Population fluctuations of moths and small rodents in relation to plant reproduction indices in southern Norway

Trophic interaction hypotheses for multiannual population fluctuations of herbivores include the predation hypothesis, the induction hypothesis and the plant stress hypothesis. Spatial synchrony is best explained by plant stress, such as high reproduction, because weather may synchronize resource allocation in plants over large areas. We used a 29‐yr time series on moth light trapping (6 selected species) and small rodent snap trapping (2 selected species) from southern Norway to test whether herbivore population fluctuations can be related to sexual reproduction of their host plants. The fluctuation pattern of the moths Parashwammerdamia lutarea and Yponomeuta padella , feeding on leaves of rowan ( Sorbus aucuparia ), could be explained by fluctuations in an annual index of rowanberry production, whereas the fluctuation pattern of Yponomeuta evonymella , feeding on leaves of bird cherry ( Prunus padus ), could not. The moth Eulithis populata , feeding on bilberry ( Vaccinium myrtillus ) leaves, fluctuated in synchrony with the bank vole ( Myodes clethrionomys ), which feeds on bilberry twigs in winter. E. populata and the bank vole also fluctuated in synchrony with a shorter index on bilberry seed crops. Finally, two moths feeding on mosses, Eudonia truncicolella and Catoptria falsella , fluctuated in synchrony with the moss‐feeding wood lemming ( Myopus schisticolor ). The observed patterns could be caused by a positive effect of the supply of highly nutritious reproductive plant tissue, but the one‐year delayed response by the rodents, which to a large extent feed on the plant species in question during winter, suggests that feeding deterrents are involved.