Interactions of Root Disease and Bark Beetles1 George T. Ferrell J. Richard Parmeter, Jr.2 Abstract: Associations between root diseases (Castello and others 1976; Harrington 1980), and bark beetles (Scolytidae) constitute some which may explain the rapid decay of sapwood of the most serious pest complexes affecting following tree-killing by bark beetles. forests in North America and elsewhere. The Despite much research, vectoring of root interactive functioning of these pests disease fungi by stem-infesting bark beetles derives from the following relationships: 1) has not yet been firmly established, but they root diseases predispose trees to bark beetle have been implicated as vectors of Peniophora infestation by lowering resistance, and gigantea (Fr.) Jul., a fungal antagonist of perhaps increasing attractiveness, of trees Heterobasidion annosum (Hunt and Cobb 1982). to the attacking beetles; 2) bark beetles may Recently, however, a species of be vectors of root disease fungi or may root-colonizing bark beetle in the genus create infection courts for them. Hylastes and root-colonizing species of the weevil genera Steremnius and Pissodes have been implicated in vectoring Leptographium (Verticicladiella) wageneri (Kendr.) Wingf., the fungus causing black-stain root disease Symbioses between bark beetles and fungi in Douglas-fir (Harrington and others 1985, are common in nature, and previous reviewers Witcosky and others 1986a). As these vectors (e.g., Graham 1967) have identified several readily breed in roots of recently cut types of these symbiotic relationships, some stumps, their populations evidently increase of which appear to be beneficial to both in stands after thinning (Harrington and (mutualism), and others which apparently others 1985; Witcosky and others 1986b). benefit only one (competition, predation, Previously, another root-colonizing species parasitism). These relationships will be of Hylastes had been implicated as a vector discussed in relation to interactions between of this fungus in ponderosa pine (Goheen and root diseases and bark beetles. Others, Cobb 1978). As other root-colonizing beetles which benefit neither (i.e., commensalism or are studied, more species will probably be cohabitation), are not considered to be true implicated as vectors of root disease fungi. interactions as defined here, and will not be further mentioned. Vectoring of H. annosum by bark beetles has not been confirmed. Nuorteva and Laine (1968) and Hodges (1969) reviewed literature VECTORING on the possibility of insect vectoring. Nuorteva and Laine showed that various Virtually all bark beetles vector fungi beetles could become contaminated with to host trees, inoculating the phloem and conidia by walking across agar cultures of H. xylem as they mine in the cambial region. annosum. Hunt and others (1976, 1982) Much studied are the pathogenic blue- and discussed the possibility of insect vectoring brown-staining fungi, vectored by stem- and of H. annosum in western North America. They twig-infesting beetles. These fungi largely concluded that even though conidia and infect above-ground host tissues and thus are basidiospores are produced on stumps, it is not true root pathogens. Bark beetles can unlikely that insects play a major role in also carry propagules of saprot fungi dissemination of H. annosum. However, circumstantial evidence and the observations that annosus root disease can appear in 1Presented at the Symposium on Research stands with no history of cutting suggest and Management of Annosus Root Disease in that the possibility of at least occasional Western North America, April 18-21, 1989, spread of H. annosum by insect vectors cannot Monterey, California. yet be excluded. Because the initiation of new centers of annosus-caused mortality in 2Research Entomologist, Pacific western timber stands appears to be a Southwest Forest and Range Experiment relatively rare event, even infrequent insect Station, Berkeley, Calif., and Professor of vectoring could be important. The initiation Plant Pathology, University of California, of one or two such centers per acre over an Berkeley, Calif. 