11. Ecology and evolution of mycophagous bark beetles and their fungal partners

Introduction Associations between bark beetles (Coleoptera: Curculionidae: Scolytinae, or family Scolytidae, depending on the classification used, Bright 1993; Marvaldi et al. 2002) and fungi are varied and well known, but mycophagy (fungal feeding) by bark beetles has received relatively little attention. This may be due to the rarity or relative unimportance of fungal feeding by bark beetles, which feed in a nutrient-rich substrate, the inner bark of trees, or it may be due to a bias in research towards the possible importance of plant pathogenic fungi carried by a few coniferous bark beetles. The best studied of the more than 3500 species of bark beetles (Wood 1982; Farrell et al. 2001) construct their egg galleries in the inner bark (secondary phloem) of living trees, especially conifers in the family Pinaceae (Raffa et al. 1993). These bark beetles kill their host and are among the most economically important of forest insects. Although much has been said about the role of fungi in aiding bark beetles in killing their tree hosts, there are inconsistencies in such associations (Harrington 1993b; Paine et al. 1997), and mycophagy may be the more important symbiosis between some of the most important tree-killing bark beetles and fungi. Other species of bark beetles feed in thin-barked branches or treetops and also exploit fungi to supplement their diet. Probably all bark beetles feed at least briefly on plant tissue colonized by fungi and could thus be considered mycophagous. However, I am here restricting the term mycophagy to grazing by larvae or young adults on fungal spores, fruiting structures or hyphae (Lawrence 1989) on the surface of galleries or pupal chambers. Mycophagy does not appear to be obligatory for bark beetles, but I hypothesize that fungal feeding allows for more efficient use of the inner bark and gives these bark beetles a competitive edge over other species of bark beetles and phloeophagous (phloem feeding) wood borers. This scenario is consistent with the hypothesized evolution of xylomycetophagous (wood and fungal feeding) ambrosia beetles from phloeophagous bark beetles (Farrell et al. 2001). Mycophagy appears to have evolved many times in the bark beetles, as it has in the xylem-feeding ambrosia beetles (Farrell et al. 2001). There are also parallels and interesting contrasts between mycophagous bark beetles and ambrosia beetles in the way they carry their fungal symbionts, the range of fungi that have been exploited by the beetles, and the morphological adaptations that some of the fungi have made to maintain the symbioses.