Population cycles of autumnal moth, Epirrita autumnata, in relation to birch mast seeding

We investigated the relationship between flowering peaks of the mountain bi rch, Betula pubescens ssp. czerepanovii, and population levels of the autum nal moth Epirrita autumnata in Ammarnas, Swedish Lapland, during 1968-2000, and in Budal, Central Norway, during 1972-2000. There was a significant co rrelation between the two moth series, both of which showed three well-defi ned population cycles during the study period. In both areas, the populatio n growth index of the moth was negatively related to population size, but a lso to the number of years since the previous flowering peak of mountain bi rch. In the northern study area, Ammarnas, there was an additional positive effect of the winter temperature index, probably due to increased mortalit y of moth eggs during cold winters. No significant relationships were found between the number of birch female catkins and larval density in the previ ous 2 years. Both in Ammarnas and in an area without moth outbreaks in sout h-eastern Norway, birch flowering was positively related to temperatures du ring flower bud formation and to the number of years since the previous flo wering peak. The results support the idea of a lower content of chemical de fence compounds in birch leaves after a mast reproduction, though we cannot exclude the possibility that the negative relationship between flowering a nd moth population levels was influenced or caused by stress associated wit h defoliation during moth outbreaks.