Borers are the destructive larval stage of certain beetles and moths that tunnel into the wood and inner bark of trees. These insects feed on the cambium layer, the living tissue responsible for transporting water and nutrients. This feeding creates internal channels, effectively girdling the tree by disrupting its vascular system. Because this damage occurs beneath the bark, an infestation often goes unnoticed until the tree is severely weakened, requiring immediate identification and treatment.
Recognizing the Signs of Borer Infestation
The presence of a borer infestation is often first detected by finding their waste material, known as “frass,” which looks like fine sawdust. Frass is pushed out of the larval tunnels and collects in bark crevices or at the base of the trunk. The shape of the exit holes is also an important clue; round or oval holes indicate one type of borer, while distinct D-shaped holes often signal metallic wood borers, such as the Emerald Ash Borer.
Trees may also exhibit excessive oozing of sap or gummy resin from the trunk as they attempt to flush out the larvae. This defense mechanism is visible as wet, discolored spots on the bark. The most alarming sign of a long-standing infestation is canopy dieback, where branches in the upper crown thin, turn brown, and die because internal damage compromises the tree’s ability to move water and nutrients.
Cultural and Physical Control Methods
The initial and most sustainable approach to managing borers involves enhancing the tree’s natural defenses, as borers preferentially attack stressed trees. Providing deep, infrequent watering improves tree vigor, encouraging robust growth and sap flow, which helps repel borers. Deep watering ensures the root zone is hydrated, while infrequent application prevents the development of shallow roots vulnerable to drought stress.
Appropriate mulching aids tree health by regulating soil temperature and retaining moisture. Apply a two to four-inch layer of organic mulch over the root zone, keeping it several inches away from the trunk flare. Avoiding this “mulch volcano” prevents moisture buildup against the bark, which can attract borers and fungal pathogens.
Physical control involves removing larvae or eliminating their habitat. Strategic pruning of heavily infested branches showing dieback removes a significant portion of the borer population. For small, active entry holes, a thin wire can be inserted into the tunnel to destroy the larva feeding inside. All infested wood must be destroyed or chipped immediately, as larvae can complete development in cut wood and re-infest nearby trees.
Chemical Treatment Options
Chemical control is an effective method for managing active borer infestations, utilizing either systemic or contact treatments. Systemic insecticides are absorbed by the tree and move through the vascular system, poisoning larvae as they feed on internal tissues. These are commonly applied as a soil drench around the trunk base or injected directly into the trunk by a professional.
Active ingredients like imidacloprid and dinotefuran are frequently used as systemic soil drenches. Emamectin benzoate is often reserved for professional trunk injection due to its effectiveness against aggressive borers. This method provides protection for one to three years, depending on the active ingredient and application rate.
Contact insecticides, such as products containing permethrin or bifenthrin, kill adult borers or newly hatched larvae as they crawl across the bark. The application of contact sprays is extremely time-sensitive, as it must coincide precisely with the period when adult borers are emerging, mating, and laying eggs, typically in spring or early summer. Once larvae tunnel beneath the bark, contact sprays are ineffective, requiring systemic treatment. Careful attention to the specific borer species’ life cycle is necessary to time the topical application correctly.
Long-Term Prevention Strategies
Preventing borer attacks begins with selecting the proper tree species adapted to the planting site, making it less susceptible to stress. Avoiding mechanical injuries to the trunk is a straightforward preventative measure. Lawn equipment, such as mowers and string trimmers, can easily wound the bark and create attractive egg-laying sites.
A protective barrier, like a plastic trunk guard or a wide ring of mulch, helps prevent these common injuries near the ground. For high-value trees susceptible to aggressive borers, such as ash trees vulnerable to the Emerald Ash Borer, preventative applications of systemic insecticides are recommended. These treatments are applied annually or biannually to maintain insecticide levels in the tree’s tissue that discourages infestation.