Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Dryocoetes confusus
(western balsam bark beetle)



Dryocoetes confusus (western balsam bark beetle)


  • Last modified
  • 20 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Dryocoetes confusus
  • Preferred Common Name
  • western balsam bark beetle
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Arthropoda
  •       Subphylum: Uniramia
  •         Class: Insecta

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Scattered tree mortality caused by D. confusus on Abies lasiocarpa.
TitleTree mortality
CaptionScattered tree mortality caused by D. confusus on Abies lasiocarpa.
CopyrightWilliam M. Ciesla
Scattered tree mortality caused by D. confusus on Abies lasiocarpa.
Tree mortalityScattered tree mortality caused by D. confusus on Abies lasiocarpa.William M. Ciesla


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Preferred Scientific Name

  • Dryocoetes confusus Swaine, 1912

Preferred Common Name

  • western balsam bark beetle

Other Scientific Names

  • Dryocoetes abietis Hopkins, 1915

International Common Names

  • English: bark beetle, mountain balsam; bark beetle, western balsam; mountain balsam bark beetle
  • French: scolyte du sapin de l'ouest; scolyte du sapin du l'ouest

EPPO code

  • DRYOCN (Dryocoetes confusus)

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Arthropoda
  •             Subphylum: Uniramia
  •                 Class: Insecta
  •                     Order: Coleoptera
  •                         Family: Scolytidae
  •                             Genus: Dryocoetes
  •                                 Species: Dryocoetes confusus


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The eggs are smooth, oval, white and translucent.


The larvae are similar to scolytid larvae in general. They are white, legless and slightly curved. They have a lightly sclerotized, pale-tan head capsule and are 3-4 mm long when mature (Garbutt, 1992). See Thomas (1957) for generic characters of Dryocoetes larvae.


The pupae are exarate, usually whitish and 3-4 mm long. They have prominent wing pads and a distinct head, thorax and abdomen.


The adults are cylindrical and 3.4-4.3 mm long. They are shiny, ranging in colour from yellow-brown to reddish-brown to black and clothed with erect setae. The head is only slightly visible from above, the frons is convex and distinctive in females, with a circular patch of very dense reddish-brown to yellow hair-like setae, as opposed to sparsely pubescent in males. The mouthparts are directed downwards. The antennae are geniculate and capitate, with the antennal club wider than it is long and the basal corneous part occupying little more than half the total length. The pronotum is 1.1 times wider than it is long and evenly convex above. The elytra are 1.6 times longer than they are wide and abruptly rounded posteriorally. The elytral declivity is convex and lacking declivital spines. For generic and specific keys including Dryocoetes, and detailed specific descriptions, see Bright (1976) and Wood (1982), see also Bright (1963) for a more detailed description of morphology.


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In addition to the countries listed, D. confusus is also present in Alaska, where it has caused widespread mortality of subalpine firs apparently stressed by changing climate (ME Schultz, United States Forest Service, Juneau, Alaska, USA, personal communication, 2002).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 17 Feb 2021

Risk of Introduction

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D. confusus has been included in the EPPO quarantine pest category 'non-European Scolytidae' (EPPO/CABI, 1992). Because it can make primary attacks on Abies lasiocarpa, A. bifolia and other Abies species, it presents a certain risk to Abies spp. throughout Eurasia. This risk has previously been assessed as relatively minor in the past, because infestations in North America occurred in relatively inaccessible subalpine and alpine areas. However, these areas are now extensively used for recreational pursuits, and in British Columbia (Canada) interior fir now comprises 10-15% of the timber harvest. Some of the timber from harvested areas may be exported offshore as unprocessed raw logs. The potential thus exists for transport of D. confusus in logs, lumber or dunnage. D. confusus vectors a pathogenic fungus that is lethal to trees and it is not necessary for D. confusus to become established in a new habitat for there to be a phytosanitary risk. D. confusus could introduce Ceratocystis dryocoetidis, which in turn could be vectored by a native species of bark beetle. The phytosanitary risk should be considered moderate.

