Tree borers are the larval stage of certain beetles and moths that tunnel underneath a tree’s bark, feeding on the inner tissues. This activity disrupts the tree’s vascular system, which transports water and nutrients between the roots and the canopy. Severe infestation can effectively “girdle” the tree, leading to the decline and eventual death of branches or the entire plant. Since borers typically attack trees already weakened by stress, immediate recognition and action are necessary to save the affected tree.
Recognizing the Signs of Infestation
Identifying a borer infestation often relies on observing external evidence that the insects have left behind on the trunk or branches. The most obvious sign is the presence of small holes in the bark, which are typically exit holes created when the adult insect emerges from the tree. The shape of these holes can offer clues about the specific pest, such as the distinct D-shaped holes left by the invasive Emerald Ash Borer or the perfectly round holes made by other types of beetles.
Another telltale symptom is “frass,” a sawdust-like material that is the excrement and wood shavings expelled by the tunneling larvae. Frass may be visible clinging to the bark, collecting in crevices, or accumulating in a small pile at the base of the trunk. Some trees will also exhibit excessive sap or gummy material oozing from the trunk where the borers have been feeding, a defensive reaction by the tree.
Look for canopy thinning or the sudden dieback of upper branches, often referred to as “flagging.” These symptoms indicate the vascular system is failing to deliver water to the crown. While visual signs confirm borers, identifying the specific species (flatheaded beetle, roundheaded beetle, or clearwing moth) is important. Control strategies are tailored to the borer’s specific life cycle and feeding habits.
Non-Chemical Methods for Control
Once an infestation is confirmed, non-chemical methods focus on physical removal and boosting the tree’s natural defenses to combat the pests. For trees with localized damage, heavily infested branches should be pruned and removed immediately, as the wood contains the feeding larvae. Proper disposal of this wood, such as burning or chipping, prevents the adults from emerging and attacking other nearby trees.
Physical probing can be effective for smaller, active tunnels. A thin, flexible wire is inserted into the borer hole to physically crush the larva inside. This method is practical when only a few holes are present and the tunnel is relatively straight. Improving the tree’s overall health, known as promoting tree vigor, is a powerful non-chemical defense, as healthy trees are less attractive to egg-laying adults.
Biological controls, such as the application of parasitic nematodes into active borer holes, offer another non-chemical option for managing the larval stage. These microscopic worms seek out and kill the borers inside their tunnels. Additionally, commercial paper or mesh wraps can be applied to the trunk of young, thin-barked trees to physically deter adult borers from laying eggs on the surface.
Chemical Treatment Options and Application
Chemical intervention involves insecticides, categorized as systemic or contact treatments, depending on the borer’s life stage. Systemic insecticides are absorbed by the tree and distributed through the vascular system, making internal tissues toxic to feeding larvae. These treatments are often applied as a soil drench or injection near the trunk base, allowing roots to absorb the active ingredient, such as imidacloprid or dinotefuran.
Soil applications are generally done in the spring or fall, allowing the insecticide time to translocate throughout the tree before larvae begin feeding aggressively. For larger trees, trunk injections of systemic compounds, like emamectin benzoate, may be necessary. Injections deliver the insecticide directly into the tree’s transport system, offering fast protection, especially for high-value trees or severe infestations.
Contact or residual sprays, often containing permethrin, are applied directly to the bark to kill adult borers or newly hatched larvae before they tunnel into the tree. Effectiveness depends on precise timing, coinciding with the brief window when adults are emerging and laying eggs. Consulting local agricultural extension offices is recommended to determine the exact adult emergence period. All chemical treatments require strict adherence to product labels regarding application rates and safety.
Long-Term Strategies for Prevention
The most effective long-term strategy against tree borers is prevention, focusing on maintaining high tree health and reducing stress factors. Borers primarily target trees weakened by drought stress, mechanical injury, or poor site conditions. Ensuring the tree receives proper, deep, and infrequent watering, especially during dry spells, helps maintain the sap flow necessary for natural resistance.
Protecting the trunk from mechanical damage is paramount, as wounds created by lawnmowers or string trimmers serve as easy entry points for egg-laying adults. Applying mulch helps conserve soil moisture and regulate temperature, but it should be kept several inches away from the trunk base to prevent bark rot. When planting new trees, selecting species well-adapted to the local soil and climate minimizes future stress and vulnerability.