Squirrel damage
Cause and general information
Squirrels eat mostly seeds and fruit but will also strip bark of conifers in winter when other food sources are scarce. There are several species in Canada: red squirrel (Tamiasciurus hudsonicus [Erxleben]), Douglas squirrel (T. douglasii [Bachman]), eastern grey squirrel (Sciurus carolinensis [Gmelin]), and eastern fox squirrel (S. niger [L.]). Squirrels in the genus Tamiasciurus are collectively called pine squirrels. Pine squirrels in particular cause damage to pine trees by clipping entire branches in the process of harvesting cones.
Distribution and species affected
Red squirrels inhabit forested areas of Canada south of the tree line in all provinces and territories, except for south coastal British Columbia. Red squirrels in western forests primarily feed on conifer seeds including those from true fir (Abies), spruce (Picea), pine (Pinus), Douglas-fir (Pseudotsuga menziesii), and hemlock (Tsuga). They will also feed on bark, shoots, and buds. In eastern Canada, depending on what types of forest they inhabit, red squirrels feed primarily on seeds of fir, spruce, pine, and hemlock, but also feed on the fruit and nuts of broadleaf trees such as oak (Quercus), walnut (Juglans), beech (Fagus), hickory (Carya), maple (Acer), and others. In the Yukon territory, red squirrels will include larvae of spruce beetle (Dendroctonus rufipennis) in their diet during beetle outbreaks. A single squirrel can eat more than 600 larvae daily. Douglas squirrels, a closely related species, are found only in the coastal southwestern corner of British Columbia, where they are dependent on conifer seeds from old growth and mature forests.
Eastern grey squirrels inhabit temperate hardwood and mixedwood forests in the United States, southwest Manitoba, northwestern and southern Ontario, southern Quebec, and New Brunswick. They can occasionally be found in urban areas as well. They have been introduced to several western urban areas, including Vancouver and Victoria, British Columbia, and Calgary, Alberta. Although they are primarily nut eaters—especially those produced by hickory, beech, butternut, walnut, and oak—they also feed on pine seeds. They also eat buds of maple and other trees in the spring.
Eastern fox squirrels inhabit edges of broadleaf and mixedwood forests and groves primarily in the eastern United States, but their northern range extends into southern Manitoba and southern Saskatchewan. They are also present on Pelee Island in Lake Erie of southern Ontario and in the Okanagan region of British Columbia, where they are considered an invasive species. They feed primarily on nuts of beech, oak, walnut, hawthorn fruit (Crataegus), and seeds from many other plants. They will also feed on tree shoots and buds in the spring.
Tree parts affected
Trunks, branches, bark, and seedlings of conifers and broadleaf trees. Cones and seeds of conifers and fruit and nuts of broadleaf trees.
Symptoms and signs
Signs of feeding on trees by all species of Canadian squirrels are very similar. End shoots and lateral branches of conifers (and sometimes oaks) are cut, and their buds clipped. Squirrels usually chew on trunks of trees that are greater than 60 centimetres diameter at chest height. Irregular patches of bark, often vertical or diagonal strips that are about 1 to 3 centimetres wide are gnawed from branches and trunks of trees. Finding bark strips under a conifer tree targeted by pine squirrels is a potential sign of crown damage due to girdling. Complete girdling of the tree trunk can result in dead tops or dead trees. The exposed sapwood usually lacks distinct tooth marks, but if found they are about 2.5 millimetres wide.
The cut branches and bark strips often litter the forest floor under the attacked tree, and they are especially evident in winter months when snow is on the ground. Squirrel signs of fallen bark differ from porcupine signs by the presence of thin bark strips (1–3 × 8 centimetres) rather than bark chunks.
Other than feeding damage on trees and branches, signs of squirrel activity include middens, which are piles of chewed conifer cones strewn around a perch (most often beneath a tree, stump, or rock) where they feed; dreys, which are large nests high in trees, made of twigs and foliage; and small oval droppings less than 1 centimetre long found on stumps near middens. Middens also serve as storage repositories for cones and nuts. Squirrel middens filled with whitebark pine (Pinus albicaulis) cones are an important food source for grizzly bears (Ursus arctos horribilis).
Damage
Squirrels reduce the natural regeneration of forest stands by consuming seeds and nuts. Red squirrels can on average harvest up to two-thirds of the annual cone crop in a conifer stand, but harvest levels may reach up to 80% in years of poor cone production. They may also cause damage by harvesting fruit and nuts from horticultural crops.
