Western Balsam Bark Beetle Joel Mcmillin1, Danielle Malesky2, Sandra Kegley3, A

Forest Insect & Disease Leaflet 184 August 2017

U.S. Department of Agriculture • Forest Service Western balsam bark beetle Joel McMillin1, Danielle Malesky2, Sandra Kegley3, A. Steven Munson4 Introduction The western balsam bark beetle (Dryocoetes confusus Swaine, is an important mortality agent of subalpine fi r Abies ( lasiocarpa) in western North America. The beetle is less aggressive than tree-killing bark beetles in the genus Dendroctonus (mountain pine beetle, D. pondero- Figure 1. Subalpine fi r mortality caused by sae; spruce beetle, D. rufi pennis; or western balsam bark beetle. Douglas-fi r beetle, D. pseudotsugae) and usually attacks weakened (e.g., Western balsam bark beetle infesta- drought stressed) or downed trees. tions have been reported since forest Commonly, less than fi ve percent of surveys began in the early 20th centu- trees in a forest stand are attacked in ry. Outbreaks have affected the subal- a year, but the cumulative impacts of pine fi r type throughout many areas of chronic infestations over time can re- British Columbia with estimated losses sult in most of the mature subalpine fi r of more than 423 million ft3 (12 mil- within a stand being killed. Outbreaks lion m3) occurring in the Prince Ru- can result in high numbers of trees be- pert Region between 1956 and 1965. ing killed in a single year More recently, area affected averaged (Figure 1). 100,000 acres (250,000 hectares) during 2003-2013 in southern British Co-

1Supervisory entomologist, U.S. Department of Agriculture, Forest Service, Forest Health Protection, Intermountain Region, Boise, Idaho. 2Entomologist, U.S. Department of Agriculture, Forest Service, Forest Health Protection, Intermountain Region, Ogden, Utah. 3Entomologist, U.S. Department of Agriculture, Forest Service, Forest Health Protection, Northern Region, Coeur d’Alene, Idaho. 4Supervisory entomologist (Retired), U.S. Department of Agriculture, Forest Service, Forest Health Protection, Intermountain Region, Ogden, Utah. lumbia with mortality from this beetle Hosts and Distribution as high as 70 percent in certain biogeoclimatic zones. Similarly, western bal- Western balsam bark beetle is common sam bark beetle infestations affected through much of the range of its prima- nearly 700,000 acres (1.7 million hect- ry hosts, subalpine fi r and corkbark fi r. ares) annually from 2001-2010 across Its distribution extends from southeast the western US, with most subalpine Alaska, British Columbia and Alberta fi r mortality recorded in Colorado and south to Arizona and New Mexico, and Wyoming. An outbreak in the Big- from Colorado westward to Oregon horn Mountains of Wyoming killed (Figure 2). Western balsam bark beetle more than 70 trees/acre (173 trees/ occasionally attacks Pacifi c silver fi r hectare) over a 6-year period reducing (A. amabilis), grand fi r (A. grandis) and tree density more than 60 percent in white fi r A.( concolor), and rarely En- infested stands. Corkbark fi r A.( lasio- gelmann spruce (Picea engelmannii), carpa var. arizonica) stand basal area white spruce (P. glauca) and lodgepole was reduced by 5 to 70 percent during pine (Pinus contorta) during outbreaks a late 1990’s outbreak in the Pinaleño occurring in mixed species stands with Mountains of southern Arizona. subalpine fi r.

Western balsam bark beetle has a close External Evidence of association with a fungus (Grosman- Infestation nia dryocoetidis = Ceratocystis dryocoetidis) that is transmitted to trees it Needles of infested trees fade from colonizes. During a 1957 survey of green to yellowish-red the year fol- subalpine fi r stands, only about 35 per- lowing successful attack by western cent of tree mortality could be directly balsam bark beetle. Needles will then credited to western balsam bark beetle turn a brick-red color for approximate- damage. Remaining trees survived ly two years on dead trees (Figure 3). attack by beetles, but apparently suc- Eventually needles become dull-red for cumbed to coalescing lesions caused by introduced fungal infection that girdled the trees. Subsequent inocula- tion tests confi rmed the pathogenicity of introduced fungi. Although host defenses often prevent initial beetle attacks, decreased tree vigor caused by fungal infection may also result in subsequent beetle attacks that are successful. Western balsam bark beetle also frequently colonizes trees weakened by root disease fungi (Armillaria spp., Heterobasidion spp.).

