Tree borers are the larval stage of certain beetles and moths that pose a severe threat to landscape trees. These pests target the inner bark, or cambium layer, where they feed and tunnel, effectively girdling the tree by disrupting the flow of water and nutrients from the roots to the canopy. Borers are often secondary invaders that attack stressed or weakened trees, so a confirmed infestation signals a serious health issue demanding immediate action. Once these larvae tunnel beneath the bark, they are protected from many external controls, making quick and informed treatment necessary to prevent the tree’s decline or death.
Recognizing the Signs of Infestation
Physical evidence on the trunk and branches is the most reliable way to confirm a borer presence. Look for small, distinct holes in the bark, which are typically where the adult insect emerged. The shape of these exit holes can provide a clue to the specific pest; for instance, flatheaded borers often leave a D-shaped hole, while roundheaded borers create a circular or oval one.
Another common sign is frass, a sawdust-like material often found near entry or exit points or collected at the base of the tree. Trees may also attempt to expel the invaders by oozing sap or gummy material from the infested areas. As the internal tunneling disrupts the tree’s vascular system, external symptoms become visible, including general thinning of the canopy and dieback of individual branches, usually starting in the upper crown.
Non-Chemical Removal and Killing Techniques
For small, localized infestations, physical intervention can effectively kill or remove borers. One technique involves using a flexible wire or specialized tool to probe into visible borer holes and galleries. By inserting the wire until resistance is felt, the larva inside can often be crushed or extracted, though this is only effective if the tunnel is relatively straight and accessible.
Pruning is another important step, as it immediately removes infested wood from the tree. Limbs showing signs of dieback or visible borer damage should be cut off several inches below the lowest sign of infestation. Proper disposal of the removed wood is equally important to prevent larvae from completing their life cycle and emerging as new adults. The infested material should be chipped or solarized by sealing it in clear plastic bags and placing them in direct sunlight to kill the remaining pests.
Maintaining the tree’s natural vigor also enhances its ability to resist borers without chemical aid. Healthy trees can often defend themselves by producing a copious flow of sap, which can drown or flush out newly hatched larvae before they successfully tunnel deep into the wood. This cultural practice makes the tree less attractive to borers.
Systemic and Contact Insecticide Treatments
Chemical control is often necessary for severe or widespread borer infestations, utilizing systemic and contact insecticides. Systemic treatments are designed to be absorbed by the tree and circulate throughout its vascular system. These are commonly applied as a soil drench around the base of the tree or injected directly into the trunk.
Common active ingredients in systemic products are neonicotinoids like imidacloprid or dinotefuran, which kill the larvae when they ingest the treated wood tissue. Soil drenches are most effective when applied in the fall or early spring, allowing the tree time to absorb the insecticide before the borer’s active season begins. Trunk injections, while more expensive and requiring specialized equipment, can be highly effective and may be preferred where soil application is restricted or impractical.
Contact or residual insecticides, such as those containing permethrin or bifenthrin, are applied as bark sprays to the trunk and main branches. These sprays do not penetrate deeply into the wood but create a protective barrier on the bark surface. The purpose of a contact spray is to kill newly hatched larvae as they crawl across the bark seeking an entry point, or to kill emerging adult borers. Timing is crucial for contact sprays, as they must be applied precisely when the adult borers are emerging and laying eggs, typically during the spring and early summer.
Long-Term Tree Health and Prevention
The best long-term defense against tree borers is maintaining the health and vigor of the tree. Proper watering techniques are foundational to tree health, requiring deep, infrequent applications that encourage strong root growth. A well-hydrated tree is better equipped to produce the defensive sap flow that can repel initial borer attacks.
Fertilization should be done judiciously, focusing on providing balanced nutrients without excessive nitrogen, which can encourage soft, vulnerable growth. Preventing mechanical injury to the trunk and root flare is also important, as wounds created by lawnmowers or string trimmers act as attractive entry points for egg-laying adult borers.
Applying a layer of organic mulch, kept a few inches away from the trunk, helps to conserve soil moisture and moderate root temperature. By focusing on these cultural practices, homeowners can strengthen their trees, making them highly resistant to future infestations and reducing the need for chemical intervention.