Hosts/Species Affected

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The main hosts of D. confusus are Abies spp., typically A. bifolia, A. lasiocarpa and their hybrid 'interior fir', A. bifolia x lasiocarpa (Hunt, 1993) but occasionally A. amabilis, A. concolor and Picea engelmannii (Wood, 1982). A. balsamea is listed as a host by Swaine (1918) but not other authors, which is consistent with a record of the pest in Lesser Slave Lake, USA (Bright, 1963).

Host Plants and Other Plants Affected

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Growth Stages

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Post-harvest, Vegetative growing stage


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Newly mass-attacked trees often have crystal-clear resin streaming down the bole from multiple new galleries. If an attack is successful, the resin flow eventually ceases and copious amounts of frass are expelled from the galleries. This can easily be seen in bark crevices and cobwebs, and encircling the root collar. In the late spring of the year following mass attack, the foliage of infested firs rapidly fades from green to a bright, brick-red colour. The red foliage can remain on a tree for up to 5 years, so aerial detection of infested trees must be followed up by ground surveys to determine the stage of attack (Garbutt, 1992). Attack on fallen trees is not associated with resin flow, but can be recognized externally by piles of frass around the entrances to galleries, or by frass coating the ground beneath the log. Verification of the attacking species must be made by removing the bark because other bark beetles such as Pityokteines minutus (Furniss and Carolin, 1977) may also attack true firs. When the bark is sloughed off previously infested trees or logs, the characteristic star-shaped galleries etched into the sapwood provide evidence of past attacks by D. confusus. If the bark is removed during handling of unseasoned timber or dunnage, galleries etched on exposed sapwood may constitute indirect evidence of living beetles in portions of the log where the bark remains intact.

List of Symptoms/Signs

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SignLife StagesType
Stems / gummosis or resinosis
Stems / internal discoloration
Stems / internal feeding
Stems / mycelium present
Stems / visible frass
Whole plant / cut at stem base
Whole plant / discoloration
Whole plant / frass visible
Whole plant / internal feeding
Whole plant / plant dead; dieback
Whole plant / uprooted or toppled

Biology and Ecology

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D. confusus, like other scolytids (bark beetles), tunnels into the bark of its hosts. In British Columbia, Canada, adults emerge from infested hosts and fly from May to August at temperatures ranging from 9 to 27°C (Stock, 1991). In Idaho and Montana, USA, D. confusus was caught in flight-monitoring traps from early June to the end of September (Gibson et al., 1997). The late summer flight is probably composed mainly of re-emergent parents. Both sexes are attracted to primary attractants from uninfested hosts (Stock and Borden, 1983), most likely monoterpenes and other terpenoid compounds (Camacho et al., 1998). Males attack first and then emit an aggregation pheromone (Stock and Borden, 1983), a 9:1 blend of (+)-exo-brevicomin to (+)-endo-brevicomin, that mediates simultaneous mass attack on a new host (Camacho et al., 1993). Broken and wind-thrown trees are preferred habitats, but standing trees are frequently attacked and overcome by pathogenic fungi. The most lethal of these fungi is Ceratocystis dryocoetidis (Kendrick and Molnar, 1965; Molnar, 1965), which is harboured in mandibular mycangia by dispersing beetles of both sexes (Farris, 1969) and is inoculated into newly attacked host trees.

Attacking males excavate a nuptial chamber in the phloem, and accept up to five females, which mine egg galleries radiating out from the nuptial chamber (Swaine, 1918; Garbutt, 1992). Both nuptial chamber and egg galleries often score the xylem tissue, leaving a characteristic star-shaped etching on the exposed wood of previously infested hosts. Eggs are laid on both sides of the egg galleries and the larvae bore irregular mines in the phloem. The life cycle usually takes 2 years (Mathers, 1931), although in warm conditions D. confusus may be univoltine (Furniss and Carolin, 1977; Garbutt, 1992). Adults that have attacked in the spring and summer overwinter in extensions of the nuptial chamber (males) and egg galleries (females) and resume mating and oviposition in the following spring. In mid-summer they may re-emerge to establish a second brood on the same tree or on a new host. Larvae and eggs produced in the first summer overwinter in the phloem and resume development in the following spring. There are four instars (Stock, 1981), which remain in the brood host for a second winter. Brood from the second spring and summer of oviposition are offset by an addition year, so that newly matured adults emerge from the same host for two successive years.