Figure 2. Distribution of western balsam bark beetle based on the range of its primary host trees subalpine fi r and corkbark fi r. 2 Figure 3. Subalpine fi r mortality caused by western balsam bark beetle. Trees with bright, brick red needles indicate trees attacked 1 to 2 years previously. Older attacked trees have dull, red needles or have lost their needles and appear gray. another 2 to 3 years, resulting in 4 to 5 Figure 4. Pitch streaming indicative of years of dead needle retention. Trees subalpine fi r tree under attack by western that successfully pitch out attacking balsam bark beetle. Beetles are often beetles but are killed by pathogenic unsuccessful in colonizing trees that exhibit such heavy pitch streaming. fungi may take more than one year to fade. The prolonged sequence of fad- occur well above breast height and into ing can make accurate assessment of the upper crown, which also hinders new tree mortality during aerial sur- diagnosis of beetle presence. Numer- veys problematic. Moreover, in areas ous streams of clear pitch can be evi- where root disease or balsam woolly dent, and may indicate unsuccessful adelgid occur, it may be diffi cult to dis- attacks (Figure 4). Pitch streams full tinguish between mortality caused by of boring dust are a sign of successful beetles versus these other agents. In attack. addition, secondary bark beetles may contribute to subalpine fi r mortality in Occasionally fi r engraver beetles, Sco- trees already attacked by western bal- lytus ventralis, will also attack subal- sam bark beetle. pine fi r and corkbark fi r; however, their horizontal egg galleries are easily dis- External evidence of attack on tree tinguished from those created by west- boles is not readily apparent, mak- ern balsam bark beetle (see description ing identifi cation of recently infested in next section). Numerous secondary trees diffi cult. During late summer, bark beetles may also be found on dead entrance holes and reddish-brown bor- or dying subalpine fi r, but they are gen- ing dust in bark crevices and around erally smaller and have different egg the base of freshly attacked trees may gallery patterns than western balsam be visible; however, precipitation will bark beetle. often remove these signs by the next spring. The majority of attacks often

3 Figure 5. Male (left) and female (right) western balsam bark beetles. Female western balsam bark beetles are identifi ed by the denser and thicker setal brush located on the forehead. Identifi cation and Life that a one-year cycle may occur under History appropriate environmental conditions. Adult beetle fl ight typically begins in Adult western balsam bark beetles are early summer after daytime tempera- shiny reddish-brown, and approxi- tures exceed approximately 60-68º F mately 1/6 inch (3.4 to 4.3 mm) long (15-20º C), but it can occur as early as (Figure 5). Their thorax is evenly late May. Male beetles initiate attacks convex, and hardened forewings are on boles of susceptible hosts; boring abruptly rounded without spines. Both into the phloem, excavating a nuptial male and female beetles have numer- chamber and releasing a pheromone ous “hairs” on the front of their fore- that attracts females. Males are po- heads (frons); however, this setal brush lygamous, mating with more than one is more prominent on females and re- female. Mated females construct egg sembles a “crew-cut”. Late-instar lar- galleries that radiate away from the vae are 1/7 inch (3-4 mm) long with nuptial chamber, forming a stellate or pale-tan head capsules and yellow- star-shaped gallery pattern (Figure 6). white bodies that are characteristically These egg galleries may score the out- c-shaped and wrinkled. Pupae are the er sapwood. Females can produce up same size as adults, are yellow-white, to three separate broods with the fi rst and many adult features such as legs occurring during initial gallery con- and wing covers are recognizable. struction in mid-summer. Many females overwinter and extend their egg Although development studies on west- galleries the following spring to create ern balsam bark beetle demonstrate a a second brood. Some then emerge in 2-year life cycle, it has been speculated summer to produce a third brood, ei- ther in the same or another tree.

