The role of foliar microfungi in mountain birch - insect herbivore relationships

We studied the effects of epiphytic and endophytic phyllosphere fungi and pathogenic birch rust fungus infection of mountain birch Betula pubescens ssp. czerepanovii trees on the larval performance of leaf beetle Phratora polaris. We assessed the effects of epiphytic fungi by growing larvae on leaves from trees with manipulated fungal densities. We also monitored larval performance and endophytic fungal densities among tree groups classified by herbivory or rust fungus densities. The differences in experimentally manipulated epiphytic fungal densities did not affect larval relative growth rates (RGR) of the species; instead we found significant tree effects. Phratora polaris RGR was higher on trees with high level of herbivory than on trees with low herbivory, nevertheless, endophyte densities between these groupings did not differ. In the rust fungus experiment, P. polaris performance was lowest on trees with low infection compared to no and high infection trees. We also did not find correlations among tree-specific endophyte densities and P. polaris performance on high and low herbivory trees and trees classified by rust fungus infection. Although antagonism among fungi and induction of tree defences cannot be excluded, we suggest that epiphytic and endophytic fungi of mountain birch have negligible effects on P. polaris larval performance under natural conditions, probably due to mountain birch variability and a loose ecological connection between mountain birch and its epi- and endophytes. Mountain birch and pathogenic birch rust have a more tightly linked relationship, which may also affect insect herbivores. Still, leaf properties may play an important role and the effects will depend on the relative timing of the rust infection, herbivore development and changes in leaf quality.