Two Birch Species Demonstrate Opposite Latitudinal Patterns in Infestation by Gall-Making Mites in Northern Europe.

Latitudinal patterns in herbivory, i.e. variations in plant losses to animals with latitude, are generally explained by temperature gradients. However, earlier studies suggest that geographical variation in abundance and diversity of gall-makers may be driven by precipitation rather than by temperature. To test the above hypothesis, we examined communities of eriophyoid mites (Acari: Eriophyoidea) on leaves of Betula pendula and B. pubescens in boreal forests in Northern Europe. We sampled ten sites for each of five latitudinal gradients from 2008-2011, counted galls of six morphological types and identified mites extracted from these galls. DNA analysis revealed cryptic species within two of six morphologically defined mite species, and these cryptic species induced different types of galls. When data from all types of galls and from two birch species were pooled, the percentage of galled leaves did not change with latitude. However, we discovered pronounced variation in latitudinal changes between birch species. Infestation by eriophyoid mites increased towards the north in B. pendula and decreased in B. pubescens, while diversity of galls decreased towards the north in B. pendula and did not change in B. pubescens. The percentage of galled leaves did not differ among geographical gradients and study years, but was 20% lower in late summer relative to early summer, indicating premature abscission of infested leaves. Our data suggest that precipitation has little effect on abundance and diversity of eriophyoid mites, and that climate warming may impose opposite effects on infestation of two birch species by galling mites, favouring B. pendula near the northern tree limit.