Types of Invasive Insects Threatening Minnesota Trees

Overview of the threat

Minnesota’s forests, street trees, and urban canopy provide ecological services, wildlife habitat, and economic value. That value is threatened by a suite of nonnative, invasive insects that either are already established in the state or represent credible risks of introduction. These insects differ in biology, host preference, and the damage they cause, but they share common traits: they spread rapidly, can kill mature trees, and are difficult or expensive to manage once established.
This article examines the most important invasive insects for Minnesota tree health, describes how to recognize an infestation, summarizes current management tools, and provides clear, practical actions that homeowners, landowners, and municipal managers can use to reduce risk.

How invasive insects damage trees

Invasive insects attack trees in several ways:

• Direct feeding on leaves reduces photosynthesis and weakens trees.
• Wood-boring and cambial feeders interrupt nutrient and water transport by creating galleries in sapwood, phloem, or cambium.
• Sap-sucking insects remove nutrients and inject toxins or pathogens.
• Some insects vector fungal pathogens that cause cankers, wilt, or root disease.

Early detection and quick response are critical. Once a large area is infested, eradication is rarely possible and management shifts to slowing spread and protecting high-value trees.

Key invasive insects in Minnesota

Emerald ash borer (Agrilus planipennis)

Emerald ash borer (EAB) is the single most destructive invasive insect affecting Minnesota’s trees today. It attacks all North American ash species (Fraxinus spp.), including green ash, white ash, and black ash.
Signs and symptoms

• D-shaped exit holes 3-4 mm wide on bark.
• S-shaped galleries beneath the bark created by larvae.
• Thinning crown, epicormic shoots on the trunk, and branch dieback.
• Woodpecker feeding flaking bark to reach larvae; increased woodpecker activity and “blonding” of bark.

Current status and impact

• EAB has been detected across much of eastern Minnesota and in several western counties; its range continues to expand.
• Infested ash typically die within 2-6 years of heavy infestation; municipal and rural ash resources have been severely reduced in many jurisdictions.

Management options

• Chemical protection: systemic insecticides (emamectin benzoate trunk injections, dinotefuran, imidacloprid) can protect individual high-value trees. Emamectin injection provides multi-year control and is currently the most reliable option for large ash.
• Biological control: three parasitoid wasps have been released in Minnesota to slow population growth; biological control reduces EAB density but does not prevent tree mortality at large scale.
• Sanitation and removal: removing and properly disposing of infested trees reduces local population pressure.

Practical takeaway

• Protect high-value ash proactively: treat before visible decline or immediately after early detection. Consult a certified arborist for correct product, dose, and timing.

Asian longhorned beetle (Anoplophora glabripennis)

The Asian longhorned beetle (ALB) is a highly destructive wood-boring insect that attacks maples, elms, birches, and other hardwoods. ALB has not become established in Minnesota but has proven capable of eliminating street and forest trees where introduced elsewhere.
Signs and symptoms

• Large round exit holes about 10-20 mm diameter.
• Silvery white to black adults with white spots and very long banded antennae.
• Sap oozing and frass (coarse sawdust) at the base of trees and branch joints.

Current status and impact

• ALB outbreaks in other states required aggressive eradication: removal and chipping of host trees within a quarantine area.

Management options

• Early detection and rapid response: eradication has been possible when ALB infestations were detected very early.
• Public reporting and inspection of high-risk materials like untreated wood packaging and firewood.

Practical takeaway

• Learn ALB identification and report suspicious large exit holes or unusual beetles to local officials. Prevent introduction by not moving untreated urban wood, pallets, or packing materials from areas with known infestations.

Spongy moth (formerly gypsy moth) (Lymantria dispar dispar)

Spongy moth is a defoliator with outbreak cycles that can cause severe stress and mortality when infections are repeated. Oaks are especially vulnerable, but many hardwoods can be defoliated.
Signs and symptoms

• Caterpillars with five pairs of blue and six pairs of red dorsal spots; hairy appearance.
• Egg masses on trunks, firewood, outdoor furniture, and structures.
• Repeated defoliation leading to branch dieback, reduced vigor, and secondary insect or disease attacks.

Current status and impact

• Minnesota experiences localized outbreaks; climate influences outbreak intensity and spread.

Management options

• Biological insecticide: Bacillus thuringiensis kurstaki (Btk) is effective against young caterpillars and has minimal non-target effects when applied at the right time.
• Mechanical control: removal of egg masses in winter, burlap bands to capture caterpillars on small trees.
• Forest-scale management: aerial or ground spraying during outbreaks when protecting high-value stands.

Practical takeaway

• Monitor for egg masses in winter and early-season caterpillars in spring. Timely Btk applications (targeting early instars) are the most environmentally friendly control for homeowners and managers.

