Insects that bore into trees

Insects that drink sap and eat plant parts typically attack the leaves or fleshy portion of the tree, which generally does not kill the plant. Boring insects, on the other hand, bore into the main truck or side branches, causing much more serious damage such as branch dieback, structural weakness or decline and death of the plant.

Some insects will bore into healthy trees, but in many cases, the borers are attracted to weak ones. Insects that only bore into weakened trees are known as “secondary attackers,” because they strike trees that have been weakened by another stress. When borers attack trees, they normally leave behind telltale signs such as holes in the tree, patches of resin, gummy substances exuding from the bark, sawdust or waste products, commonly known as “frass.”

Woodpeckers strip the bark of an ash tree infected with the Emerald Ash Borer

Credit: Jan Barrow, Myersville, MD

Here are some insect borers that are of local concern:

  • Emerald ash borer: First identified near Detroit in summer 2002, this insect is native to Eastern Russia, Northern China, Korea and Japan. In its native habitat, the emerald ash borer is held in check by parasitic wasps and birds so that populations rarely build to the point that they do extensive damage. In our country, where no natural controls exist, populations of this insect have exploded such that most species of native ash have come under attack. The emerald ash borer is a metallic green beetle that emerges from ash trees in May when the black locust trees are in bloom. The adult has two to three months to disperse, mate and lay eggs on ash trees before dying. The eggs hatch in about two weeks, and then the grub-like larvae burrow into the tree and feed on the inner bark and sapwood of the host ash tree. This feeding activity cuts off the transport of water and nutrients inside the tree, effectively girding the tree. Infested trees generally die in one to three growing seasons. The emerald ash borer attacks both healthy and stressed ash trees. The insect was detected in Maryland in 2003. It moved into Frederick County in 2012. Trees infested with emerald ash borer display thinning in the upper third of their canopy, then die back. Woodpeckers will often rip apart the bark of ash trees to feed on the grubs. Another telltale sign of the disease is D-shaped exit holes in the bark. There are some insecticides that are effective in protecting a tree as long as they are applied before the tree is heavily infested. It is best to hire a certified arborist to treat ash trees for this pest.

  • Bronze birch borer: The bronze birch borer is a native member of the flat-headed borer family. This insect normally attacks birch trees that are stressed due to drought or a birch that is a northern variety planted out of its native range. Birch such as paper, white, gray or European birch are highly susceptible to bronze birch borer in Maryland. The river birch is the most resistant species of birch to bronze birch borer, and this is the most common birch planted in a landscape setting. The adult birch borer emerges from a tree in June, mates, lays eggs on the bark of a suitable tree, then perishes. The newly hatched larvae bore into the tree, where they tunnel under the bark, feeding on sapwood. This feeding activity results in the death of the host birch tree in one to three years. Choosing a variety of river birch for landscape plantings will greatly reduce the chances of a bronze birch borer attack.

  • Two-lined chestnut borer: This insect is a secondary attacker of weak oak trees, especially red, black and scarlet oak. Like most flathead borers, the larvae feed on the inner bark, creating meandering galleries underneath this protective covering. Two-lined chestnut borer is one of the primary causes of oak decline (along with shoestring root rot and hypoxylon canker). Some of the factors that weaken oak trees and make them susceptible to two-lined chestnut borer and oak decline are defoliation by gypsy moth and other insects, construction damage, drought, forest fires, age and pollution.

  • Dogwood borer: The dogwood borer is a native, clear-winged moth that attacks flowering dogwoods, pecans, oaks and dwarf apple trees. The adult emerges from the host tree in late June, mates, then lays its eggs on the bark of a suitable tree, attracted to dogwoods that have wounds or cracks in the bark. After the eggs hatch, the young larvae burrow into the tree and begin feeding. Red frass is often found around the opening of the tunnel. The larvae develop through multiple instars and pupate inside the tree. The adults emerge the next spring and the cycle begins again. Flowering dogwoods growing in direct sunlight develop cracks in their bark that attract dogwood borer. If you are planting a flowering dogwood on your property, look for a location that offers partial shade and plenty of air flow to guard against dogwood borer and anthracnose of dogwood.