4 A B

Figure 6. Stellate (star-shaped) egg galleries of western balsam bark beetle on the inner bark (A) and etched on the outer sapwood (B) of infested trees. Larval feeding galleries are smaller in width and perpendicular to egg galleries (seen in A).

Upon hatching, larvae tunnel away regime. Adult fl ight in northern Idaho from egg galleries to feed throughout and western Montana generally began the summer and into fall until freezing in early- to mid-June and continued temperatures occur. A range of larval through September; with peak fl ights instars is found overwintering beneath occurring in late-June/early-July and the bark. Larval development contin- late-July/early-August. In northern ues the next spring and summer with Utah, peak fl ights occurred between the second winter spent as adults. mid-June to early-July, then 4-7 weeks later in August. In north-central Wyo- Studies from across the range of west- ming, adult fl ight initiated in early-July ern balsam bark beetle suggest there are with peak fl ight activity observed in two principal fl ight periods each year; mid-July and late-August. In a fi ve- the fi rst typically larger and comprised year study of adult fl ight in Colorado, of newly emerged young adults and beetles commonly began dispersing in the second being primarily re-emerged early- to mid-June with fi rst peak fl ight second-year females. Initial fl ight is occurring in early- to mid-July. Occur- dominated by male beetles followed by rence of a second peak varied between an increasing percentage of females or years and elevation. Based on limited a relatively equal number of both gen- studies in northern Arizona, fl ight ini- ders. In British Columbia, peak fl ights tiation occurred the fi rst week of July occurred in mid- to late-June and in and peaked in mid-July. mid-August through early September depending on latitude and moisture

5 Factors affecting tree and were older, but not necessarily bigger. stand susceptibility and Increased susceptibility of older trees seemed to be associated with effects of outbreaks senescence and declining host vigor. Generally, this bark beetle attacks Trees with low vigor are slower grow- small groups of host trees at a some- ing and produce less secondary resin what low, but steady level each year in which results in beetles having higher infested stands. During endemic pop- successful attack rates compared to ulation phases, western balsam bark vigorous, faster growing trees with beetle infests old, stressed or dam- more resin production. Pioneering aged trees and fresh host material on male beetles attacking vigorous trees the ground, such as wind breakage and may be overwhelmed by host defenses blowdown. Outbreaks typically begin and therefore unable to release aggre- as a result of certain environmental gation pheromones needed to success- conditions (extended drought) or large- fully colonize trees. Although western scale disturbances such as windthrow balsam bark beetle may show prefer- (Figure 7). ence for larger-diameter size classes/ older trees; during intense outbreaks Susceptibility of individual trees to at- or as larger diameter host trees are tack by western balsam bark beetles depleted, smaller-diameter trees may may be related to specifi c traits. In a also be attacked. This pattern of attack comparison of attacked and unattacked is typically observed during periods of subalpine fi r in British Columbia, suc- prolonged drought when host tree resin cessfully attacked trees typically had production has been signifi cantly re- slower growth and smaller crowns and duced. In addition, smaller diameter