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Beauveria bassiana Pathogen Adults/Eggs/Larvae/Nymphs/Pupae

Notes on Natural Enemies

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Stock (1981) reared Lasconotus intricatus, Corticeus sp., Thanasimus undatulus, Xylophagus abdominalis and Medetera aldrichii (all assumed to be predators of D. confusus) in bolts cut from trees, mass attacked by D. confusus. Beauvaria bassiana is a naturally occurring fungal pathogen of D. confusus in British Columbia, Canada (Whitney et al., 1984).

Means of Movement and Dispersal

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All life stages of D. confusus can be readily transported from forested sites in infested timber because of the pest's subcortical habitat. As with other scolytids, e.g. Ips pini (EPPO/CABI, 1997), D. confusus could be introduced in unseasoned raw logs, waney lumber and dunnage. No studies of adult flight capability have been done, but similarity of infestation dynamics to that of other aggressive bark beetles suggests a flight range of several kilometres.

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Bark adults; eggs; larvae; nymphs; pupae Yes Pest or symptoms usually visible to the naked eye
Stems (above ground)/Shoots/Trunks/Branches adults; eggs; larvae; nymphs; pupae Yes Pest or symptoms usually visible to the naked eye


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Furniss and Carolin (1977) describe D. confusus as "the most destructive species in the genus". It does not cause rapid catastrophic mortality as do some other aggressive bark beetles, with infestations typically comprising less than 5% of a given stand. However, D. confusus can kill all of the mature and semi-mature firs in a stand over time (Garbutt, 1992). Mortality may be associated with trees infected with root disease (James and Goheen, 1981). In addition to being a pest of commercial timber, D. confusus can also be a major disruptive agent of alpine and subalpine protection and amenity forests (Hansen, 1996). In British Columbia (Canada) in 2001, over 1 million hectares sustained mortality caused by D. confusus (Westfall, 2002) and in 1996 D. confusus killed an estimated 51,000 subalpine firs in one US Forest Service region (Gibson et al., 1997).

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Tamiasciurus hudsonicus grahamensis (Mount Graham red squirrel)USA ESA listing as endangered speciesArizonaEcosystem change / habitat alterationUS Fish and Wildlife Service (2007)

Risk and Impact Factors

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Impact outcomes
  • Ecosystem change/ habitat alteration

Detection and Inspection

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Any unseasoned logs, lumber or dunnage with all or part of the bark still adhering to the wood should be thoroughly inspected at ports of entry for fresh or residual frass around entrance holes or in bark fissures. Fresh, brown frass is usually certain evidence of insects boring in the inner bark or phloem. Entrance holes are not always visible because they may be initiated deep in crevices in the bark. Living brood beetles may be present in galleries below entrance holes from which frass is no longer being produced. Therefore, all bark should be removed to confirm whether or not there is any bark beetle infestation. An alternative measure is to assume that any wood with the bark on may harbour living bark beetles and to destroy it through chipping or burning. For detection of infestation by introduced beetles (usually around suspected ports of entry), see 'Symptoms'.

Similarities to Other Species/Conditions

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All Dryocoetes species of a similar size look superficially similar. For example, D. confusus can be confused with Dryocoetes betulae and Dryocoetes autographus because of size. The latter species is holarctic in distribution, also infests Abies spp. and without reference to published descriptions can easily be mistaken for D. confusus.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

D. confusus infestations can be controlled by sanitation-salvage harvesting (Garbutt, 1992), which removes infested trees from a stand and kills the beetles during the manufacturing process in a saw mill or pulp mill. Before harvesting, infestations can be contained and concentrated by baiting uninfested trees with the aggregation pheromone exo-brevicomin (Stock et al., 1994; Greenwood and Borden, 2000). There is no other published information on the control of this species.

