Porcupine damage
Cause and general information
Porcupines (Erethizon dorsatum L.), found throughout Canada in forested areas, are known to cause occasional damage to conifers due to winter feeding of sapwood. Feeding wounds can be large enough to girdle the tree, and wounds also act as infection courts for fungal pathogens. Bark beetles may breed in dead or dying tops above the girdling and spread to other nearby trees when adults emerge following pupation. Porcupines also cause damage to maples (Acer) and other broadleaf trees in the spring when sap begins to rise.
Distribution and species affected
In Canada and the United States, there is only one species of porcupine. This large, quilled rodent lives in forested areas of every province and territory, except for the islands on either coast (it is present but rare on Cape Breton Island). Its distribution also extends south throughout the western United States. During winter months porcupines prefer to climb into the forest canopy to feed on the inner bark of conifers, especially pine (Pinus), hemlock (Tsuga), and spruce (Picea), but other species as well. Feeding damage to broadleaf trees and shrubs occurs in the spring when sap begins to run in the trees and new buds and foliage are available. During the spring, porcupines prefer the catkins and sweet inner bark (phloem) of maple (Acer). They will also eat terminal twigs, buds, and foliage of alder (Alnus), poplar (Populus), willow (Salix), and other species. They feed primarily on herbaceous plants in the summer. In the fall, they will eat nuts from oak (Quercus) and beech (Fagus), and orchard fruit.
Tree parts affected
Physical damage due to winter and spring feeding on bark, twigs, foliage and fruit; seedlings may also be eaten.
Symptoms and signs
Conifer crown symptoms include dead or bushy deformed tops. In winter, trunks and main branches of conifers damaged by porcupines will have bark removed in large patches high in the canopy. Porcupines may also initially taste a tree, making only a small wound, and move to another tree if not satisfactory to them. The exposed sapwood is deeply gnawed and grooved by tooth marks about 5 millimetres wide, made in vertical and horizontal directions on the trunk. The bark at the wound edges is not shredded but gnawed away. Branches are also occasionally gnawed or cut. If there is snow on the ground under trees where porcupines are feeding, the dropped chunks of bark and branches are easily detected. Signs of nearby porcupine tracks, only visible in snow, are another clue. The tracks are trough-like due to the dragging tail and marked with "brush strokes" due to the quills. Porcupine droppings are 2.5 centimetres long, curved, with rounded ends. Distinctive porcupine guard hairs (white base, dark central section, buff-coloured tip) may also be found stuck in pitch in conifer wounds.
Porcupines prefer stands that are open (a lower density of stems) and are less likely to cause winter damage to conifers in mature stands or plantations with a closed canopy.
Squirrels do not gnaw the sapwood as deeply, and their feeding patches are smaller and narrower, with narrower tooth marks (although this may be difficult to discern high up in the canopy). Squirrel signs below trees include small strips of bark and clipped shoots.
Damage
The greatest damage to conifers is caused by girdling or partial girdling of trees. Girdling is caused by circumferential bark removal, which kills the branches or the entire tree above the damage by preventing the upward movement of water and food. Conifers may die if the section of the trunk below the branches is girdled. In a study site near Prince Rupert, British Columbia, more than half the western hemlock (Tsuga heterophylla) trees with diameters at breast height (1.3 metres above the ground) greater than 27 centimetres were found to be girdled by porcupines. They preferred these larger-diameter hemlock over smaller hemlock, grand fir (Abies grandis), and western redcedar (Thuja plicata). They are also known to prefer stands with thin canopies because of the increased amount of understory vegetation available in them. Bark damage to both conifers and broadleaf trees also provides a site for forest pathogens to become established, resulting in wood decay or cankers. Trunk and upper branch girdling can lead to spike tops or deformed crowns. Porcupines have also been observed feeding on conifer seedlings. Bark beetles (species of Ips) may breed in dead or dying tops of conifers above the girdling and spread to other nearby trees as the population builds.
Porcupines are also an occasional nuisance to homeowners when they chew on bark of ornamental trees or raid fruit-bearing trees.
Prevention and management
It is illegal to poison porcupines, but rodent deterrents (regulated in the same manner as pesticides) may also work on porcupines, although this is not well-documented. In rural areas, electric fences provide deterrence. High-value trees can be protected by completely wrapping the bottom 70 centimetres of the trunk with a smooth band of a sheet metal, such as aluminum flashing.
Pesticides registered for use against porcupines under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Databasefor currently registered pesticides and product information for use against this pest. 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 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
Burleigh, J.T.; Ebata, T.; White, K.J; Rusch, D.; Kope, H., editors. 2014. Field guide to forest damage in British Columbia (3rd edition). Crown Publications, Province of British Columbia. Joint publication No.17. 368 p.
Curtis, J.D. 1941. The silvicultural significance of the porcupine. Journal of Forestry 39(7): 583–594. https://doi.org/10.1093/jof/39.7.583
Curtis, J.D.; Kozicky, E.L. 1944. Observations on the eastern porcupine. Journal of Mammalogy 25(2): 137–146. https://doi.org/10.2307/1375011
Harestad, A.S.; Bunnell, F.L.; Sullivan, T.P.; Andrusiak, L. 1986. Key to injury of conifer trees by wildlife in British Columbia. Research Branch, British Columbia Ministry of Forests Publication. Victoria, British Columbia. Wildlife Habitat Research Publication WHR-23. 38 p.
Hiratsuka, Y.; Langor, D.W.; Crane, P.E. 2004. A field guide to forest insects and diseases of the prairie provinces (2nd edition). Natural Resources Canada, Canadian Forest Service, Northwest Region, Northern Forestry Centre. Edmonton, Alberta. Special Report 3. 297 p. https://www.ubcpress.ca/field-guide-to-forest-insects-and-diseases-of-the-prairie-provinces
Internet Center for Wildlife Damage Management. Identifying wildlife damage to trees, shrubs, & bushes: Where is the damage occurring? Available at: https://icwdm.org/identification/inspection/outdoors/trees/ [Accessed March 2024]
Reeks, W.A. 1942. Notes on the Canada porcupine in the maritime provinces. The Forestry Chronicle 18(4): 182–187. https://doi.org/10.5558/tfc18182-4
Sullivan, T.P.; Jackson, W.T.; Pojar, J.; Banner, A. 1986. Impact of feeding damage by the porcupine on western hemlock—Sitka spruce forests of north-coastal British Columbia. Canadian Journal of Forest Research 16(3): 642–647. https://doi.org/10.1139/x86-110
Woods, A.J.; Zeglen, S. 2003. Impact of feeding damage by the porcupine on western hemlock-Sitka spruce forests of north-coastal British Columbia: 15-year results. Canadian Journal of Forest Research 33(10): 1983–1989. https://doi.org/10.1139/x03-122