Figure 7. Dense stands of corkbark fi r impacted by western balsam bark beetle during an extended drought on the Sandia Mountains, New Mexico. 6 trees can be attacked as a result of els of infestation. Corkbark fir basal spillover from successful attacks on area was reduced by more than 50 per- neighboring large-diameter trees. cent in many stands due to a western balsam bark beetle outbreak in south- Tree mortality caused by western bal- eastern Arizona following defoliation sam bark beetle may also be related to caused by looper defoliation, Nepytia forest stand conditions and site char- janetae, and winter storm damage. acteristics. Significant positive linear Similarly, western balsam bark beetle relationships were found between the population increase and associated amount of tree mortality and percent- tree mortality was observed following age of subalpine fir trees in a stand, a 1993 storm that caused windthrow subalpine fir basal area, and subal- and tree breakage over a large area pine fir stand density index following in northern Wyoming. Egg galleries a western balsam bark beetle outbreak were observed in all diameter classes during the late 1990s and early 2000s of fir blowdown greater than 4 inches in the Bighorn Mountains of Wyoming. (10 cm). Felled trees in Colorado had These findings suggest that inter-tree highest attack rates on bole sides and, competition caused by high tree den- to a lesser extent, on the underside dur- sity and basal area results in increased ing endemic conditions, although there susceptibility to western balsam bark was no surface preference for attack beetle. Forest stands impacted during on felled trees exposed to high popula- the outbreak in Wyoming were domi- tions in British Columbia. nated by subalpine fir (>80 percent), which may also explain the high rate Management Options and level of mortality (>50 percent of subalpine fir tree density and basal Vegetation management area). In stands with a smaller component of subalpine fir (<50 percent), fac- Generally, subalpine fir grows at high- tors such as abundance of large-diam- elevation sites with a limited season eter host trees and level of root disease for vegetation management activities. may be more important in determining Lack of accessibility can prevent re- the amount and rate of tree mortality. moval of currently infested trees to During a similar timeframe in southern suppress building beetle populations. British Columbia, the highest levels of In addition, difficulty in identify- tree mortality occurred in the driest, ing currently infested trees can affect coldest Engelmann spruce-subalpine timely removal. If sanitation treat- fir biogeoclimatic zone compared with ments are implemented, they should wetter, warmer sites. occur following flight activity in Au- gust or before adult flight in June. Disturbance events (windthrow, ava- Prompt removal or destruction of host lanches) and host stress caused by material generated from windthrow or drought, defoliation and root disease logging slash is recommended to pre- have all been implicated in the ini- vent population increase or suppress tiation of western balsam bark beetle populations. Similarly, removing trees outbreaks or maintaining chronic lev- weakened by causes other than root

7 disease will mitigate successful at- bark beetle have been identified and tacks. Caution should be exercised in shown to be effective in manipulat- areas of known root disease pockets ing beetle populations and dispersal. because partial cutting can increase Commercially available attractant root disease spread. pheromone lures or baits containing exo-brevicomin can be used to moni- Because western balsam bark beetle tor beetle flight or concentrate dispers- prefers less vigorous trees, silvicultur- ing beetles into stands scheduled for al treatments that improve tree growth harvest. Based on studies in British and vigor may be an effective manage- Columbia, baiting two trees per active ment strategy. Treatments that lower spot is recommended for containment subalpine fir basal area or stand den- and concentration of small isolated in- sity index, percentage of fir in a stand, festations, while baiting trees on a 164 and stand age could reduce inter-tree foot (50 meter) grid is recommended competition for limited resources, in- for larger areas. One study demon- crease individual tree vigor and lower strated that stands can be successfully stand susceptibility to damage caused co-baited for both western balsam bark by western balsam bark beetle. In- beetle and spruce beetle when trying creasing age class diversity and pro- to manage populations of both bark moting younger age classes would also beetle species. Baited trees less than presumably limit stand level impacts 8 inches (20 cm) in diameter were not caused by western balsam bark beetle. typically attacked. Therefore, select Because subalpine fir stands over 100 larger, older or less-vigorous trees for years old in British Columbia sustain baiting. continuous attack from western balsam bark beetle, it is recommended to Although a trap out strategy to sup- use a rotation age less than 100 years. press western balsam bark beetle Although encouraging a healthy fir populations has not been documented, component through active manage- high quantities of beetles collected in ment in high-value sites such as devel- funnel traps and attraction of beetles to oped ski areas makes intuitive sense, freshly felled trees suggest this might specific silvicultural guidelines in fir be a feasible tactic. This suppression dominated stands that result in lower- strategy could be used to address a ing susceptibility to beetle attack have small, local population of insects. All yet to be fully developed. Further- baited standing trees, felled trap trees more, thinning in spruce-fir stands can and adjacent infested trees from spill- increase windthrow risk thereby pro- over effects must be removed or treat- viding a preferred habitat for western ed on site (e.g., burned or bark peeled) balsam bark beetle development. to kill all beetle life stages before adult flight occurs. Pheromones Western balsam bark beetle response to Pheromones may provide another al- its aggregation pheromone, exo-brevi- ternative for managing beetle popula- comin, was inhibited by endo-brevi- tions. Pheromones for western balsam comin in both laboratory assays and

