Differential host utilization by two parasites in a population of ponderosa pine.

We compared phloem characteristics of individual Pinus ponderosa attacked by the dwarf-mistletoe Arceuthobium vaginatum (Viscaceae) or by the beetle Dendroctonus ponderosae (Scolytidae) or by neither species. We quantified total nonstructural carbohydrates and a broad range of chemical elements for these three categories of trees. There were significant differences between trees parasitized by Arceuthobium, trees parasitized by Dendroctonus, and non-infected trees. Discriminant function analysis of trees attacked by either Arceuthobium or Dendroctonus correctly predicted group membership for 59 of 60 trees tested. Some of the differences detected may be induced, but many probably are not. Given that the accumulation of certain elements and compounds is under genetic control, and that both parasites often cause severe reductions in fitness, including death of their host, our results suggest that the two parasites may generate diversifying selection in ponderosa pine populations.