Healthy Bark, Healthy Tree Part 1: Wildlife Damage

Part 1: Wildlife Damage

Robinia pseudoacacia tree in October. Photo © 2019 Mary Free

Robinia pseudoacacia (black locust) tree in October.
Photo © 2019 Mary Free

By Mary Free, Certified Extension Master Gardener

Winter and early spring are good times to look at your trees and shrubs with a critical eye to assess their health. The density of the canopy and the appearance of the leaves should be evaluated during the growing seasons to detect signs of disease, and during hot and dry periods for signs of heat stress or dehydration. Surface roots and soil drainage can be checked in any season. However, winter allows an unobstructed view of a deciduous tree trunk, which is central to tree health.

Trees have numerous ways to protect themselves. They can emit chemicals that repel invaders or attract predators to the invaders. Birch trees use the compound betulin, which gives the tree its white color, to reflect sun rays and reduce winter sunscald as well as to ward off invaders. If trees are wounded, they can try to compartmentalize the disease, by isolating or “walling off” the infected area and halting the spread of the infection. But it is bark that provides the first line of protection for a tree.

The inner layers of a tree reveal its life history, and the color and width of tree rings provide a glimpse into past climate conditions. © 2019 Mary Free

The inner layers of a tree reveal its life history, and the color and width of tree rings provide a glimpse into past climate conditions. © 2019 Mary Free

Outer bark, which is comprised primarily of dead cells, protects a tree against the weather elements (rain, snow, hot and cold temperatures) and foreign bodies (insects, fungi, and bacteria). When the outer bark is damaged, the tree becomes vulnerable to pests and diseases. It exposes the inner bark (phloem) to injury. Phloem carries sap (water mixed with the sugars made through photosynthesis by the leaves) throughout the tree to feed it. Damaged phloem in turn endangers the next layer, which is growing tissue called vascular cambium.

Once deciduous trees lose their leaves it is easier to see the shape of the tree, especially the angles, arrangement, and spacing of the branches, and any abnormalities or damage to the bark. Some potential bark problems to be on the lookout for are described below.


Holes

Adult locust borers, like this one at Simpson Gardens in Alexandria, Virginia, usually emerge from tunnels in a locust tree by early August (depending on the temperature) and often feed on Solidago (goldenrod) in late summer to early fall. © 2019 Mary Free

Adult locust borers, like this one at Simpson Gardens in Alexandria, Virginia, usually emerge from tunnels in a locust tree by early August (depending on the temperature) and often feed on Solidago (goldenrod) in late summer to early fall. © 2019 Mary Free

For “D”-shaped emergent holes, the type of tree often provides the best clue as to the culprit. For example, if it’s ash, beware the emerald ash borer. If it’s birch, beware the bronze birch borer. For these, the first symptoms of infection are cracks in the highest branches (ash) or wilting (birch), followed by thinning and dying of the upper crown. Larvae cause this damage under the bark and adults make holes through the bark as they emerge. For black locust trees, the locust borer larvae, not the adults, chew through the bark and continue into the heartwood, leaving “sawdust” accumulated at the bottom of the tree. Wind and ice can break branches weakened by the tunneling, creating a hazard. Stressed trees (due to drought, water-logged soil, etc.) are prone to pest attack, so make sure to maintain trees properly. Your local extension office can provide information on chemicals labelled for pest control, but unfortunately, depending on the pest and extent of infestation, the likely outcome for some trees is death.

This oak was damaged by lightning strikes, exposing the inner wood, which attracts insects that attract woodpeckers. © 2019 Mary Free

This oak was damaged by lightning strikes, exposing the inner wood, which attracts insects that attract woodpeckers. © 2019 Mary Free

Large and jagged or straight line holes probably indicate woodpecker activity. Woodpeckers are protected migratory species so you cannot harm them to protect your trees. Most woodpeckers drill into the bark in search of insects (downy and hairy woodpeckers are natural enemies of locust and other borers), while yellow-bellied sapsuckers tap the tree sap and eat the cambium. It is unlikely a woodpecker would kill a tree, but copious feeding could weaken it.


