Ergosterol content of fungi associated with Dendroctonus ponderosae and Dendroctonus rufipennis (Coleoptera: Curculionidae, Scolytinae)

Insects require sterols for normal growth, metamorphosis, and reproduction, yet they are unable to synthesize these organic compounds and are therefore dependent upon a dietary source. For phloephagous species, such as Dendroctonus bark beetles, whose food does not necessarily contain appropriate types or adequate quantities of sterols, fungal symbionts may provide an alternative source in the form of ergosterol. We determined and compared the relative amounts of ergosterol in the primary fungal associates of Dendroctonus ponderosae Hopkins and Dendroctonus rufipennis Kirby. Ergosterol content of host tree phloem naturally infested with larvae (and their fungal symbionts) of both species also was compared with ergosterol contents in uninfested phloem tissue. Mycelia of Ophiostoma montium (Rumfold) von Arx and Ophiostoma clavigerum (Robinson-Jeffrey & Davidson) Harrington isolated from D. ponderosae mycangia, and Leptographium abietinum (Peck) Wingfield isolated from the exoskeleton of D. ruifpennis contained relatively large quantities of ergosterol, although no significant differences in content were found among these fungal species. Phloem colonized by larvae of both species contained significantly more ergosterol than did uninfested host phloem tissue. Our results suggest that larval life stages of D. ponderosae and D. rufipennis may obtain vital nutrients not only from the host tree phloem but also from fungal symbionts, in the form of ergosterol, while mining larval galleris.