During conifer cone harvesting, red squirrels often clip entire shoots, which may also bear immature cones and buds which, in the case of pine species, could have produced the cones for the next two years. Other conifers complete cone development in a single year, so only the current crop and next year’s crop could be affected. Red squirrels will also clip or simply hollow out buds on branches, including pollen buds, which also reduces reproduction of the trees. Shoot clipping and bud removal reduces the growth of very young saplings and changes their shape. Strips of bark are gnawed from the branches and main stems of saplings and mature trees, both of which may be completely girdled because the squirrels return to the same tree repeatedly, removing bark each time. Bark wounds serve as entrance courts for canker pathogens, wood decay fungi, and insects.
Prior to the devastation caused by mountain pine beetle (Dendroctonus ponderosae) in western Canada, thousands of hectares of managed pine forests in interior regions of British Columbia were susceptible to squirrel damage. Estimations of damage levels in forests ranged from 30 to 96% of trees with bark stripping, suggesting a potential for future squirrel damage in regenerating forests. Young conifer stands regenerating after forest fires are more susceptible to squirrel damage than those regrowing after harvest, due to greater shrub protection from predators in the latter.
Studies in spaced lodgepole pine (Pinus contorta) forests in central British Columbia noted that after spacing, the fast-growing trees remaining are especially susceptible to red squirrel bark stripping damage. Bark-feeding damage is most severe when stands are spaced (thinned to improve growth rates) at the ages of 20 through 25 years. Older trees have thicker bark that is harder to feed on and do not grow as quickly in response to thinning.
Prevention and management
Because most bark stripping by squirrels occurs in the early spring when stored food is depleted and new growth has not yet flushed, diversionary feeding can be effective. Sunflower seeds spread both manually and aerially in lodgepole pine stands in central British Columbia significantly reduced red squirrel damage.
Regulations for hunting and trapping squirrels vary in Canada, depending on the squirrel species and the province. For example, red squirrels are protected in British Columbia and cannot be hunted, but the eastern grey squirrel and fox squirrel are considered invasive species in the province and can be trapped and killed. Squirrel damage in yards, gardens, and orchards is often very difficult to control. In areas with high populations, new squirrels arrive quickly to replace the ones that have been removed by shooting or trapping.
In natural forests, squirrels are beneficial in several ways. Seeds and nuts harvested by squirrels, which are buried but not eaten, may germinate, thus helping to disperse these tree species. Squirrels eat many different food sources apart from seed and fruit. They will harvest and dry fungi, both mushrooms and underground truffles, and spread their spores in their droppings, as well as burying them in their caches. Many of these fungi are important symbionts of forest trees, in particular conifers, forming associations with their roots. These colonized roots are called mycorrhizae, and they are highly beneficial to the tree, increasing uptake of important nutrients from the forest soil. Squirrels are also important prey food for predators such as weasels, hawks, owls, and foxes.
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. Victoria, British Columbia. Joint publication No.17. 368 p.
Harestad, A.S.; Bunnell, F.L.; Sullivan, T.P.; Andrusiak, L. 1986. Key to injury of conifer trees by wildlife in British Columbia. British Columbia Ministry of Forests, Wildlife Habitat Research. Publication WHR-23. 38 p.
Internet Center for Wildlife Damage Management. 2024. Identifying wildlife damage to trees, shrubs, & bushes: Where is the damage occurring? Available at: https://icwdm.org/identification/inspection/outdoors/trees/ [Accessed March 2024]
Obbard, M.E. 1987. Red squirrel. Chapter 24 in M. Novak; J.A. Baker; M.E. Obbard; and B. Malloch, editors. Wild furbearer management and conservation in North America. Ontario Ministry of Natural Resources. 1150 p.
Pretzlaw, T.; Trudeau, C.; Humphries, M.M.; LaMontagne, J.M.; Boutin, S. 2006. Red squirrels (Tamiasciurus hudsonicus) feeding on spruce bark beetles (Dendroctonus rufipennis): energetic and ecological implications. Journal of Mammalogy 87(5): 909–914. https://doi.org/10.1644/05-MAMM-A-310R1.1
Sullivan, T.P.; Klenner, W. 1993. Influence of diversionary food on red squirrel populations and damage to crop trees in young lodgepole pine forest. Ecological Applications 3(4): 708–718. https://doi.org/10.2307/1942102
Sullivan, T.P.; Krebs, J.A.; Diggle, P.K. 1994. Prediction of stand susceptibility to feeding damage by red squirrels in young lodgepole pine. Canadian Journal of Forest Research 24(1): 14–20. https://doi.org/10.1139/x94-003
Sullivan, T.P.; Vyse, A. 1987. Impact of red squirrel feeding damage on spaced stands of lodgepole pine in the Cariboo Region of British Columbia. Canadian Journal of Forest Research 17(7): 666–674. https://doi.org/10.1139/x87-109