Bark Beetles: Beware the ‘Tree Killers’

by Kathleen Curthoys, Fairfax Master Gardener

elm bark beetle

Native elm bark beetle

“The Tree Killer” is how the National Park Service has described a single type of bark beetle that attacks trees in our region and throughout the eastern U.S., recently sparking alarm as its range expands northward.

This infamous machine of destruction is the southern pine beetle. It is one of most familiar of the many types of bark beetle, tiny but mighty, that infest trees and forests, leaving swaths of dead, gray trees in their wake.

Bark beetles are known to target trees under stress from any number of causes: disease, damage, drought, air pollution or root damage from construction or disturbance. Trees under stress may already be weakened or dying, making them more susceptible to damage from invaders such as beetles.

Even that pile of firewood you have ready for cozy winter evenings may be infested. Bark beetles are drawn to wood that was recently cut and still has its bark.

Signs of damage
How do you know if your trees or shrubs are under attack? Initially, in the case of a pine tree, for example, a bark beetle will bore into the bark, causing resin to ooze out at the spot where the beetle entered. As the resin appears, it will create a gob or tube that may push beetles out with it. This works as a defense by capturing the beetles in the sticky resin, or pitch, where they may die.

The tubes of resin or pitch may be white-ish and mixed with reddish dust from the bore holes. Healthier trees may be able to produce enough resin to fend off the beetles, but a heavy infestation may overwhelm stressed trees and lead to their death. You may also see dust accumulating from where the beetles bored into the tree. The dust may appear on the bark and at the base of the tree where it falls as the beetles work through the bark to establish tunnels or “galleries.”

A pattern of holes in the outer bark of a tree is evidence of beetles maturing, boring their way out of the infested tree, and leaving that tree to establish new breeding sites in other trees. You need to look closely. Adults are 0.125 to 0.33 inch long, or 3 to 8 mm. Most are brown or black with oblong bodies and hard shells. The members of the Curculionidae family have hundreds of species, most looking similar to each other.

As adults bark beetles bore into a tree, it may take them weeks or months to get through the bark. They may be on the job from spring through fall.

Life cycle
There’s more bad news when it comes to their proliferation. Bark beetles may have one generation per year, or several, depending on species. After adult beetles get through the bark, the female will create a tunnel under the bark and lay eggs there. The four life cycle stages — egg, larva, pupa, and adult — can be found under the bark. The thin layer of plant tissue between the bark and the wood, called the cambium, is where bark beetles feed as well as reproduce.

Pitch tubes

Pitch tubes produced by Southern pine beetles

When the small white grubs hatch, they bore out of the tunnel to make their meals out of the bark tissue (phloem) and cell layers of wood (xylem). A gallery is created when egg tunnels and larvae tunnels are formed under the bark. When a gallery is established, the bark beetle will release a pheromone that acts as a signal for other beetles to take up occupancy in the tree. Invading beetles will girdle the tree with galleries.

It gets worse: Beetles may carry fungi that can grow in the gallery spaces. The fungi, often referred to as blue stain fungi, act as a pathogen and impede water from getting to the top of the tree as it should. Then the foliage becomes discolored, changing from green to light green and yellow, then red before fading to brown. Then defoliation begins. Once numerous bark beetles infest a tree this way, chances are the tree will die.

Better news
Depending on the type of bark beetles, a death sentence may be avoided. Some types of bark beetles go for twigs and branches that are already dying on a generally healthy tree due to factors such as damage or shade from other trees. The twigs, branches and limbs that are dying or recently dead may still have enough moisture to host larvae of some species of beetles that don’t breed in healthy parts of trees. In these cases, the overall health of the tree may not be under threat. Generally, bark beetles won’t infest trees that have been dead for at least a season and have dried out.

Types of bark beetles
Several of the most common bark beetles found in Virginia are listed below. These are a few among hundreds of species of bark beetles that may be found in trees and wooded areas.

Conifer bark beetles: Pines and other conifers are susceptible to attack by a variety of bark beetles that prey on trees that are declining and under stress. Live trees that have been transplanted are more prone to beetle infestation, leading to a recommendation that spring and early summer transplants by treated with insecticide spray as a protection just after planting.

Ips beetles: (Genus Ips) These beetles are usually found in the crown of trees but can attack an entire tree. These “engraver” beetles form galleries in pine trees. They look different from other bark beetles due to a “scooped-out” shape in their posterior section.

Southern pine beetle: (Dendroctonus frontalis) Outbreaks involving this destructive population have prompted forest management strategies to stem the rapid killing of vast areas of pine forest. At one-fifth of an inch long, this beetle targets the middle or upper trunk of a pine tree. Tunnels from larvae may appear around the trunk in random patterns.

Black turpentine beetle: (Dendroctonus terebrans) The base of pine trees and newly cut stumps are victim to this relatively large bark beetle measuring one-third of an inch long. Large numbers of half-inch white globs of pitch on the bark are a common indicator of this beetle’s presence.

Elm bark beetles: Recently dead or dying elm wood is prone to attack by two species of bark beetles, and both can transmit Dutch elm disease to healthy trees. The native elm bark beetle (Hylurgopinus rufipes) goes for the thick bark of trunks and limbs and bores egg tunnels across the grain of the wood. The non-native European elm bark beetle (Scolytus multistriatus) goes to the crotches of one- to three-year-old twigs, creating egg tunnels parallel to the grain.

Common bark beetles also include: the shothole borer that is attracted to fruit trees, wild cherry, serviceberry and elm; the peach bark beetle that looks for stone fruits, mountain ash, elm and mulberry; the birch bark beetle, which also seeks beech, wild cherry and red gum; the ash bark beetle; and the hickory bark beetle.

Prevention and control
Keeping trees healthy and vigorous is the first step to preventing beetle infestations. Trees should be given adequate water to keep them from stress. During times of drought, which Fairfax County has seen recently, trees may be more prone to bark beetle infestation. Keeping trees well-watered helps them produce sap needed as a defense against borers.

As promptly as possible, get rid of trees, limbs and tall stumps that have recently been cut or have died to lessen the chance of bark beetles arriving to lay eggs. Consider thinning crowded groups of trees and keep the ground cleared of debris. If storing wood, try to remove the bark. Avoid transporting cut wood across long distances in areas that may not yet be exposed to bark beetles.

Keep an eye on your trees’ health and watch for signs of bark beetles. A single tree may be attacked by several species of bark beetles.

Chemical treatment: As a preventive measure, trees that are not yet infested but already under stress, or near a known site of infestation, can be treated with residual insecticides. You may want to limit chemical applications to selected high-value trees.

Once trees are infested with large numbers of bark beetles, it’s probably too late. Most trees won’t recover and chemical treatments usually won’t save them. Insecticidal sprays aren’t effective against beetles or larvae that are inside the tree and won’t reverse the damage the beetles have already done. However, an option is to use chemical controls on infested trees to kill adult beetles as they exit the bore holes they have created and prevent them from infesting other trees nearby.

Firewood indoors
You can inadvertently invite bark beetles into the house when you bring in firewood. Wood stored indoors may harbor several types of beetles that can emerge when they’ve been in a warm environment for a few days.

You can take precautions by examining firewood while outside and checking for small holes in the bark, looking for sawdust under the firewood, and disposing of any beetles you find. Consider removing bark from firewood, or putting a tarp over wood that still has bark and bringing in only enough firewood to burn within a day or two. If firewood has been sprayed with chemicals, it shouldn’t be burned.