  • Peachtree borer: The greater peach tree borer is a clear-winged moth that can be very destructive to peach, cherry, plum and apricot trees. As such, it is a major pest to orchards. The female moth lays eggs on host trees near ground level, looking for broken or rough bark. The larvae hatch, then enter the tree where they slowly girdle the tree by feeding between the outer bark and sapwood. Often, a tree under attack will display gummosis, the oozing out of a sticky gum-like substance. To protect their trees, orchardists will deploy pheromone traps to gauge when the adults are active; then they spray the base of the tree with an insecticide to prevent the newly hatched larvae from entering.

  • Pine bark beetles: There are numerous species of beetles that attack pine trees throughout the country. Most of these insects are attracted to pine trees that are under stress due to age, previous damage, fires, densely stocked pine plantations or pollution. The adult beetle attacks the tree and feeds under the bark, usually killing the host tree. Healthy pine trees can fend off attack by exuding pitch to push the intruders back out of the tree. Unhealthy trees are unable to mount these defenses. The southern pine beetle is the most destructive pine bark beetle found in Maryland, where it can attack trees by the millions, causing extensive damage in unhealthy pine plantations. Fortunately, Frederick County does not witness southern pine bark beetle attacks very often. The Ipps beetle is most commonly found in Frederick County, attacking various pine trees under stress including Scotch, pitch and white pine.

  • Ambrosia beetle: Ambrosia beetles are insects that bore into trees, carving out galleries while depositing fungal spores on the inside of the tree. Once the spores are planted and grow, the beetle feeds on this fungus. The Asian ambrosia beetle was introduced into this country in 1974 in Charleston, South Carolina, where it has spread and become a significant pest, attacking nearly 200 species of trees, including magnolias, red maple, oaks, elms, cherries and persimmon. Sometimes long toothpick-like projections extend out of trees under attack by the ambrosia beetle. These projections consist of the wood that the insect excretes while making its tunnels.

  • Asian long-horned beetle: The Asian long-horned beetle is a native of China and Korea that first appeared in Brooklyn in 1996. This is a serious insect pest whose damage normally results in the death of the tree. The Asian long-horned beetle favors maples, birch, elm, ash, golden rain tree, katsura, willows and sycamore. The adult beetle is fairly large and cannot fly very far. The female beetle lays a single egg on the tree after excavating some bark. A female can plant up to 90 eggs per season. Once the eggs hatch, the larvae bore deep into the sapwood of the tree, feeding on the wood and leaving sawdust and frass that can collect underneath the tree. The larvae pupate inside the tree, and the large adults emerge, creating a fairly large ⅜ -inch exit hole. The adult will often begin egg-laying on the host tree. That the adult cannot disseminate very far has helped confine this insect somewhat and has made eradication efforts effective. Nonetheless, the Asian long-horned beetle is still found in the country, where it can be a menace to forest and landscape trees. So far, this insect has not been found in Maryland.

  • Walnut twig beetle: The walnut twig beetle is an insect native to the Southwest. This insect bores into the branches of Western walnuts. This tunneling activity does not do much harm to the walnut. The harmful part of this feeding activity is that the twig beetle can carry the fungal spores of thousand canker disease. Thousand canker disease has impacted walnuts in the West since the 1980s, and may eventually cause the death of the host walnut tree. This disease complex was confined to the West until 2010, when it was observed in some black walnuts growing in Tennessee. Following this observation, most states have been monitoring for the presence of walnut twig beetle or the fungus that causes the disease. In 2013, walnut twig beetles were trapped in Cecil County; in 2014, the fungus was identified growing in one of the impacted walnut trees. Cecil County is now under quarantine for walnuts.

Article by Mike Kay, Frederick County Forestry Board

Nature Note for 3/19/22