Secondary Impacts to Bark Beetle Invaded Conifers
Bark beetles evolved a symbiotic relationship with microorganisms. Bacteria are present in the gut and in the phloem tissues surrounding the galleries. The role of the bacteria is not yet known but is suspected to aid in digestion within the gut and possibly to alter the nutrient content of the phloem prior to ingestion. Yeasts are also associated with bark beetles and are found within the phloem tissues near the galleries. Again, they are likely to play a role in nutrition but the effect on the tree is unclear. Bark beetles carry a variety of fungal species which adhere to the exoskeleton, or within specialized structures on their heads. The benefit of this relationship to the beetle is not understood but is likely to be related to nutrition. The benefit to the fungi is clear. The beetles transport the fungi to new host trees where the fungi become established. Fungi are detrivores, consuming dead and dying plant tissues. The action of fungi on dead and dying trees is beneficial to the forest, releasing nutrients back into the soil for re-use. The impact of a fungal invasion into a bark beetle infested tree is thought to contribute to tree death by preventing defense reactions, clogging the phloem and xylem, and aiding in beetle brood development. The degree to which the fungi impact the tree isdependent upon the fungal species.
Many of the fungi associated with bark beetles are in the genus Ophiostoma and are referred to as blue staining fungi. These species cause a blue stain to appear in the sapwood. A cross section or "round" of a tree will reveal the presence of the fungi as blue wedge shaped stain running from the outer edge of the section towards the center. The presence of the fungi does not alter the structural integrity of the wood but many mills downgrade the wood due to its appearance. Some retailers have taken advantage of the altered appearance and sell blue stained lumber at higher cost do to its pleasing aesthetic qualities. In either case, the wood retrieved from bark beetle killed trees can be used in the same manner as trees cut specifically for milling purposes.