Bronze birch borer (Agrilus anxius)

Bronze birch borer (BBB) is a native North American insect in some parts of the continent but acts invasively in stressed birch populations and where planted birches are common.
Signs and symptoms

• Crown thinning starting at the top, dieback.
• D-shaped emergence holes and serpentine galleries under bark.
• Infested trees often show stress, such as drought or root issues, before borer attack.

Current status and impact

• Minnesota’s planted birches (especially river birch and white birch) are susceptible where trees are stressed by site conditions.

Management options

• Maintain tree vigor: proper watering, mulching, and avoiding root damage.
• Chemical treatments: systemic insecticides can protect high-value birches; timing is critical to interrupt the larval stage.

Practical takeaway

• Avoid planting birch on sites prone to drought or compacted soils; preserve birch vigor through cultural care to reduce risk.

Thousand cankers disease (Pityophthorus spp. beetles + Geosmithia morbida fungus)

Thousand cankers disease (TCD) is a complex disease of black walnut and other Juglans species caused by a tiny bark beetle (walnut twig beetle) that vectors a fungus which forms cankers.
Signs and symptoms

• Branch and crown thinning, yellowing foliage, and progressive dieback.
• Small pinhole beetle entry points and numerous small sunken cankers under the bark.

Current status and impact

• TCD has devastated walnut in parts of the West and has appeared in some eastern states; Minnesota remains at risk because walnut is present in the state and TCD can spread via infested wood and beetle movement.

Management options

• Monitoring for dieback and reporting suspect trees.
• Removal of severely affected trees and proper disposal (chipping or burning where allowed) to reduce local inoculum.

Practical takeaway

• Black walnut owners should vigilantly watch for sudden dieback and consult extension or forestry personnel if TCD is suspected.

Pine shoot beetle (Tomicus piniperda)

Pine shoot beetle attacks the shoots and stems of pines, killing new growth and causing malformation. It is an established pest in parts of the U.S.; Minnesota is within potential range for concern in pine stands and plantations.
Signs and symptoms

• Browned or undersized new shoots, premature needle shedding.
• Small round holes and galleries under the bark of young shoots and stems.

Management options

• Sanitation: avoid moving infested pine material; follow quarantine rules where they apply.
• Maintain stand vigor and diversify planting age classes to reduce widespread damage.

Practical takeaway

• Avoid moving slash or firewood from pine stands that could harbor beetles. Monitor new shoot growth in spring.

What to look for: common diagnostic signs

• D-shaped or round exit holes in bark.
• Serpentine galleries visible under peeled bark.
• Increased woodpecker activity and bark scaling.
• Thinning crown, epicormic shoots, or dieback.
• Presence of frass (sawdust-like material) at the base of trees or in bark crevices.
• Unusual insect bodies (beetles with long banded antennae, metallic green beetles, hairy caterpillars) or egg masses on trunks and materials.
• Sudden dieback in a particular species across an area.

Immediate steps if you suspect an infestation

1. Note and photograph diagnostic signs (exit holes, galleries, adult insects, egg masses, frass).
2. Do not move firewood, logs, or potentially infested materials away from the site.
3. Mark the tree(s) and limit further stress (watering, mulching).
4. Contact local extension, state forestry, or a certified arborist for confirmation and next steps.
5. If an invasive species is confirmed, follow recommended quarantine and disposal procedures to avoid spreading the pest.

Long-term strategies to reduce risk

• Diversify urban and forest tree species to avoid monocultures that are vulnerable to single pests.
• Implement proactive monitoring programs and rapid response plans for municipalities.
• Use preventive chemical treatments for high-value trees in areas with known pests, applied by licensed professionals.
• Maintain tree vigor through proper planting, mulching, pruning, and irrigation.
• Educate the public about not moving firewood, buying nursery stock from reputable sources, and recognizing early signs.
• Support biological control where scientifically vetted and permitted; release programs can help suppress populations over time but are not a standalone solution.

Closing summary

Minnesota faces a suite of invasive insects that threaten the state’s ash, maple, birch, walnut, pine, and other tree species. Emerald ash borer is already widespread and poses immediate economic and ecological impacts, while pests such as the Asian longhorned beetle and thousand cankers disease remain high-risk threats that could cause major additional losses if introduced. Early detection, prudent cultural practices, strategic chemical protection for high-value trees, responsible disposal of wood, and public cooperation against moving potentially infested materials are the most effective tools available.
Vigilance, diversified planting, and quick action when a suspect insect or tree decline is observed will reduce long-term damage and costs. If you care for trees in Minnesota — as a homeowner, landowner, or municipal manager — learning to recognize the signs and having a plan for reporting and response are essential steps in protecting our shared canopy.