The exclusion of any unseasoned logs, lumber and dunnage at ports of entry is probably adequate to prevent introductions of D. confusus or Ceratocystis dryocoetidis. A requirement for all lumber to be kiln-dried would be a reasonable additional precaution. A potential problem could arise if traps baited with racemic exo-brevicomin are used as a monitoring tool for tree-killing Dendroctonus species at ports of entry. Although the pheromone for D. confusus is a 9:1 blend of (+)-exo-brevicomin to (+)-endo-brevicomin (Camacho et al., 1993), both D. confusus and Dryocoetes autographus respond to racemic exo-brevicomin (Jeans-Williams, 1999). Unless catches of Dryocoetes species in monitoring traps baited with exo-brevicomin are identified to species, it is thus possible that captured D. confusus could be mistaken for D. autographus, or may remain undetected among a more numerous catch of D. autographus.


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Brasier CM, Gibbs JN, 1973. Origin of the Dutch elm disease epidemic in Britain. Nature, UK., 242(5400):607-609

Bright DE Jr, 1976. The insects and arachnids of Canada. Part 2. The bark beetles of Canada and Alaska. Coleoptera: Scolytidae. The insects and arachnids of Canada. Part 2. The bark beetles of Canada and Alaska. Coleoptera: Scolytidae. Canada Department of Agriculture. Ottawa Canada, 241 pp.

Bright DE, 1963. Bark beetles of the genus Dryocoetes (Coleoptera: Scolytidae) in North America. Annals of the Entomological Society of America, 56:103-115.

Camacho AD, Pierce HD Jr, Borden JH, 1993. Geometrical and optical isomerism of pheromones in two sympatric Dryocoetes species (Coleoptera: Scolytidae), mediates species specificity and response level. Journal of Chemical Ecology, 19(10):2169-2182

Camacho AD, Pierce HDJr, Borden JH, 1998. Host compounds as kairomones for the western balsam bark beetle Dryocoetes confusus Sw. (Col., Scolytidae). Journal of Applied Entomology, 122(6):287-293; 49 ref.

EPPO, 1992. Quarantine pests for Europe. Wallingford, UK: CAB International, 210-212.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization.

Farris SH, 1969. Occurrence of mycangia in the bark beetle Dryocoetes confusus (Coleoptera: Scolytidae). The Canadian Entomologist, 101:527-532.

Furniss RL, Carolin VM, 1977. Western Forest Insects. Washington DC, USA: US Department of Agriculture Forest Service, Miscellaneous Publication No. 1339.

Garbutt R, 1992. Western balsam bark beetle. Victoria, British Columbia, Canada: Forestry Canada, Forest Pest Leaflet Number 64.

Gibson K, Kegley S, Oakes B, 1997. Western balsam bark beetle activity and flight periodicity in the Northern Region. Missoula, Montana, USA: US Department of Agriculture, Forest Service. Cooperative Forestry and Forest Health Protection Report Number 97-3.

Greenwood ME, Borden JH, 2000. Co-baiting for spruce beetles, Dendroctonus rufipennis, and western balsam bark beetles, Dryocoetes confusus (Coleoptera: Scolytidae). Canadian Journal of Forest Research, 30(1):50-58; 30 ref.

Hansen EM, 1996. Western balsam bark beetle, Dryocoetes confusus Swaine, flight periodicity in northern Utah. Great Basin Naturalist, 56(4):348-359; 9 ref.

Hunt RS, 1993. ABIES. In: Morin NR, ed. Flora of North America North of Mexico, Volume 2. New York, USA: Oxford University Press, 354-362.

James RL, Goheen DJ, 1981. Conifer mortality associated with root disease and insects in Colorado. Plant Disease, 65(6):506-507

Jeans-Williams NL, 1999. Evaluation of pheromone baits for the western balsam bark beetle, Dryocoetes confusus Swaine (Coleoptera: Scolytidae) and partial elucidation of the pheromone for Dryocoetes autographus Ratzeburg. Master of Pest Management Thesis. Burnaby, British Columbia, Canada: Simon Fraser University.

