Spruce beetle
- French common name: Dendroctone de l'épinette
- Other common names: Spruce bark beetle, Engelmann spruce beetle
- Scientific name: Dendroctonus rufipennis (Kirby)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Coleoptera
- Family: Curculionidae
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Partial list of synonyms:
- Dendroctonus obesus (Mannerheim)
- Dendroctonus piceaperda Hopkins
General information and importance
Spruce beetle is one of the most widespread native bark beetles in North America. Spruce beetle attacks large-diameter spruce (Picea) trees with weakened defences, including recent windthrown and senescing trees, fresh stumps, and standing trees stressed by dry or cold soils, competition, insect defoliation, or disease. Populations of beetles that build up in stressed trees may become sufficiently abundant to successfully transition to attacking healthy trees, making this insect a damaging pest of mature spruce. Significant outbreaks of spruce beetle often follow large-scale disturbances such as major windstorms, damage to root systems, drought, or accumulation of logging residue, such as high stumps and large-dimension woody debris. Several outbreaks lasting 2 to 5 years have been recorded during the past century in different parts of the insect’s range. The most devastating recent outbreaks have been in western North America. Between 1990 and 2000, extensive mortality occurred on more than 1.2 million hectares of forests in Alaska, United States, and more than 350,000 hectares in the adjacent forests of the Yukon, Canada. Significant outbreaks have been mapped on more than 300,000 hectares of forests annually in British Columbia, Canada, between 2017 and 2021.
The generic scientific name Dendroctonus means “tree destroyer.” The specific name rufipennis refers to the reddish-brown, hardened outer forewings (elytra) of the adult beetle.
Distribution and hosts
Spruce beetle occurs across the transcontinental range of spruce species in Canada and the United States. This range includes Alaska and the west coast of North America, as far south as California. It spans throughout the interior western montane forests, along the Rocky Mountains eastward through the northern spruce forests to the Atlantic coast, and south along the Appalachian Mountains in the eastern United States.
All native species of spruce and their hybrids, as well as some introduced spruce species, are potential hosts. The most common hosts are white spruce (Picea glauca) in boreal forests, white and red spruce (P. rubens) in the east, Engelmann spruce (P. engelmannii) in the interior west, and Sitka spruce (P. sitchensis) on the west coast. Black spruce (P. mariana), especially those growing on drier, upland sites, are also susceptible. During intense outbreaks, spruce beetle may attack non-spruce species, but beetle populations are not sustained in these hosts.
Tree parts affected
Stems: Adult spruce beetles bore through the outer bark into the cambium and phloem to construct galleries and lay eggs. The adults also introduce blue-stain fungi, which spread subsequently throughout the tree’s vascular system, compromising its function. Feeding larvae mine galleries under the bark. If spruce beetle brood populations become high enough to girdle the tree, starved needles in the crown first turn yellow, then red, and ultimately fall off. This process begins at the top of the tree.
Symptoms and signs
Initial attacks by spruce beetle are concentrated in shaded locations on windfall and stumps. Pitch tubes may be seen around attack sites on living trees. It is more common, however, to have light-brown boring dust resembling coarse sawdust accumulating in bark crevices on the lower bole of the tree near the root collar, and even on the ground at the base of the tree. The sapwood in successfully colonized trees becomes blue-stained by fungi introduced by the adult beetles. Egg galleries excavated by adult beetles follow the grain of the wood and are 15 centimetres long, on average. About 80 pearly white, 1-millimetre, oblong eggs are laid into grooves cut by the female in alternate sides of these galleries. Larvae are legless and have cylindrical, wrinkled white bodies and a pale tan head. Fully-grown larvae are 6 to 7 millimetres in length. Their feeding tunnels radiate away from the egg galleries across the wood grain. Oval pupal chambers are formed at the end of the feeding tunnels. Pupae resemble adults but are creamy white, with visible wings, legs, and antennae. Emerging adult beetles leave round exit holes in the outer bark.
Woodpeckers have often been observed flaking off the outer bark scales in search of overwintering spruce beetle, leaving a pile of bark flakes on the snow around infested trees.
Conspicuous change of foliage colour may not appear in the year of attack. Foliage usually fades to yellowish green in early summer of the year following attack and needles begin to fall throughout the rest of the second growing season. Once all needles have dropped, the bare twigs give a reddish appearance to the crown. When the fine twigs fall off, trees appear grey.
Life cycle
Spruce beetle requires 2 years to complete its life cycle across most of its range, overwintering the 1st year in an immature stage. It completes its development the next summer and overwinters the 2nd year as an adult. A 1-year life cycle is possible in warmer years or regions if beetles reach the adult stage in the 1st year and overwinter as an adult. Some adult beetles may survive a second winter following a season of reproduction. If these are adults of a 2-year cycle, this means some beetles have a 3-year life span.