8 field studies. Endo-brevicomin is not References commercially available and additional field studies are required to determine Bleiker, K.P., Lindgren, B.S., its efficacy and deployment methods. and Maclauchlan, L.E. 2003. Combining non-host volatiles with Characteristics of subalpine fir endo-brevicomin may enhance repel- susceptible to attack by western balsam lency effects but further research is bark beetle (Coleoptera: Scolytidae). required to document the effectiveness Canadian Journal of Forest Research of this technique. If pheromones are 33: 1538-1543. used for manipulating behavior or distribution of beetles, treatments should Bleiker, K.P., Lindgren, B.S., and be deployed by early-June for the Maclauchlan, L.E. 2005. Resistance northern Rockies and late-June for the of fast- and slow-growing subalpine central Rockies and Southwest. fir to pheromone-induced attack by western balsam bark beetle Dryocoetes Acknowledgements confusus (Coleoptera: Scolytidae). Agricultural and Forest Entomology 7: Earlier drafts of this leaflet were im- 237-244. proved through thoughtful reviews provided by Kathy Bleiker, Ken Gib- Borden, J.H., Pierce, A.M., Jr, Chong, son, Matt Hansen, Lorraine Maclauch- L.J., Stock, A.J., and Oehlschlager, lan and Art Stock. A.C. 1987. Semiochemicals produced by the western balsam bark beetle, Photo Credits Dryocoetes confusus Swaine (Coleoptera: Scolytidae). Journal of Authors’ photos Chemical Ecology 13: 823–836.

Sandra Kegley (Figure 4), Joel McMil- Bright, D.E., Jr. 1963. Bark Beetles lin (Figure 6A) of the Genus Dryocoetes (Coleoptera: Scolytidae) in North America. Annals Non-authors’ photos Entomological Society America. 56: 103-115. USDA Forest Service - Ogden Archive (Figure 1), Elbert L. Little, Jr., USFS Doidge, D.F. 1981. Western balsam (Figure 2), Kenneth Gibson, USFS bark beetle in British Columbia. FHP R1 (Figure 3), Thomas A. Shah- Forest Pest Leaflet FPL-64. Canadian an, Oregon Department of Agriculture Forest Service, Pacific Forest Research (Figure 5), David Beckman, Idaho De- Centre, Victoria, BC. 4 p. partment of Lands (Figure 6B), Daniel Ryerson, USFS FHP R3 (Figure 7). Garbutt, R. 1992. Western balsam bark beetle in British Columbia. Forest Pest Leaflet FPL-64. Canadian Forest Service, Pacific Forest Research Centre, Victoria, BC. 4 p.

