Tree borers are the larval stage of certain beetles and moths that feed beneath a tree’s bark, burrowing into the living tissues of the trunk and branches. This internal feeding damages the vascular system, specifically the xylem and phloem, which transport water and nutrients throughout the tree. The destruction of these tissues restricts the tree’s ability to move resources, leading to decline, structural weakness, and eventually death. When infestations are severe, chemical control becomes necessary to protect high-value trees.
Identifying the Target Pest and the Damage
Before treatment is considered, confirming the presence of borers and identifying the specific species is crucial. Different borer species have distinct life cycles, meaning the timing and method of insecticide application must be tailored to the pest.
Homeowners should look for sawdust-like material, known as frass, which is the excrement and chewed wood pushed out of the tunnels by the larvae. This frass may be visible in bark crevices or accumulating at the base of the tree. Another clear sign is the appearance of emergence holes left by adult insects after they exit the tree.
The shape of the exit hole can help narrow down the identification, such as the distinct D-shaped holes left by the Emerald Ash Borer or the metallic-colored flatheaded borers. Other borers, like the clearwing moths or Asian Longhorned Beetle, leave behind round or oval exit holes. Oozing sap from the trunk is a common response to the tunneling injury and can be another sign of borer activity.
As the infestation progresses and the vascular system is damaged, the tree will exhibit visible decline symptoms, such as thinning foliage or dieback in the crown and branches. Common wood-boring pests include the Bronze Birch Borer and the Emerald Ash Borer. Identifying the underlying stressor is also important for long-term management, since most borers only attack trees that are already weakened.
Categories of Chemical Control Agents
Insecticides used to combat tree borers generally fall into two main categories: systemic and contact/residual. Systemic insecticides are absorbed by the tree and move internally through the vascular system, poisoning the larvae as they feed on the treated wood or bark tissue. This method is effective against larvae protected deep inside the tree’s wood.
Common active ingredients in systemic products include neonicotinoids like Imidacloprid and Dinotefuran, as well as Emamectin Benzoate. Imidacloprid is often applied as a soil drench and is effective against certain flatheaded borers that feed just beneath the bark. Dinotefuran, sometimes applied as a basal trunk spray, moves more rapidly into the tree’s system compared to soil-applied products.
Emamectin Benzoate is typically used in professional trunk injection applications and provides long-lasting control against pests like the Emerald Ash Borer, often lasting for two years per treatment. The second category is contact or residual insecticides. These function by coating the bark surface, killing newly hatched larvae or adult insects as they crawl over the treated area.
Active ingredients used in contact sprays include synthetic pyrethroids such as Permethrin and Bifenthrin, as well as Carbaryl. These products are sprayed directly onto the trunk and main branches to create a toxic residue. They must be applied when the adult borers are active and laying eggs. Unlike systemic products, residual sprays have little effect on larvae that have already tunneled beneath the bark.
Timing and Methods for Effective Application
The success of chemical control relies heavily on precise timing, as the insecticide must be present when the larvae are at their most vulnerable stage. This vulnerable period is typically when the adult borers are emerging, mating, and laying eggs, or when the larvae are newly hatched and attempting to chew their way into the bark. Applying treatment outside of this window reduces effectiveness because the target pest is protected inside the tree.
To accurately predict the emergence of a specific borer species, professionals rely on degree-day models, which track the accumulation of heat units over time to forecast insect development. This method provides a more accurate prediction of pest activity than relying on a fixed calendar date alone. This is useful for timing residual insecticide applications to coincide with adult flight and egg-laying.
Application methods vary depending on the type of insecticide being used. Systemic products like Imidacloprid are often applied as a soil drench or granule, mixed with water and poured around the base of the tree for root uptake. This method requires the soil to be moist for the insecticide to move effectively, and the uptake process can take several weeks.
Contact and residual products like Permethrin are applied as a protective cover spray directly to the trunk and lower branches, soaking the bark to runoff. This application must be done with thorough coverage to ensure the entire surface is protected from newly hatched larvae.
Trunk injections use highly concentrated systemic products like Emamectin Benzoate. These bypass the soil and are injected directly into the tree’s vascular tissue, providing the fastest uptake and longest residual control.
Strict adherence to the product label is paramount for efficacy, safety, and environmental protection. Systemic treatments, especially those containing neonicotinoids, should be applied with caution on trees that bloom, or only after the flowering period is complete, to minimize the risk of harm to pollinators. Professional applicators are required for products that require specialized equipment or restricted-use chemicals.