80-year rotation could have serious impacts. USDA Forest Service Gen. Tech. Rep. PSW-116 105 PREDISPOSITION COMPETITION The role of root diseases in predisposing Compared to mutualistic relationships, trees to bark beetles was largely ignored in competitive or other antagonistic early studies of bark beetle population interactions between bark beetles and root dynamics and control. More recent studies disease fungi have not been widely reported. (Partridge and Miller 1972, Cobb and others But in stumps or at the base of trees, in 1974, Lane and Goheen 1979, Alexander and galleries of stem-infesting beetles invaded others 1980) have revealed that, where bark by H. annosum, the beetles are sometimes beetle populations are not in outbreak phase, observed to be dead and enveloped by mycelium a high proportion of trees killed by bark of the fungus (Stark and Borden 1965, Hunt beetles also are root-diseased. However, and Cobb 1982). However, root disease Cobb and others (1974) found that, where bark infections rarely spread far upward in beetle outbreaks are in progress, the stems. Thus, on a tree-wide basis, beetle proportion of killed trees that also are survival would likely not be much reduced by root-diseased tends to be lower, suggesting this mortality except for a species like that root-diseased trees act as foci from Dendroctonus valens LeConte which seldom which bark beetle outbreaks can spread during attacks its host pines more than two meters droughts or other widespread tree stress. above ground. Competitive or other antagonistic Physiologically, the predisposition of interactions may also occur between the root-diseased trees to bark beetle attack has fungal symbionts of stem-infesting bark been shown to involve such indications of beetles and root disease fungi. Such subnormal physiology as increased moisture interactions could be particularly important stress and reduced resin exudation pressure, in reducing stump colonization by H. annosum both of which are known to signify reduced in trees felled for bark beetle control. tree resistance to bark beetle attack (Stark Hunt and Cobb (1982) observed that fungal and Cobb 1969, Ferrell and Smith 1976). symbionts of bark beetles frequently co-occur There is also some evidence that ponderosa with H. annosum in pine stumps in pines with black-stain root disease are California. They concluded that although attractive to certain species of Dendroctonus these fungi may reduce the amount of H. bark beetles before they initiate mines and annosum colonization somewhat, they had not concomitantly produce attractants (Goheen and colonized enough stump surface or volume to others 1985). Similarly, in Douglas-fir, reduce H. annosum colonization in pine stumps segments of roots with black-stain root to a level acceptable to forest managers. disease were more attractive to the Hylastes and Steremnius vectors of the black-stain Other interactions may be envisioned. fungus than were segments of uninfected roots For example, root systems of trees killed by (Witcosky and others 1987). bark beetles might deteriorate too rapidly to provide suitable substrate for root disease Other than the vectoring of pathogenic fungi. If so, it is possible that early fungi to trees, there is presently little killing of infected trees or their healthy evidence supporting the idea that bark neighbors by bark beetles might reduce the beetles predispose their host trees to root rate of enlargement of root disease centers. diseases. However, even if root-colonizing CONCLUSION beetles are not vectoring fungal pathogens, Our understanding of the various ways in their feeding wounds in roots may create which bark beetles and root diseases interact infection courts for pathogenic fungi present remains limited. Interactions in addition to in soil or other, infected, roots. For those discussed above may also be important. example, in precommercially thinned For example, whether insectan natural enemies Douglas-fir stands, Harrington and others of bark beetles might vector root disease (1985) found numerous feeding wounds of fungi has not been widely assessed, but it Hylastes beetles in roots of residual trees. may be important to do so because of their None of these wounds was observed to be potential introduction into new environments infected by black-stain disease, but field as biological control agents for bark observations have long implicated lesions in beetles. With the increased recognition of the root cortex as infection courts for H. the importance of interactions between bark annosum (see review by Nuorteva and Laine beetles and root diseases, it is likely that 1968). Hendrix and Kuhlman (1964) found that this field of research will receive increased pine roots could be infected by H. annosum attention. through root wounds, including wounds made by Additional research on insect/annosus insects. It is therefore likely that insect interactions could greatly benefit forest wounds in roots are important infection managers. If spread of H. annosum by insects courts for H. annosum. to roots of trees or stumps is common in some 106 USDA Forest Service Gen. Tech. Rep. PSW-116 tree species, reduction of insect activity Graham, K., 1967. Fungal-insect mutualism in could be an effective adjunct to treatment of trees and timber. Annual Review of stump surfaces to prevent infection by the Entomology 12: 105-126. fungus. Conversely,