Kendrick WB, Molnar AC, 1965. A new Ceratocystis and its Verticicladiella imperfect state associated with the bark beetle Dryocoetes confusus on Abies lasiocarpa. Canadian Journal of Botany, 43:39-43.

Mathers WG, 1931. The biology of Canadian bark beetles. The seasonal history of Dryocoetes confusus Sw. The Canadian Entomologist, 63:247-248.

Molnar AC, 1965. Pathogenic fungi associated with a bark beetle on alpine fir. Canadian Journal of Botany, 43:563-570.

Smith IM, McNamara DG, Scott PR, Holderness M, 1997. Quarantine pests for Europe. Second Edition. Data sheets on quarantine pests for the European Union and for the European and Mediterranean Plant Protection Organization. Quarantine pests for Europe. Second Edition. Data sheets on quarantine pests for the European Union and for the European and Mediterranean Plant Protection Organization., Ed. 2:vii + 1425 pp.; many ref.

Stock AJ, 1981. The western balsam bark beetle, Dryocoetes confusus Swaine: secondary attraction and biological notes. Master of Science Thesis. Burnaby, British Columbia, Canada: Simon Fraser University.

Stock AJ, 1991. The western balsam bark beetle, Dryocoetes confusus Swaine: impact and semiochemical-based management. PhD Thesis. Burnaby, British Columbia, Canada: Simon Fraser University.

Stock AJ, Borden JH, 1983. Secondary attraction in the western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae). Canadian Entomologist, 115(5):539-550

Stock AJ, Borden JH, Pratt TL, 1994. Containment and concentration of infestations of the western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae), using the aggregation pheromone exo-brevicomin. Canadian Journal of Forest Research, 24(3):483-492

Swaine JM, 1918. Canadian bark beetles, Part II. Preliminary classification, with an account of the habits and means of control. Ottawa, Ontario, Canada: Printer to the King's Most Excellent Majesty. Dominion of Canada, Department of Agriculture, Entomological Branch, Technical Bulletin Number 14.

Thomas JB, 1957. The use of larval anatomy in the study of bark beetles (Coleoptera: Scolytidae). The Canadian Entomologist, Supplement Number 5, 3-45.

US Fish and Wildlife Service, 2007. In: Mount Graham Red Squirrel (Tamiasciurus hudsonicus grahamensis). 5-Year Review: Summary and Evaluation. US Fish and Wildlife Service, 24 pp.

Westfall J, 2002. 2001 Summary of forest health conditions in British Columbia. Victoria, British Columbia, Canada: British Columbia Ministry of Forests, Forest Practices Branch.

Whitney HS, Ritchie DC, Borden JH, Stock AJ, 1984. The fungus Beauveria bassiana (Deuteromycotina: Hyphomycetaceae) in the western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae). Canadian Entomologist, 116(10):1419-1424

Wood SL, 1982. The bark and ambrosia beetles of North and Central America (Coleoptera: Scolytidae), a taxonomic monograph. Great Basin Naturalist Memoirs, No. 6:1359 pp.

Distribution References

Bright D E Jr, 1976. The insects and arachnids of Canada. Part 2. The bark beetles of Canada and Alaska. Coleoptera: Scolytidae. In: The insects and arachnids of Canada. Part 2. The bark beetles of Canada and Alaska. Coleoptera: Scolytidae. Ottawa, Canada: Canada Department of Agriculture. 241 pp.

Burns K S, Klopfenstein N B, Kim M S, 2016. First report of the Armillaria root disease pathogen, Armillaria sinapina, on subalpine Fir (Abies lasiocarpa) and quaking aspen (Populus tremuloides) in Colorado. Plant Disease. 100 (1), 217-218. DOI:10.1094/PDIS-07-15-0837-PDN

CABI, Undated. CABI Compendium: Status as determined by CABI editor. Wallingford, UK: CABI

EPPO, 2020. EPPO Global database. In: EPPO Global database, Paris, France: EPPO.

Wood S L, 1982. The bark and ambrosia beetles of North and Central America (Coleoptera: Scolytidae), a taxonomic monograph. In: Great Basin Naturalist Memoirs, 1359 pp.

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