Types Of Wood-Boring Beetles That Threaten Illinois Trees

Overview: why wood-boring beetles matter in Illinois

Wood-boring beetles are a broad group of insects whose larvae feed beneath bark or through the sapwood of living and dead trees. In Illinois, native and nonnative species can kill or weaken valuable trees in urban, suburban, and forest settings. These pests reduce shade, increase storm hazards, lower property values, and raise management costs for homeowners and municipalities.
Understanding which beetles are present, how they damage trees, and what you can do about them improves early detection and reduces long-term impacts. This article describes the major types and representative species that threaten Illinois trees, how to recognize infestations, and practical steps for control and prevention.

How wood-boring beetles damage trees

Wood-boring beetles attack trees at different life stages and in different parts of the tree. Some species attack healthy trees; others attack stressed or dying trees. Common ways they damage trees include:

• Tunneling under bark that disrupts phloem and cambium and blocks nutrient transport.
• Excavating galleries in sapwood and heartwood, weakening structural integrity.
• Introducing secondary fungi and pathogens that accelerate decline.
• Creating entry points for other pests and pathogens through wounds.

Damage accumulates over one or several generations of larvae and often becomes obvious only after canopy thinning, epicormic shoots, bark splitting, or large numbers of exit holes appear.

Life cycles and signs to watch for

General life cycle patterns

Most wood-boring beetles share a similar sequence: adult beetles lay eggs on bark or in crevices; eggs hatch into larvae that bore into and feed within wood; larvae mature over months to years; then larvae pupate and emerge as adults through characteristic exit holes. The length of the life cycle depends on species and environmental conditions.

Common signs of infestation

• Crown thinning, branch dieback, or sudden tree decline.
• Sawdust-like frass at branch crotches, trunk bases, or on bark.
• Characteristic exit holes (shape and size vary by group).
• Serpentine or meandering galleries visible beneath peeled bark.
• Bark splits, D-shaped or round holes, and galleries in exposed wood.

If you observe these signs, early detection can make a difference in control options.

Major wood-boring groups and species of concern in Illinois

This section focuses on families and species that are most important for Illinois trees. Information includes typical hosts, damage pattern, identification clues, and management considerations.

Emerald Ash Borer (Agrilus planipennis) — Buprestidae (metallic wood-borers)

Emerald ash borer (EAB) is the single most destructive wood-boring beetle to hit Illinois in recent decades. It is an invasive buprestid that attacks all North American ash species.

• Hosts: Green, white, and black ash and related species.
• Damage pattern: Larvae feed in the cambial region leaving serpentine galleries that sever the flow of water and nutrients. Trees can die within 2 to 4 years of heavy infestation.
• Identification: Adults are metallic green and slender; larvae are white, flat-headed. Exit holes are D-shaped and about 3-4 mm across. Bark splitting and S-shaped galleries are diagnostic.
• Management: Preventive and curative systemic insecticides can protect high-value trees for several years (emamectin benzoate trunk injections are highly effective; neonicotinoid soil drenches or trunk sprays can also help). Biological control agents have been released in some areas but do not replace other measures. In advanced infestations, removal and replacement are recommended to prevent spread.

Asian Longhorned Beetle (Anoplophora glabripennis) — Cerambycidae (longhorned beetles)

Asian longhorned beetle (ALB) is a high-risk invasive longhorned beetle that attacks many hardwood species. Illinois has experienced ALB detections historically and remains at risk from new introductions.

• Hosts: Maples, elms, birch, poplar, willow, and several other hardwoods.
• Damage pattern: Larvae bore deeply into sapwood and heartwood, creating large round exit holes and heavy structural damage.
• Identification: Adults are large (up to 1.5 inches), black with white spots and very long antennae banded in black and white. Exit holes are round, about 10-20 mm.
• Management: Because of the high risk of spread and tree mortality, the standard response for ALB is aggressive quarantine and removal and destruction of infested and nearby host trees. Homeowner chemical treatment is generally not recommended as the primary control; professional eradication programs are the appropriate route.

Bronze Birch Borer (Agrilus anxius) — Buprestidae

Bronze birch borer is a native metallic buprestid that is an important pest of birch trees in the Midwest.

• Hosts: Paper birch, river birch, and other birch species.
• Damage pattern: Larvae create galleries in the cambial region, causing branch dieback that often begins in the upper crown. Infested trees may have thinning crowns and epicormic shoots.
• Identification: Adults are slender metallic beetles; larval galleries and D-shaped holes (small) are typical. Damage often follows environmental stress such as drought or salt injury.
• Management: Maintain tree vigor through watering, mulching, and avoiding trunk injury. Systemic insecticides applied preventively or at early infestation can be effective. Remove and replace severely infested trees when necessary.

Two-lined Chestnut Borer (Agrilus bilineatus) — Buprestidae

A native buprestid that targets oaks weakened by drought or other stress.