Bark Stripping

Even though this small fruit tree was surrounded by fencing to protect it from foraging white-tailed deer, it fell victim to a fawn who was able to lean under the barrier and nibble at the bark. © 2018–2019 Mary Free

Even though this small fruit tree was surrounded by fencing to protect it from foraging white-tailed deer, it fell victim to a fawn who was able to lean under the barrier and nibble at the bark. © 2018–2019 Mary Free

Besides insects, a number of mammals use trees as a source of nutrition. The timing and type of bark damage can often identify the offender. In Northern Virginia these include beavers, who peel bark with their teeth up to 6 feet in order to eat young bark, cambium and sapwood. Black bears strip bark, mostly from conifers at the 3–5-foot level, looking for insects and sap, usually in early spring, leaving behind teeth and claw marks. Deer bucks rub their antlers against small, 1–3-inch-diameter shrubs and trees at about a 4-foot height, early fall to late winter. Deer also browse tree branches to a height of 6 feet, doing more serious damage during harsh winters or times of overpopulation. Rabbits gnaw on bark up to their standing height when food is scarce, especially during winter snow cover. Primarily in spring and fall, squirrels gnaw on branches and trunks to get to the inner bark, possibly to relieve calcium deficiencies—the Calcium Hypothesis (Nichols et al., 2016). Voles and mice chew off bark at the ground level or below the snow line during winter. It is often difficult to prevent wildlife damage, but the references below offer some suggestions. Make sure you learn the laws and regulations regarding vertebrate pest control in your state and locality before taking any action.

The bark on the branch of this declining Amelanchier arborea in the Sunny Garden in Bon Air Park in Arlington, Virginia may have been gnawed by a squirrel. The small circular marks visible at the lower part of the tree might be where the squirrel “taste-tested.” © 2019 Mary Free

The bark on the branch of this declining Amelanchier arborea in the Sunny Garden in Bon Air Park in Arlington, Virginia may have been gnawed by a squirrel. The small circular marks visible at the lower part of the tree might be where the squirrel “taste-tested.” © 2019 Mary Free


Girdling

A tree is likely to survive bark damage that is less than 25 percent of the trunk’s circumference. But the more bark stripped around the tree, the more it is at risk of death. If a tree is girdled—a portion of bark has been stripped around the entire circumference of the trunk—then the tree is likely to die because the phloem has been destroyed and there is no way to feed the tree. In this case, if a tree’s diameter is less than 2 inches, it may be cut just below the girdled area above a bud. Otherwise it may be bridge-grafted. Neither remedy guarantees survival.

This tree will not survive the damage obviously inflicted by beavers. Photo by Peggychoucair, Pixabay

This tree will not survive the damage obviously inflicted by beavers. Photo by Peggychoucair, Pixabay


Egg Masses

Malacosoma americanum (Eastern tent caterpillar) are common native pests. They overwinter as egg masses on twigs (fall and winter are the best times to kill them by pruning off the twig or rubbing them off by hand). Eggs hatch as the trees leaf out in early spring. Larvae favor wild cherry and ornamental crabapple as hosts, but will attack trees like maple and oak as well. They construct silvery-white, conical tents in branch crotches or forks. The larvae emerge on warm or sunny days to feed on leaves, but usually retreat back to the tents on rainy days and at night.

Caterpillars are visible within the silky tent woven around a branch crotch on a tree bordering the Quarry Shade Garden in Bon Air Park. © 2019 Mary Free

Caterpillars are visible within the silky tent woven around a branch crotch on a tree bordering the Quarry Shade Garden in Bon Air Park. © 2019 Mary Free

Severe defoliation may weaken less vigorous and younger trees. Birds and predatory insects normally act as natural controls, but if you see a few colonies early in the spring, you can remove tents by hand. Make sure to destroy all small caterpillars. Do not attempt to burn nests as this could seriously injure the tree. Maturing in about 6 weeks, caterpillars sport a single white stripe with a row of blue dots on either side. They crawl to places that offer protection, like nearby buildings, to spin cocoons from which they emerge in about 3 weeks as moths. Each female then lays a mass of eggs like a “collar” around a twig and the cycle begins anew.

Tent caterpillars are sometimes mistaken for Lymantria dispar (gypsy moth). The eggs of this non-native pest are laid in buff-colored masses in July or August and hatch the following spring, late April through early May. Although oak is the preferred host, larvae also favor apple, birch, poplar, and willow, and during heavy infestations will defoliate other deciduous trees and even some conifers.

