Bark beetles are small but highly destructive forest pests that target trees weakened by environmental stress, disease, or injury. These insects burrow beneath the bark to lay eggs, and the resulting larval feeding quickly girdles the tree by destroying the tissues responsible for transporting water and nutrients. This guide covers identifying, preventing, and treating bark beetle infestations.
Recognizing an Active Infestation
Recognizing an active infestation requires identifying specific visual evidence on the tree’s trunk and base. The most common sign is the appearance of pitch tubes, which are small masses of resin the tree extrudes to push the invading beetle out. A large, white or clear pitch tube indicates a successful defense. Conversely, a small, reddish-brown pitch tube suggests the beetle successfully bored through the bark, mixing the resin with boring dust and signifying colonization.
Another indicator is “frass,” a fine, sawdust-like material that accumulates in bark crevices or at the base of the tree. Frass is often reddish-brown or yellowish, depending on the tree species. Tiny, circular boreholes on the trunk are the entry or exit points for the beetles, sometimes giving the bark a “shotgun” appearance. The most advanced sign is crown discoloration, known as “flagging,” where the needles or leaves turn yellow, then red or brown.
Proactive Measures to Protect Trees
Maintaining tree health is the most effective defense against bark beetles, as stressed trees emit chemical signals that attract the pests. Proper watering is a foundational practice, requiring deep, infrequent irrigation to encourage root growth. During periods of drought, supplemental watering is important to support the tree’s natural defense mechanism of producing resin.
Physical injury to the trunk or bark should be avoided, as wounds serve as easy entry points for beetles or other pathogens. This includes being careful with lawnmowers, string trimmers, and construction activity near the tree’s base. Pruning should be timed during the dormant season (late fall to late winter) when beetle activity is low. Proper spacing between trees also reduces competition for light, water, and nutrients, increasing overall tree vigor.
Wood debris management is a preventative step, especially when dealing with recently cut logs or dead wood, known as “slash.” Slash piles from susceptible tree species attract beetles, creating a breeding ground that can lead to attacks on healthy, standing trees. All infested or fresh wood should be removed, chipped, or destroyed before the main beetle flight season in the spring and early summer.
Chemical Control Strategies
Chemical control is primarily a preventative measure, targeting high-value, uninfested trees located near an active infestation. These treatments create a toxic barrier on the bark surface to kill adult beetles before they can bore into the tree. Preventative sprays typically utilize long-lasting residual insecticides, such as pyrethroids (permethrin or bifenthrin) or the carbamate insecticide carbaryl.
Application timing is important; the insecticide must be applied to the entire trunk and large branches before the adult beetles begin their flight and attack, typically in the late winter or early spring. These products are generally not available for homeowner use due to the specialized, high-pressure equipment needed for thorough coverage up to the mid-crown. The chemicals remain active on the bark surface for a full season, killing beetles on contact.
Systemic insecticides, often containing emamectin benzoate, are applied as trunk injections for prevention. These treatments are absorbed by the tree and circulate in the vascular system, offering protection that can last for two to three years. However, systemic treatments are not uniformly effective against all bark beetle species and may not prevent the introduction of blue-stain fungi, which the beetles carry and can lead to tree decline.
Non-Chemical and Cultural Treatment Options
Several cultural and mechanical treatments can help manage beetle populations for property owners avoiding synthetic insecticides. Pheromone traps and repellents offer targeted approaches to monitoring or disrupting beetle behavior. Anti-aggregation pheromones, such as verbenone, signal to incoming beetles that a tree is occupied, diverting them elsewhere. Pheromone traps are used primarily to monitor population levels, not for mass-trapping and eradication.
The immediate destruction of infested wood is a direct way to eliminate developing beetle broods before they emerge to attack other trees. This involves promptly removing the infested tree or branches and processing the wood to kill the larvae. Chipping the wood into pieces less than one inch thick is an effective method, as the small size and subsequent drying destroy the beetle larvae.
Larger infested logs can be treated using solarization, where they are tightly wrapped in thick, clear plastic and left in direct sunlight for several weeks. The intense heat generated beneath the plastic kills the larvae and pupae within the bark. Debarking the infested wood and immediately burning or chipping the bark itself will also destroy the majority of the beetle population.
When to Remove an Infested Tree
Once a bark beetle infestation is successful and widespread throughout the trunk, the tree is generally considered beyond saving. The decision to remove a tree is based on the extent of the colonization and the need to prevent the spread of the next generation of beetles. If a majority of the trunk circumference is covered with successful pitch tubes and frass, or if significant crown dieback is apparent, the tree should be removed.
Reddish-brown foliage, a late-stage symptom, indicates that the tree’s water transport system has been compromised and the tree is dead. Prompt removal and proper disposal of the wood are necessary to ensure that the developing larvae do not complete their life cycle and emerge to attack nearby healthy trees. Disposal methods include burning, deep burial, or chipping the wood.