Birch trees (Betula) are widespread deciduous species recognized for their distinctive bark and ecological value across the Northern Hemisphere. Species like paper birch (B. papyrifera) and yellow birch (B. alleghaniensis) serve as a significant food source for a wide variety of wildlife. The trees’ nutritional appeal, from energy-rich sap to woody tissues, makes them a regular target for animals and insects seeking sustenance throughout the year.
Mammalian Herbivores Targeting Bark and Twigs
Medium to large mammals frequently target the woody structure of birch, especially in winter when softer foliage is unavailable. Deer, moose, and elk browse on the tender twigs and foliage of saplings. This feeding involves tearing or pulling, leaving a ragged, splintered end on clipped branches. Heavy browsing can severely stunt or eliminate regeneration in young stands.
Larger ungulates like deer and elk also inflict damage through antler rubbing in the autumn to shed velvet and mark territory. This action scrapes away the thin bark of birch trunks, often targeting trees one to four inches in diameter, causing deep wounds that reach the vascular tissue. If the bark is stripped completely around the circumference (girdling), the tree’s ability to transport nutrients is cut off, potentially leading to death.
Smaller mammals consume bark closer to the ground. Rabbits and hares gnaw on the bark and clip saplings, leaving a clean, angled cut. Voles, active under snow cover, often chew the bark at the base of small trees. Beavers consume the cambium layer, particularly in fall and winter, and use the wood for dams and lodges. Porcupines climb trees to feed on the cambium, leaving long, chiseled tooth marks where they strip away bark.
Insect Pests and Larval Defoliation
Insects pose a threat to birch trees, causing damage both inside the wood and on the leaves. The Bronze Birch Borer (Agrilus anxius) is a significant pest. Its destructive stage is the larva, a creamy-white grub that tunnels extensively beneath the bark in the cambium layer. This larval feeding creates winding galleries that effectively girdle branches and the trunk, disrupting the flow of water and nutrients and causing dieback, typically starting in the upper crown. Adult borers emerge by chewing a distinctive, D-shaped hole in the bark, but they cause minimal damage by feeding only on leaf edges.
Defoliating insects, primarily the larvae of sawflies known as birch leafminers, feed on the leaf tissue, which weakens the tree. The adult sawfly deposits eggs into young birch leaves in the spring. Once hatched, the larvae consume the cells between the upper and lower leaf surfaces, resulting in brown, inflated blotches called mines.
Repeated, heavy defoliation reduces the tree’s ability to photosynthesize, stressing the tree and making it vulnerable to other threats, including the Bronze Birch Borer. While leafminer damage rarely causes mortality alone, the cumulative stress can precede fatal borer infestation. The leafminer has multiple generations per year, with larvae dropping to the ground to pupate before emerging as new adults.
Birds and Small Mammals Seeking Sap and Buds
Some species target the nutrient-rich liquids and dormant tissues of the birch tree. Sapsuckers, a type of woodpecker, drill small, uniform holes (sapwells) in rows to access the sugary sap and the insects it attracts. Birch is a preferred host due to its thin bark and high sap sugar content.
Repeated drilling can cause stress; heavy sapsucker feeding has been linked to high mortality rates in species like gray birch. The flowing sap is also a food source for red squirrels, which lap the liquid directly from the wounds. Squirrels also gnaw the bark of saplings to initiate sap flow.
Small mammals, including squirrels, feed on developing flower buds in late winter and early spring before leaf-out, providing succulent food when other vegetation is scarce. Birds like the American Goldfinch feed on the small nutlets within the birch catkins during the spring.
Protecting Birch Trees from Consumption
Protecting birch trees from herbivore and insect damage involves physical barriers and maintenance practices. To mitigate browsing damage, physical exclusion is the most reliable method, using wire cylinders or fencing to prevent deer and rabbits from accessing the trunk and lower branches. Tree guards or plastic wraps can be placed around young trunks to deter gnawing by rabbits and voles. Chemical repellents can also deter deer from browsing or rubbing on young trees.
For insect pests, maintaining the tree’s overall health is the primary defense. Birch trees require consistent moisture due to their shallow root systems, so regular watering and applying mulch are important cultural controls. Avoid pruning during late spring and early summer when adult Bronze Birch Borers are active, as fresh wounds attract egg-laying females.
In cases of existing borer or leafminer infestations, systemic insecticides can be applied as a soil drench or trunk injection. This treatment is most effective when applied preventatively or at the earliest signs of damage, before the vascular system is compromised. To deter sapsuckers, wrapping the affected trunk area with a physical barrier like burlap can prevent new sapwells. Planting resistant varieties, such as River Birch (B. nigra), is a long-term strategy.