprised observations of multivoltine species made in the Finnish moth monitoring scheme 'Nocturna' trap sites during the period 1993-2006 along an exceptionally long latitudinal gradient of 1000 km. To compare recent changes in moth multivoltinism with a longer time period, we gathered older time series of moth observations from five locations. We used generalized linear mixed models (GLMMs) to detect possible temporal and geographical trends in the annual occurrence of multivoltinism. We also identified areas where the recent changes in multivoltinism have been greatest. Monthly average temperatures of spring and summer periods and annual sum of growing degree days above 5 degrees C (GDD5) were used as explanatory variables to distinguish the main climatic correlates of moth multivoltinism. Results We observed a clear increase in the occurrence of moth multivoltinism during the period 1993-2006. The incidence as well as the recent increase in multivoltinism were highest in southernmost Finland and decreased towards the north. We also detected a weaker, although significantly positive, trend of moth multivoltinism in southern Finland during the period 1963-92, suggesting that this increasing trend might already have begun earlier. The most important climatic correlates for the annual occurrence of moth multivoltinism were the mean summer temperature (periods May-July and June-August) and GDD5, but all the tested climatic variables showed a significant univariate relationship with the occurrence of moth multivoltinism. All climatic variables showed an increasing trend during the period 1993-2006. Main conclusions The occurrence of multivoltinism has increased in northern European moth communities during recent decades, apparently as a response to increasing temperatures during the spring and summer seasons. The increase in multivoltinism was greatest in the southernmost parts of Finland, whereas in the northern landscapes recent warming has triggered multivoltinism in only relatively few moth species.