491CHAPTER21BARKBEETLES ANDTHEIRMANAGEMENTINTRODUCTIONBark beetles belong to the family Scolytidae, a family that contains extremelydestructive forest insects. Scolytid outbreaks frequently kill hundreds of squarekilometers of coniferous forests. In this context they influence tree age, size, andspecies distributions across the landscape. The bark beetles, together withpathogens, wind, and fire are a preeminent factor in forest succession. The Scolytidae form two ecological groups: the bark beetles and ambrosia bee-tles (latter to be discussed in Chapter 22). Bark beetle broods feed on subcorticaltissues (phloem tissues), and ambrosia beetles are wood-boring insects whose lar-vae feed on fungi (Figure 21.1). Scolytids can be classified on the physiologicalcondition or vigor of their hosts (Rudinsky 1962). Beetles that normally select vig-orous hosts are primary scolytids, whereas those that normally select weakenedand thus susceptible hosts are secondary scolytids. Most of the economically impor-tant bark beetles are considered secondary: During latent (endemic or low) popu-lation levels, they preferentially attack downed trees, logs, fire-damaged trees,moribund trees, or trees temporally weakened by drought, defoliation, root rots, orother destructive agents (Table 21.1). When populations of secondary bark beetlesincrease, however, they can become primary and kill healthy trees. For example,the Douglas-fir beetle (Dendroctonus pseudotsugae) and Ips spp. are secondaryscolytids that preferentially attack windthrown trees, logs, and fire-damaged trees.When there is a surplus of brood material, their populations can increase and massattack living trees.Associations with FungiScolytids have symbiotic relationships with fungi (Berryman and Ashraf 1970,Barras and Perry 1972, Berryman 1972). Fungi associated with aggressivescolytids have an active role in killing trees as developing mycelia block vascu-lar tissues. In scolytids invading pines, successful penetration of the mutualisticfungi produce a blue stain in the sapwood. The association between bark beetlesCh21 10/4/99 2:38 PM Page 491and blue-staining fungi arose from the requirement that both the fungi and the bee-tles need susceptible trees for their development. For instance, when ponderosapines are unable to muster up their resinous defense systems (e.g. , a reduced ole-oresin exudation pressure), a condition brought on by water stress, the blue-staining fungi can successfully develop in sapwood of these trees. Spores of thesefungi are introduced into the succulent phloem of these trees by invading Den-droctonus brevicomis, D. ponderosae, or Ips spp. Beetles either transport sporesadhering to their bodies, in specialized pits, or within specialized structures,termed mycangia. Most staining fungi associated with bark beetles belong to thegenus Ophiostoma—for example, O. pini is carried by Ips calligraphus, I. grandi-collis, and I. pini; on the other hand, the fir engraver beetle (Scolytus ventralis)vectors the brown-staining fungus, Trichosporium symbioticum.Fungal interactions with subcortical tissues also serve to enhance the palata-bility and nutrient content of scolytid food. There are scolytid and also some wee-vil (Curculionidae) species that infest roots of stumps, logging debris, abandonedlogs, or the bases of moribund hosts. Some of these Rhynchophora (a taxonomicclassification that contains the weevils and bark beetles) include the bark beetles,Hylastes nigrinus, Hylastes ater, and Hylurgus lignipurda (latter two native toEurope); examples of weevils include Hylobius pales (pales weevil), Pissodes fas-ciatus, and Steremnius carinatus. Brood development in decaying material posestwo problems: (1) the fermenting substrate may not provide sufficient energy forreproductive development and (2) larvae developing in this decaying substratemay form relationships with pathogens. In the former case, emerging adults feedat the root crown of newly planted seedlings for sexual maturation and kill theseedlings—as is the case with H. ater, H. pales, P. fasciatus, and S. carinatus. Inthe latter example, H. nigrinus, P. fasciatus, and S. carinatus are vectors of theblack-stain root disease fungus, Leptographium wagenerii (see Chapter 12).When Douglas-fir stands are thinned in southwest Oregon, the fungus is trans-mitted to new stumps as spore-laden insects move into this new food source492 CHAPTER 21Bark beetles• Except for adults, life is entirelyunder the bark in the phloem• Feed in/on subcortical tissues• While boring into a tree or log,leave brownish debris that ispushed out of entrance hole• New adults bore out of tree or logAmbrosia beetles• Except for adults, life is entirelyin the xylem• Feed on ambrosia fungus• While boring into a tree or log,leave whitish debris that is pushedout of entrance hole• New adults leave by original entranceThe Scolytidae and Its Two Ecological GroupsFigure 21.1The two ecological groups found in the family Scolytidae: the bark beetles and theambrosia beetles.Ch21 10/4/99 2:38 PM Page 492(Goheen and Hansen 1978). Studies in Oregon demonstrated that 15-year-olddense stands, thinned to 600 stems per hetacre, developed black-stain root diseasein 15% of the leave trees during the next 6 years. Plantation mortality by theserhynchophorans is of concern to intensive silvicultural applications around theworld and a major issue in implementation of quarantine regulations.In the case of Scolytus multistriatus, vector of the Dutch elm disease (causedby the fungus Ophiostoma ulmi), association between bark beetle and fungusallows for spread of the fungus and concomitant weakening of elm trees. Asinfected elms become moribund, they are susceptible hosts for attack of S. mul-tistriatus (see Chapter 14). Mortality of elms in European and North Americancities is of major economic and aesthetic concern to urban forestry. Biology of Bark BeetlesBark beetles have common traits and habits (Wood 1982). They all excavate egggalleries in fresh phloem. The larvae feed away, at right angles from the egggallery, and feed in the succulent phloem tissues. Patterns formed by both the orig-inal egg galleries and the larval mines are characteristic for each bark beetlespecies, as shown in Figure 21.2 for five species that infest southern pines.Bark beetles complete their life cycle under the bark of their hosts, with theexception of a short flight period. A flight period is essential to bark beetles