Tree borers are the larvae of certain beetles and moths that tunnel into the wood of trees. These insects pose a serious threat to the structural integrity and long-term health of nearly all woody plants. Once hatched, the larvae feed on the cambium and phloem layers located just beneath the bark. These layers are responsible for transporting water and nutrients throughout the tree. This internal feeding effectively girdles the tree from the inside, leading to a slow decline and eventual death if the infestation is left unchecked. Understanding the signs of an infestation, implementing preventative care, and knowing the appropriate treatment options are necessary steps to maintain tree vitality.
Recognizing the Signs of Borer Activity
Diagnosing a borer infestation requires a careful inspection of the trunk and branches for specific physical evidence. The most definitive signs are the exit holes left behind when the adult insect emerges from the wood. The shape of these holes can help identify the type of borer involved, such as a distinct D-shape indicating a flatheaded borer, or a perfectly circular hole suggesting a roundheaded or clearwing borer.
A sawdust-like material known as frass often accumulates near these holes or gathers at the base of the tree. Frass is the insect’s waste and chewed wood fragments, and its presence indicates active feeding inside the tree. Clearwing moth borers often leave wet frass mixed with gummy sap, while flatheaded borers tend to pack their fine frass tightly into their tunnels.
Secondary symptoms appear in the canopy as the tree’s vascular system fails. Look for branch dieback, where upper limbs begin to wilt and turn brown, often starting at the top of the tree. On fruit trees, excessive oozing of sap or gummy residue, called gummosis, may be visible on the trunk where the larvae are feeding. Cracked, loose, or peeling bark also suggests extensive tunneling damage beneath the surface.
Proactive Measures to Prevent Infestation
The most effective defense against borers is maintaining a high level of tree health, as most borers are opportunistic pests that target stressed or weakened specimens. Ensuring the tree receives adequate water is paramount, especially during periods of drought or in the first year after planting. Water-stressed trees emit chemical signals that attract egg-laying adults. Mulching is beneficial for moisture retention, but it must be applied correctly in a wide, thin layer that does not touch the base of the trunk, which can trap moisture and invite pests.
A tree’s susceptibility to borers increases significantly when the bark is mechanically damaged. Protecting the trunk from injuries caused by lawnmowers, string trimmers, or construction equipment eliminates easy entry and egg-laying sites for adult insects. Any accidental wound should be managed quickly, as open injuries are particularly attractive to borers.
Pruning should be timed to avoid the peak flight season of adult borers in your region, which typically occurs from late spring through mid-summer. Pruning during this time creates fresh wounds that release attractive volatile compounds, increasing the risk of eggs being laid on the new cuts. For young, thin-barked, or newly planted trees, applying a protective tree wrap to the trunk can provide a physical barrier against egg-laying during the first few vulnerable years.
Active Treatment and Removal Strategies
Physical and Cultural Control
For small-scale or localized infestations, physical removal methods can be effective. Heavily infested branches should be pruned away well below the point of visible damage to ensure the removal of active larvae. After making a cut, inspect the severed branch for borer tunnels. If a tunnel is present, make another cut further down the branch toward the trunk until no more tunnels are evident. The infested wood should be destroyed or removed from the property to prevent adult emergence and reinfestation.
If an active borer tunnel is visible on the trunk, particularly one with moist frass or a clear opening, manual extraction may be possible. A flexible, stiff wire can be inserted into the tunnel to physically probe and kill the larva inside. This method is most practical for larger, more open tunnels created by clearwing moth larvae. After removal or treatment, improving post-infestation tree care, such as providing deep watering and appropriate fertilization, helps the tree recover and seal off internal wounds.
Chemical Control
Chemical treatments are often necessary for established infestations or for preventative care of high-value trees. These treatments fall into two main categories: protective bark sprays and systemic insecticides. Protective sprays are contact insecticides applied to the bark to kill newly hatched larvae before they bore into the wood. These sprays are only effective if applied precisely when the adult borers are flying and laying eggs, typically from late spring to early summer.
Systemic insecticides are absorbed by the tree and move throughout the vascular system, poisoning any larva that feeds on the internal tissue. They can be applied as a soil drench around the base of the tree or injected directly into the trunk. Soil drenches are best applied in mid-April to mid-May, allowing four to six weeks for the chemical to be fully absorbed and translocated throughout the canopy before larvae establish.
Trunk injections provide faster uptake, typically applied from mid-May to mid-June, and are often reserved for larger trees or species sensitive to soil-applied products. Systemic treatments are most effective against flatheaded borers, like the Emerald Ash Borer, but are less reliable against moth larvae. Due to the potential risk to pollinators, systemic insecticides should not be applied to trees that are actively flowering or heavily visited by bees, such as Lindens or Crabapples. If a tree displays more than 50% canopy dieback, the damage is too severe for treatment, and the tree should be safely removed.