Adult beetles emerge in the spring and disperse to new host trees. In British Columbia, emergence and colonization of new trees typically occurs in late May and June, when temperatures in forests exceed 16°C. In warmer seasons, parent beetles may exit the tree after establishing one brood. They may reattack the same tree, or another, and establish a second brood.
Female adult spruce beetles initiate attack, boring through the outer bark of stressed or downed trees. They are joined by a male under the bark where they mate. Females lay eggs on alternating sides of the gallery. The eggs hatch and the legless, white larvae mine feeding galleries away from the egg gallery at right angles in the phloem. The larval galleries coalesce and larvae feed gregariously until they are about two-thirds grown. They feed individually for the remainder of their development. Pupation occurs in chambers enlarged at the ends of the feeding tunnels. The adults may overwinter under the bark on the bole of the tree where they developed. Alternatively, a proportion may emerge, drop to the base of the tree, bore under the bark again, and overwinter in groups. The groups are presumably protected from extreme temperatures by the snowpack.
Survival and population trends of spruce beetle are affected by many ecological factors, including the availability of fallen or weakened trees for brood, rate of deterioration of the brood resource, and weather and climatic conditions. The relationship with weather is complex, but warm autumns and mild winters favour survival. Drought-stressed trees favour successful attacks, but such environmental conditions also accelerate deterioration in the suitability of the food source required by developing broods. Nonetheless, recent trends in weather patterns in North America are favouring survival of spruce beetle.
Damage
Under normal conditions, spruce beetle preferentially establishes broods in weakened, senescing trees. This hastens the decomposition of large, dead material in forest stands and recycles material to a state more easily utilized by other decay organisms. However, attacks by many beetles under outbreak conditions may overcome the defences of healthy trees. In these situations, the largest, slowest-growing trees are the most likely to be killed. The rate of tree mortality can be very high, significantly reducing the average size of trees and potentially changing the species composition of the forest. The dramatic reduction in forest canopy decreases transpiration rates and increases water runoff. In forests where tree mortality is less severe, retention of advanced regeneration and increased growth rates in surviving trees provide some resiliency for forest recovery.
Prevention and management
The prevention of spruce beetle outbreaks emphasizes forestry practices designed to reduce stand and tree susceptibility by removal of dead and senescing trees. These practices prevent the accumulation of fresh windfall and logging residue and help monitor and respond to indicators of population increase. Annual surveys using pheromone and flight traps, combined with site susceptibility indicators, guide rapid responses to incipient outbreaks, such as sanitation logging and the deployment of trap trees. Longer term goals involve strategic planning to harvest susceptible stands in areas of high beetle pressure, reduce the time between harvests (i.e., rotation time) of spruce forests, and develop forests that have mixed species and/or age classes (where possible).
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.
Pheromones are defined as pest control products and are regulated in Canada. Products registered for use against spruce 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. Pest control products 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.
Photos
Adult spruce beetle (length: 5–6 millimetres) on resin of the pitch tube at the point of attack on white spruce.
Thérèse Arcand
Adult spruce beetle on resin at entrance hole of an egg gallery.
Thérèse Arcand
Adult spruce beetle on resin at entrance hole of an egg gallery in white spruce.
Thérèse Arcand
Brown boring dust after attack by spruce beetle (the adult is visible at the right top edge of the dust).
Thérèse Arcand
Resin on white spruce at the entrance hole of a spruce beetle egg gallery.
Thérèse Arcand
Close-up of base of an infested spruce trunk, showing flow of resin and exit holes made by adults.
Claude Monnier
Base of an infested white spruce trunk, showing resin flow at spruce beetle attack points.
André Lavallée
A large white spruce severely infested by spruce beetle. Note discoloured foliage in comparison to nearby trees.
Christian Hébert
Base of an infested white spruce trunk, showing resin flow and a few exit holes made by adults of spruce beetle.
Claude Monnier
Selected references
Bleiker, K.P.; Brooks, J.E. (editors) 2021. Spruce beetle: a synthesis of biology, ecology, and management in Canada. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 116 p.
Bleiker, K.P.; Willsey, T. 2020. Experimental evidence supporting an obligate adult diapause for spruce beetle (Coleoptera: Curculionidae) from British Columbia. Environmental Entomology 49(1): 98–103. https://doi.org/10.1093/ee/nvz152
Garbutt, R.; Hawkes, B.; Allen, E. 2006. Spruce beetle and the forests of the southwest Yukon. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Information Report BC-X-406. 68 p.
Safranyik, L.; Linton, D.A. 1999. Spruce beetle (Coleoptera: Scolytidae) survival in stumps and windfall. The Canadian Entomologist 131(1): 107–113. https://doi.org/10.4039/Ent131107-1
Werner, R.A.; Holsten, E.H.; Matsuoko, S.M.; Burnside, R.E. 2006. Spruce beetles and forest ecosystems in south-central Alaska: a review of 30 years of research. Forest Ecology and Management 227(3): 195–206. https://doi.org/10.1016/j.foreco.2006.02.050