Douglas-fir beetle
- French common name: Dendroctone du Douglas
- Other common names: Douglas fir beetle
- Scientific name: Dendroctonus pseudotsugae (Hopkins)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Coleoptera
- Family: Curculionidae
General information and importance
Douglas-fir beetle is a native bark beetle of western North America. It occurs throughout the natural range of its primary host, Douglas-fir (Pseudotsuga menziesii), from western Alberta and central British Columbia in Canada, south through the Pacific Northwest to central California of the United States, and south along the Rocky Mountains to the montane forests of northern Mexico.
As with most related bark beetles in the genus Dendroctonus, Douglas-fir beetle colonizes older, weakened, and recently windthrown trees. Adult beetles are unable to overcome the defences of vigorously growing trees to establish their broods. Mature forests normally have sufficient downed and moribund trees to support endemic bark beetle populations. Douglas-fir often grows in mixed-species forests with variable age classes. Susceptible host trees are dispersed in the landscape. The distribution of trees killed by Douglas-fir beetles is patchier than with other bark beetles attacking conifers such as spruce and pine. Spruce and pine both grow in denser, more even-aged stands. However, when there is an abundance of suitable host material due to disturbances (windstorms, logging, or biotic stress from defoliating insects or root rot), Douglas-fir beetle populations can increase to the point where mass attacks by adults overcome tree defences. This leads to outbreaks that kill both stressed and healthy trees.
Distribution and hosts
This beetle is found throughout the range of Douglas-fir in western North America. It ranges from the foothill forests of western Alberta through British Columbia to the Pacific coast in the north, south along the coast to California, east through the interior of the Pacific Northwest states, and south along the Rocky Mountains, from Montana to west Texas in the United States. The fragmented montane forests of northern Mexico appear to have distinct subspecies of both Douglas-fir and Douglas-fir beetle.
Douglas-fir is the principal host. Douglas-fir beetle can be found in all geographically distinct varieties of the host tree; coastal, Interior or Rocky Mountain, and Mexican. It also utilizes big-cone Douglas-fir (P. macrocarpa) in southern California. Western larch (Larix occidentalis) and western hemlock (Tsuga heterophylla) are sometimes attacked. However, they are not preferred hosts.
Tree parts affected
Symptoms and signs
Populations of Douglas-fir beetle normally persist in an endemic state, unable to overcome the defences of healthy trees. They are restricted to larger, windthrown, damaged, and moribund trees, with relatively slow growth rates. Trees scorched by fire are also more susceptible to attack. There is evidence that insect defoliation (e.g., Douglas-fir tussock moth [Orgyia pseudotsugata]) predisposes trees to attack by Douglas-fir beetle. A strong association has been noted between successful colonization by beetles and the presence of root rots, such as Armillaria ostoyae and Phellinus weirii. Both of these conditions also reduce tree vigour and increase windfall, contributing more material for beetle brood. In situations where there is an abundance of susceptible material, beetle populations will increase to outbreak levels. However, successful colonization of live trees still requires mass attacks by many beetles.
Early detection is problematic because discoloration of foliage may not occur until the year after attack. Douglas-fir beetle adults tend to attack the least vigorous trees, so resin flow from attack sites is often not present. Adults are 4 to 7 millimetres long. They are very dark brown, with reddish wing coverings (elytra). Attacks occur first at the upper mid-bole of trees and progress both upward and downward from the original site. Earliest evidence of successful attack is reddish boring dust on the bark of the tree near entrance holes. Resin streams flowing from these entrance holes are diagnostic. Beetles introduce blue-stain fungi (Ophiostoma pseudotsugae and Leptographium abietinum), which discolour the sapwood, interrupt water transport, and assist the beetle in overcoming the defences of living trees. Female beetles lay up to about 40 elliptical, pearly white eggs that are 1 to 1.5 millimetres long. The eggs are laid in excavated galleries that are mostly straight and parallel to the grain of the wood, and at least 20 to 30 centimetres long. Larvae are legless, white grubs with brown heads. The grubs can be up to 6 millimetres in length when full grown. The larvae tunnel through the phloem away from their egg sites, producing fan-shaped groups of tunnels on either side of egg galleries. Pupal cells are formed at the end of the feeding tunnels. Pupae are white and have rudimentary adult features.
Foliage discoloration from green to pale yellow-green can occur late in the season of the original attacks. Most attacks, however, are not evident until the following spring. Needles turn red and remain on trees for 2 years before falling.
Life cycle
There is one generation of Douglas-fir beetle per year but often two broods. The insect usually overwinters as either immature, dormant adults, or mature adults in physiological diapause. Most adults emerge and fly in the spring when temperatures are greater than 18oC. This can be as early as April or as late as July, depending on location. Flights of adults that emerge later in the summer may be beetles that overwintered at an earlier developmental stage and completed their development in early summer. It could also be the adults emerging from the first set of galleries to initiate a second brood.
Egg galleries are initiated by the female adult. As the female mines the galleries, males join and pack lower areas of the gallery with frass, blocking the entrance against other beetles. Eggs are laid in groups, specifically in grooves on alternate sides of the gallery. After hatching, larvae tunnel individually through the phloem, perpendicular to the egg gallery. Unless populations are very dense, feeding tunnels rarely intersect. Pupation occurs in cells at the end of the tunnels.
Earliest attacks of the season are slow at first but become progressive. The first colonizing females release an aggregation pheromone (frontalin). In combination with volatile tree resins emanating from the attack site, this pheromone attracts males and other female beetles. Adult beetles also carry blue-stain fungi on their bodies, which infect the tree and hamper defensive responses, facilitating further beetle attacks. As the mass attack progresses, aggregative volatiles are masked by sonic and chemical communication among pairs of beetles that repel further attacks.
