The term “borer” refers not to a single species but to the destructive larval stage of various insects that tunnel into woody plants and trees. These insects primarily belong to the orders Coleoptera (beetles) and Lepidoptera (moths), though some wasps also fit the description. The damage occurs when the grub-like larvae chew through the plant’s vascular tissues. This internal feeding activity weakens the plant’s structural integrity and can eventually lead to its decline and death. Borers are serious pests because their life cycle protects them inside the wood, making detection and control challenging.
The Borer Life Cycle
The life cycle of a borer involves complete metamorphosis: egg, larva, pupa, and adult. The adult insect, typically a beetle or a moth, emerges from the tree to mate, and the female seeks a host plant to lay her eggs in bark crevices, wounds, or on leaves.
Once the larva hatches, it immediately chews its way into the plant, beginning its destructive phase. The larvae tunnel through the wood or just beneath the bark, feeding on the cambium layer. This stage can last from one year to several years, depending on the species and environmental conditions.
The larvae eventually enter the pupal stage, transforming inside the wood near the surface. The adult insect then emerges, chewing a characteristic exit hole to leave the tree and restart the reproductive cycle. Most plant damage occurs during the concealed larval stage.
Recognizing Infestation Signs
Identifying a borer infestation relies on recognizing the physical evidence left behind, as the larvae remain hidden inside the plant tissue. A telling sign is the presence of exit holes on the bark, which vary in shape and size depending on the insect family. Flatheaded borers, like the Emerald Ash Borer, leave distinctive D-shaped holes, while roundheaded borers create circular or slightly oval holes.
Another sign is “frass,” a sawdust-like material consisting of wood shavings and insect excrement. Frass texture can be an identification clue, ranging from coarse and pelletized to fine and powdery. It is often visible protruding from entry points, collecting in bark crevices, or accumulating at the base of the plant.
Larvae create winding tunnels, or galleries, that disrupt the tree’s vascular system. This internal destruction manifests externally as a decline in health, leading to wilting, yellowing, and branch dieback. Stone fruits may respond by exuding excessive sap or gum (gummosis) from the attack site, which indicates a borer entry point.
Common Borer Families and Targets
Borers are categorized by their biological family, which dictates their preferred host plants and the damage they inflict. Flatheaded borers (family Buprestidae) are characterized by their metallic, flattened adult form and target trees stressed by drought or injury. The Emerald Ash Borer attacks ash trees and leaves a D-shaped exit hole.
Roundheaded borers, or long-horned beetles (family Cerambycidae), produce larger, rounder exit holes, and their larvae create galleries deeper into the heartwood. These borers, such as the Roundheaded Appletree Borer, produce coarser frass and infest hardwoods like apple, hawthorn, and maple. Their tunneling can compromise the structural strength of the trunk or large branch.
Another group is the clearwing moths (family Sesiidae), whose adults often mimic wasps. The larvae, like the Peachtree Borer, primarily attack the lower trunk and root crown of shrubs and fruit trees, including peach, plum, and cherry. Infestation is accompanied by the oozing of gum mixed with frass near the soil line.
Prevention and Management Strategies
Managing borers involves cultural controls focused on prevention and timely direct management. Maintaining the health of trees and shrubs is the first line of defense, as many borer species are attracted to chemicals released by stressed or wounded plants. Proper watering, appropriate fertilization, and avoiding mechanical injuries to the trunk reduces a plant’s susceptibility.
Once borers are established, direct chemical control is necessary, but timing is crucial because the larvae are protected by the wood. Systemic insecticides are the preferred method, applied to the soil or injected into the trunk, allowing the chemical to move throughout the vascular system. These treatments must be applied before eggs hatch or when larvae are young and feeding beneath the bark, typically in spring or early summer.
Physical management involves pruning and destroying infested limbs and branches to remove developing larvae and pupae. For light infestations, carefully removing the bark to expose and destroy larvae in their galleries can be attempted. For severe, widespread infestations that have resulted in significant trunk girdling, removal and destruction of the plant is necessary to prevent adult borers from emerging and spreading the infestation.