9 Gibson, K., Kegley, S., and Oakes, systems. Journal of the Entomological B. 1997. Western balsam bark beetle Society of British Columbia 100: 27- activity and flight periodicity in the 41. Northern Region. USDA Forest Service Forest Health Protection M a cl a u c h l a n , L . E . 2 016 . Q u a n t i fi c a t i o n Report 97-3. 5 p. of Dryocoetes confusus-caused mortality in subalpine fir forests of Greenwood, M.E, and Borden, J.H. southern British Columbia. Forest 2000. Co-baiting for spruce beetles, Ecology and Management 359: 210- Dendroctonus rufipennis, and western 220. balsam bark beetles, Dryocoetes confusus (Coleoptera: Scolytidae). Mathers, W.G. 1931. The biology of Canadian Journal of Forest Research Canadian bark beetles. The seasonal 30: 50-58. history of Dryocoetes confusus Sw. Canadian Entomologist 68: 247-248. Hansen, E.M. 1996. Western balsam bark beetle, Dryocoetes confusus McMillin, J.D., Allen, K.K., and Long, Swaine, flight periodicity in northern D.F. 2001. Evaluation of western Utah. Great Basin Naturalist 56: 348– balsam bark beetle flight periodicity on 359. the Bighorn National Forest, Wyoming. USDA Forest Service Biological Huber, D.P.W., and Borden, J.H. 2003. Evaluation R2–02–01, Golden, CO. 14 p. Comparative behavioral responses of Dryocoetes confusus Swaine, McMillin, J.D., Allen, K.K., Long, Dendroctonus rufipennis (Kirby), and D.F., Harris, J.L., and Negrón, J.F. Dendroctonus ponderosae Hopkins 2003. Effects of western balsam bark (Coleoptera: Scolytidae) to angiosperm beetle on spruce-fir forests of north- tree bark volatiles. Environmental central Wyoming. Western Journal of Entomology 32: 742-751. Applied Forestry 18: 259-266.

Kendrick, W.B., and Molnar, A.C. Molnar, A.C. 1965. Pathogenic fungi 1965. A new Ceratocystis and its associated with a bark beetle on alpine Verticicladiella imperfect state fir. Canadian Journal of Botany 43: associated with the bark beetle 563-570. Dryocoetes confusus on Abies lasiocarpa. Canadian Journal of Negrón, J.F., and Popp, J.B. 2009. The Botany 43: 39-43. flight periodicity, attack patterns, and life history of Dryocoetes confusus Maclauchlan, L.E. 2003. Impact Swaine (Coleoptera: Curculionidae: of the western balsam bark beetle, Scolytinae), the western balsam bark Dryocoetes confusus Swaine beetle, in north central Colorado. (Coleoptera: Scolytidae), at the Western North American Naturalist Sicamous Creek research site, and 69: 447–458. the potential for semiochemical based management in alternative silviculture

10 Smith, S.M., and Hulcr, J. 2015. Scoly- of infestations of the western balsam tus and other economically important bark beetle, Dryocoetes confusus bark and ambrosia beetles. pp. 495- (Coleoptera: Scolytidae) using 531. In: Vega, F.E. and Hofstetter, R.W. the aggregation pheromone exo- (Eds.), Bark Beetles. Biology and Ecol- brevicomin. Canadian Journal of ogy of Native and Invasive Species. Forest Research 24: 483-492. Academic Press, London. Stock, A.J., Borden, J.H., Pratt, T.L., Stock, A.J. 1991. The western bal- Pierce, Jr. H.D., and Johnston, B.D. sam bark beetle, Dryocoetes confusus 1995. Enantiomeric composition Swaine: Impact and semiochemical- and release rates of exo-brevicomin based management. Ph.D. Thesis, Si- influence aggregation of the western mon Fraser University, Burnaby, B.C., balsam bark beetle, Dryocoetes 133 p. confusus Swaine (Coleoptera: Scolytidae). Canadian Entomologist Stock, A. J., Borden, J.H., Pratt, T.L., 127: 449-456. Pierce, Jr., H.D., and Johnson, B.D. 1990. Endo-brevicomin: An anti- Wilson, J.L., Fitzgibbon, R., and aggregation pheromone for the western Dudley, S. 1999. Evaluation of a balsam bark beetle, Dryocoetes spruce and western balsam bark confusus Swaine (Coleoptera: beetle outbreak on Mt. Graham, Scolytidae). Canadian Entomologist Safford Ranger District, Coronado 122: 935-940 National Forest. USDA Forest Service Southwestern Region Biological Stock, A.J., Borden, J.H., and Pratt, T.L. Evaluation. 24 p. 1994. Containment and concentration

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Published by USDA Forest Service, Pacific Northwest Region (R6), Portland, Oregon FS/R6/RO/FIDL#184-17/XXX 12