• Hosts: Oaks (particularly red and other stressed oaks), chestnuts.
• Damage pattern: Attacks weakened trees and causes branch flagging and crown thinning. It can kill trees that are already stressed.
• Identification: Narrow, winding galleries under the bark; timing of adult emergence corresponds with late spring and summer.
• Management: The most effective control is preventing or relieving stress. Avoid drought stress, manage competing vegetation, and remove and destroy heavily infested material.

Locust Borer (Megacyllene robiniae) — Cerambycidae

Locust borer is a longhorned beetle that specifically attacks black locust trees and can be locally important.

• Hosts: Black locust (Robinia pseudoacacia).
• Damage pattern: Larvae bore into the sapwood of branches and trunks, weakening trees and making them more susceptible to breakage.
• Identification: Adults are striking with yellow bands on a dark body and emerge in late summer and early fall. Exit holes are round and somewhat larger than buprestid holes.
• Management: Remove infested wood, maintain tree vigor, and consider targeted insecticide sprays on high-value trees at adult flight times when practical.

Bark beetles and ambrosia beetles (Scolytinae and related groups)

This is a diverse group that includes small bark beetles and ambrosia beetles that attack stressed trees, nursery stock, or recently planted trees.

• Hosts: Many hardwoods and ornamentals; some species have broad host ranges.
• Damage pattern: Tunneling under bark and into sapwood; galleries often carry fungal symbionts that cause staining and additional tree decline. Ambrosia beetles bore into sapwood and introduce fungal gardens.
• Identification: Tiny round holes, small dust-like frass, quick onset of decline, and staining in wood cores. Ambrosia beetles often attack during spring when trees are recently planted or stressed.
• Management: Prevent stress, avoid planting susceptible species in poorly drained or saturated soils, avoid fresh pruning wounds when beetles are active, and remove infested nursery stock. Preventive insecticide sprays may be used by professionals on very valuable specimens.

How to detect and confirm an infestation

Early detection is critical. Practical steps you can take:

• Regularly inspect trunks and the lower crown for D-shaped or round exit holes, bark splits, and frass.
• Look for thinning or flagging of branches, especially starting in the upper canopy.
• Peel back small bark sections on weakened trees (with permission/professional help) to reveal galleries.
• Note the timing of emergence and adult beetle sightings–metallic EAB adults appear in late spring and early summer; many longhorns emerge mid to late summer.
• Photograph suspected beetles, holes, and galleries and retain samples if possible. Contact your local cooperative extension office, certified arborist, or state Department of Natural Resources for identification and reporting recommendations.

Practical control and management strategies

No single tactic solves all beetle problems. Integrate the following approaches as appropriate:

• Maintain tree vigor: proper watering, mulching, correct pruning, and avoiding compaction and trunk injuries are the best long-term defense.
• Sanitation: promptly remove and properly dispose of heavily infested, dead, or dying trees to reduce local pest pressure. Follow local regulations for quarantine pests (EAB and ALB may be regulated).
• Chemical control: systemic insecticides (emamectin benzoate injections, neonicotinoid soil drenches or trunk sprays, dinotefuran treatments) can prevent or reduce larval damage for several years when timed correctly. Hire a licensed arborist for effective trunk injections.
• Biological control and monitoring: for some invasive species, biological control agents have been introduced and can help reduce populations but are generally supplemental rather than curative.
• Quarantine and reporting: if you suspect ALB or a new invasive species, do not move wood and report the find to authorities per local instructions.
• Species selection: in new plantings, diversify tree species to reduce the risk that one pest will decimate the entire landscape.

Prevention: best practices for homeowners and managers

• Do not move firewood. Many wood-boring pests spread on infested firewood and nursery stock.
• Choose diverse, regionally appropriate tree species and avoid planting large monocultures of a single genus (for example, all ash or all maple).
• Water and mulch newly planted and young trees to reduce stress and susceptibility.
• Inspect newly purchased nursery stock before planting and reject material with holes, frass, or other signs of boring.
• Maintain records of treatments and inspections for high-value trees and schedule regular assessments with a certified arborist.

When to call a professional and how to report suspects

If you suspect an infestation of emerald ash borer, Asian longhorned beetle, or an unusual wood-boring beetle:

• Contact a certified arborist for assessment and a plan. Chemical treatments and trunk injections should be done by licensed professionals.
• Report suspected invasive detections to your state natural resources or agriculture department and your local extension office. Early reporting helps containment and eradication efforts.
• If a municipal or utility tree is affected, contact your city tree department or utility provider; they handle removals and follow regulatory protocols.

Take action early: delaying response often reduces the available management options and increases costs.

Final takeaways

• Illinois faces threats from both invasive and native wood-boring beetles. Emerald ash borer remains the most destructive, but longhorned beetles, bronze birch borer, two-lined chestnut borer, locust borer, and various bark and ambrosia beetles also cause significant damage.
• Regular inspection, maintaining tree health, and early professional intervention are the most effective responses.
• Avoid moving wood, diversify plantings, and use systemic treatments judiciously on high-value trees when warranted.

Proactive care, informed monitoring, and timely professional help will preserve tree health, protect public safety, and limit the spread of destructive wood-boring beetles in Illinois.