Buff-colored egg masses were laid on this maple tree in southeastern Connecticut in the summer of 2016. The spring of 2017 saw a large outbreak of gypsy moths in this area. Parts of desiccated caterpillars lay near the masses, which have since turned white. © 2019 Mary Free

Buff-colored egg masses were laid on this maple tree in southeastern Connecticut in the summer of 2016. The spring of 2017 saw a large outbreak of gypsy moths in this area. Parts of desiccated caterpillars lie near the masses, which have since turned white. © 2019 Mary Free

The most recent outbreak in the mid-Atlantic Region began in 2014. (Tatman 2016) In 2016, southwest Virginia saw the most destruction from gypsy moths since 2008 (Ward 2016) and the Maryland Department of Agriculture did some aerial spraying to combat this pest on the Eastern Shore. The most severe gypsy moth outbreak in Connecticut since the early 1980s occurred in 2017, due to nearly 3 years of drought, which prevented the growth of the fungus Entomophaga maimaiga, the most important agent in suppressing gypsy moth activity. (Stafford 2018, 9) With record or near record rains in 2018, the mid-Atlantic and Northeast may enjoy a break from this destructive pest. If masses are apparent on trees, there are some physical options for removal and control.  Consult your local extension office before using chemical controls.


Fungi

Fungi serve essential functions in the ecosystem, as symbionts (for example, mycorrhizae grow in symbiotic, protective relationships with plant roots) and as decomposers (like the wood-decaying bracket and conk fungi found on both dead and living trees). Fungi paired with algae to form lichens are self-sustaining and do not harm the trees to which they attach themselves. (Refer to Are You Lichen The Bark? Part 3) Some fungi, such as Aleurodiscus oakesii, are harmless to a tree. It affects the outer (non-living) white oak bark, which scales off in thin plates, revealing smooth patches below.

Lichens do not obtain their nutrients from trees and are not the cause of the decline of this Amelanchier arborea in the Sunny Garden. In this case, they are opportunists and symptomatic of the tree’s unhealthy state. © 2019 Mary Free

Lichens do not obtain their nutrients from trees and are not the cause of the decline of this Amelanchier arborea in the Sunny Garden. In this case, they are opportunists and symptomatic of the tree’s unhealthy state. © 2019 Mary Free

However, other fungi can be deadly to a tree, and to find them growing on a tree trunk or by the tree’s base is an ominous sign. For example, Daedalea quercina (maze bracket) is a gray to light-brown leathery shelf mushroom with a maze-like lower surface that often grows on oak trees. It enters the tree through wounds in the bark, where bare wood is exposed, often due to improper pruning. It causes brown heart rot, feeding on and decaying the heartwood, or center of the branches or trunk. The rot might be stopped by pruning the affected branch, but if the mushrooms appear on the trunk itself, the softened heartwood will structurally weaken the tree, with a likely fatal result.

During limb removal, the pruning cut was made too close to the trunk of this (circa) 70-year-old Quercus hemisphaerica (Darlington oak), damaging the branch collar and preventing the wound from sealing. That left it vulnerable to what appear to be maze bracket mushrooms, which have spread onto the trunk. © 2019 Mary Free

During limb removal, the pruning cut was made too close to the trunk of this (circa) 70-year-old Quercus hemisphaerica (Darlington oak), damaging the branch collar and preventing the wound from sealing. That left it vulnerable to what appear to be maze bracket mushrooms, which have spread onto the trunk. © 2019 Mary Free


Summary

A vigorous native tree may be host to many insects, which provide food for birds and omnivorous mammals. A stressed tree is most susceptible to pests and disease. However, even a dying or dead tree may have a place in a wildlife habitat as a snag unless it poses a hazard or has an infestation that could spread, in which case it should be removed as soon as possible. Infested trees should be destroyed on site so that the infestation is not carried to other areas. In cases of non-infestation and when appropriate, a felled trunk can be left to decay in place, providing nesting sites, etc. for wildlife.

If you have concerns about a tree, the online resources listed below may be helpful. You can also contact your local extension office. In Arlington and Alexandria, Virginia, you can contact the Extension Master Gardener Help Desk at (703) 228-6414 or mgarlalex@gmail.com


Sources

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