Damage
Douglas-fir beetle selects older, moribund trees and windthrow for their brood. They are considered primary decomposers, hastening the replacement of damaged and senescent trees. The dispersed nature of tree mortality results in little change in local fuel loads. This means that the likelihood of fires is no different than from background levels. However, outbreaks can have a significant effect on older trees, with the net effect of shifting forest character from closed canopy to more open, parkland conditions.
Prevention and management
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
Preventative measures mostly involve sanitation. Disposing of log and slash residue and cutting stumps as low as possible during logging operations are important. Mechanical damage to residual trees in harvested areas should be minimized. Trees with damage to stems or roots should be removed. Single tree and small block harvesting have reduced problems associated with the beetle. Restricting road construction to late summer after beetle flight has finished for the season has also been effective.
Early detection by aerial surveys is unreliable and emphasizes the importance of hazard rating systems and ground surveys in susceptible forests. Forests with large, old trees where growth has slowed are most susceptible. Sites that have experienced low-level fires, soil disturbances, and drought contain trees with increased susceptibility. Forests with significant root rot or recent insect defoliation are also at a higher risk. Beetle pressure (the density and proximity of trees recently killed by beetles) is the best predictor of new attacks because of common landscape features mentioned above, and the limited dispersal capacity of adult beetles.
Trap trees (felled or girdled) can be utilized for management of beetle populations. These trees must be removed in April before beetle emergence. Effectiveness of trap trees can be enhanced with the use of pheromones, which attract more beetles to the trap site. Trap logs must be peeled and slash burned to kill broods. Limited accessibility to sites early on in the season limits this management approach.
Products that mimic the chemical communications (pheromones) used by the beetles to either attract (aggregation) or repel (antiaggregation) other beetles have been used to manage populations. Results suggest that great care must be taken with the location of traps because of the possibility of increasing, rather than reducing, attack rates. Pheromones are defined as pest control products and are regulated in Canada. Products registered for use against Douglas-fir beetle under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered products and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Selected references
Duncan, B. 1987. An illustrated guide to the identification and distribution of the species of Dendroctonus Erichson (Coleoptera: Scolytidae) in British Columbia. Journal of the Entomological Society of British Columbia 84: 101–112.
Furniss, M.M. 2014. The Douglas-fir beetle in western forests: a historical perspective — Part 1. American Entomologist 60(2): 84–96. https://doi.org/10.1093/ae/60.2.84
Furniss, M.M. 2014. The Douglas-fir beetle in western forests: a historical perspective — Part 2. American Entomologist 60(3): 166–181. https://doi.org/10.1093/ae/60.3.166
Furniss, M.M.; Kegley, S.J. 2014. Douglas-fir beetle (revised). United States Department of Agriculture, Forest Service. Pacific Northwest Region (R6). Portland, Oregon. Forest Insect and Disease Leaflet 5. 7 p.
Gugger, P.F.; González-Rodríguez, A.; Rodríguez-Correa, H.; Sugita, S.; Cavender-Bares, J. 2011, Southward Pleistocene migration of Douglas-fir into Mexico: phylogeography, ecological niche modeling, and conservation of ‘rear edge’ populations. New Phytologist 189(4): 1185–1199. https://doi.org/10.1111/j.1469-8137.2010.03559.x
Harvey, B.J.; Donato, D.C.; Romme, W.H.; Turner, M.G. 2013. Influence of recent bark beetle outbreaks on fire severity and postfire tree regeneration in montane Douglas-fir forests. Ecology 94(11): 2475–2486. https://doi.org/10.1890/13-0188.1
Hood, S.; Bentz, B. 2007. Predicting postfire Douglas-fir beetle attacks and tree mortality in the northern Rocky Mountains. Canadian Journal of Forest Research 37(6): 1058–1069. https://doi.org/10.1139/X06-313
Humphreys, N. 1995. Douglas-fir beetle in British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 14. 4 p.
Ross, D.W.; Solheim, H. 1997. Pathogenicity to Douglas-fir of Ophiostoma pseudotsugae and Leptographium abietinum, fungi associated with the Douglas-fir beetle. Canadian Journal of Forest Research 27(1): 39–43. https://doi.org/10.1139/x96-167
Ruiz, E.A.; Víctor, J.; Hayes, J.L.; Zújñiga, G. 2009. Molecular and morphological analysis of Dendroctonus pseudotsugae (Coleoptera: Curculionidae: Scolytinae): an assessment of the taxonomic status of subspecies. Annals of the Entomological Society of America 102(6): 982–997. https://doi.org/10.1603/008.102.0608
Schmitz, R.F.; Gibson, K.E. 1996. Douglas-fir beetle. United States Department of Agriculture, Forest Service. Forest Insect and Disease Leaflet 5. 7 p.
Shore, T.L.; Safranyik, L.; Riel, W.G.; Ferguson, M.; Castonguay, J. 1999. Evaluation of factors affecting tree and stand susceptibility to the Douglas-fir beetle (Coleoptera: Scolytidae). The Canadian Entomologist 131(6): 831–839. https://doi.org/10.4039/Ent131831-6
Simard, M.; Powell, E.N.; Raffa, K.F.; Turner, M.G. 2012. What explains landscape patterns of tree mortality caused by bark beetle outbreaks in Greater Yellowstone. Global Ecology and Biogeography 21(5): 556–567. https://doi.org/10.1111/j.1466-8238.2011.00710.x
Sturdevant, N.; Haavik, L.; Negrón, J.F. 2022. Douglas-fir tree mortality caused by the Douglas-fir beetle in thinned and unthinned stands in Montana, USA. Forest Science 68(2): 145–151. https://doi.org/10.1093/forsci/fxac006