**Tree Borers** is a broad common name for the larvae (grubs or caterpillars) of various insect groups (primarily **Beetles**—e.g., Metallic Wood-boring Beetles, Longhorned Beetles; or **Moths**—e.g., Clearwing Moths) that tunnel into the wood or inner bark (cambium/phloem) of trees. The conflict is severe structural damage and tree mortality: their tunneling destroys the vascular tissues, effectively **girdling the tree** (blocking nutrient/water flow) and **weakening the wood**, leading to branch dieback, crown thinning, and eventual death of the entire tree.
Taxonomy and Classification
Tree Borers include species from the orders Coleoptera (Beetles) and Lepidoptera (Moths). The larvae are the destructive stage, which spends months or years protected inside the tree tissue.
Physical Description
Larvae are highly variable, 1/4 to 2 inches long.
- **Beetle Larvae (Grubs):** White, legless, often flattened or cylindrical grubs with strong mandibles. Key examples: **Emerald Ash Borer** (flat-headed grub) and **Asian Longhorned Beetle** (round-headed grub).
- **Moth Larvae (Caterpillars):** Cream-colored caterpillars with small legs, boring deep into the wood.
- **Damage Sign (Key ID):**
- **Exit Holes:** D-shaped (flat-headed borers) or round (round-headed borers) exit holes on the trunk or branches.
- **Frass/Pitch:** Sawdust-like frass mixed with sap or pitch accumulating near the base of the tree or coming out of bark cracks.
- **Crown Decline:** Sudden thinning of the tree canopy, yellowing leaves, and upper branch dieback.
- **Conflict:** Severe tree mortality and structural damage.
Distribution and Habitat
Tree Borers are found globally, often specializing in a small group of host trees (e.g., Ash, Oak, Birch, Pine). Their habitat is the cambium and xylem tissues of the trunk and major branches.
Behavior and Conflict
The conflict is dominated by the hidden damage and targeting of stressed trees.
- **Hidden Pest:** The larvae are protected inside the tree, making topical insecticide sprays ineffective.
- **Targeting Stress:** Many native borers (e.g., Two-lined Chestnut Borer) only successfully attack trees that are already stressed by drought, injury, or disease, making tree health a critical defense.
- **Invasive Threat:** Invasive species like the Emerald Ash Borer can attack and kill perfectly healthy trees, posing a catastrophic threat.
Management and Prevention
Control requires prevention, tree health maintenance, and systemic treatments.
- **Cultural Control:** Maintain optimal watering, mulching, and fertilization to ensure the tree can fend off attacks (by producing sap to “pitch out” the borers).
- **Trunk Injection/Soil Drench:** Use a professional, systemic insecticide (e.g., containing dinotefuran or emamectin benzoate) that is absorbed by the tree and circulated to the wood tissue to kill feeding larvae.
- Avoid injuring the bark (e.g., lawnmower damage) and do not prune trees during the peak adult flight period (when wounds attract females).
Conservation and Research
Tree Borers are managed as major forest and urban landscape pests. Research focuses on host-tree resistance, optimizing systemic insecticide timing and efficacy, and developing biological controls (e.g., parasitic wasps) against invasive borers.