Types Of Invasive Pests Threatening New Hampshire Trees

Overview

New Hampshire forests and urban trees face a growing array of invasive pests that damage canopy species, alter ecosystems, and impose significant economic costs. Some of the most destructive agents are insects and nematodes that attack specific tree genera, while others facilitate secondary problems like disease and windthrow. Understanding which pests pose the greatest risk, how to spot them early, and what practical steps landowners and communities can take is essential to slow spread and preserve tree health.

Why invasive pests matter in New Hampshire

Invasive pests can:

• Rapidly kill large numbers of a preferred host species and reduce forest diversity.
• Increase storm and ice damage risk as weakened trees break or fall.
• Reduce wildlife food and habitat when dominant species decline.
• Raise costs for municipalities, landowners, and utilities for removals, treatments, and replanting.

Early detection and coordinated action can limit impacts, but many pests spread through human activity, especially by moving firewood, nursery stock, or untreated wood products.

Major invasive pests to watch

Emerald ash borer (EAB) – Agrilus planipennis

Emerald ash borer is one of the most destructive invasive insects in North America for ash trees.
Signs and symptoms

• Epicormic sprouting, thinning crowns, and branch dieback in ash trees.
• S-shaped larval galleries under the bark and D-shaped exit holes on the bark surface.
• Woodpecker feeding activity can be concentrated on infested ash.

Host and impact

• Attacks all North American ash species. In infested areas, mortality rates can approach 100% without treatment.

Management options

• Prevent movement of infested wood; follow local quarantine guidance.
• Trunk injection or soil-applied systemic insecticides (emamectin benzoate, dinotefuran, imidacloprid) can protect high-value ash for several years when applied by professionals.
• Biological control with introduced parasitoids has reduced population growth in some areas but is not a stand-alone solution.

Hemlock woolly adelgid (HWA) – Adelges tsugae

HWA targets eastern and Carolina hemlocks and has already altered many New England hemlock stands.
Signs and symptoms

• White, woolly egg masses at the base of hemlock needles, often visible year-round.
• Needle loss, twig dieback, thinning crowns, and tree mortality within 4-10 years for severely infested trees.

Host and impact

• Primarily affects eastern hemlock and Carolina hemlock. Loss of hemlock changes stream shading, water temperature, and wildlife habitat.

Management options

• Systemic insecticides (soil or trunk-applied imidacloprid, or trunk-injected emamectin benzoate) are effective when used correctly and timed properly.
• Biological control agents (predatory beetles) are being used in some areas to reduce HWA densities over time.
• Prioritize treatment of hemlocks that provide critical shade for streams or are of high aesthetic or cultural value.

Spongy moth (formerly gypsy moth) – Lymantria dispar

Spongy moth outbreaks cause severe defoliation on a range of hardwoods and create stress that allows other pests and diseases to take hold.
Signs and symptoms

• Caterpillars with distinct blue and red dots feed on leaves in late spring and early summer.
• Severe defoliation may occur across large areas; trees may resprout but repeated defoliation can kill trees.
• Silken webs and egg masses on trunks, branches, under bark, and on outdoor objects.

Host and impact

• Prefers oaks but will feed on many hardwoods and some conifers. Repeated outbreaks lead to long-term forest composition changes.

Management options

• Remove and destroy egg masses in winter when accessible.
• Use burlap bands in spring to trap and remove caterpillars on small trees.
• For larger infestations, biological or chemical controls may be applied by professionals. Bacillus thuringiensis kurstaki (Btk) targets young caterpillars and is used in some public and private applications.

Asian longhorned beetle (ALB) – Anoplophora glabripennis

ALB attacks a wide range of hardwoods and has led to eradication efforts where detected.
Signs and symptoms

• Large, round exit holes in the trunk and branches and sawdust-like frass at gallery sites.
• Adult beetle appearance: smooth, glossy black with white spots and very long banded antennae.

Host and impact

• Attacks maples, birches, elms, willows, and many others. Infested trees must often be removed to prevent spread.

Management options

• Rapid detection and reporting are critical; follow local eradication protocols if ALB is suspected.
• Do not move firewood, nursery stock, or untreated wood from suspected areas.

Winter moth – Operophtera brumata

Winter moth caterpillars emerge early in spring and chew emerging buds and leaves, especially on maples, oaks, and fruit trees.
Signs and symptoms

• Early-season defoliation and bud damage, stunted leaf growth, and reduced vigor with repeated defoliation.
• Small inchworm-like caterpillars in spring; pupae in soil.

Host and impact

• Can cause repeated defoliation and weaken trees over several years, making them susceptible to secondary pests.

Management options

• Pheromone disruption and biological controls (parasitic flies) have been used in parts of New England.
• Sticky bands and burlap wraps can reduce caterpillar numbers on small trees.

Beech leaf disease and beech bark disease complex

Beech leaf disease appears as patterned dark bands between leaf veins and leads to leaf curling and decline. Beech bark disease results from the combined effect of a scale insect and a fungal pathogen.
Signs and symptoms

• Distorted, dark-striped leaves for beech leaf disease.
• Cracked and pitted bark followed by fungal fruiting in beech bark disease; canopy decline and mortality are common.

Host and impact

• American beech is highly susceptible. Loss of beech reduces mast production for wildlife and changes forest structure.

Management options

• Monitor and remove badly affected trees to limit spread and for safety.
• Maintain tree vigor through proper pruning, watering, and avoiding injury.

Detection and monitoring: what to look for

Early detection is crucial. Regularly inspect trees for the following:

• Bark damage, unusual exit holes, sawdust-like frass, or galleries under bark.
• Unexpected thinning of crowns, epicormic shoots, or widespread dieback.
• Presence of unusual insects, caterpillars, or conspicuous egg masses.
• White woolly deposits at needle bases on hemlock.
• Clusters of defoliation matching pest life cycles in spring or summer.

Use a simple routine: walk the property every spring and late summer, inspect high-value trees, and remove or report suspicious signs to local extension or forestry authorities.

Practical management strategies for landowners

1. Prioritize assets.
2. Identify high-value trees (shade trees, specimen trees, streamside trees) that merit monitoring and treatment.
3. Keep thorough records of treatments, dates, and tree conditions.
4. Follow an integrated approach.
5. Combine sanitation (removing dead wood and infested materials), chemical protection when appropriate, and biological controls where available.
6. Timing matters.
7. Apply soil or trunk systemic insecticides before pest feeding periods for best effect; injectables are often timed in spring or early summer.
8. Use qualified professionals.
9. Many systemic treatments require licensed applicators. Work with certified arborists to ensure correct dosing and minimal environmental impact.
10. Prevent spread.
11. Never move firewood or untreated logs. Buy firewood locally and season it properly, or use certified kiln-dried wood.
12. Diversify plantings.
13. Avoid monocultures of a single genus when replanting; mix species and ages to increase landscape resilience.

Community and policy actions

Coordinated community responses are more effective than individual efforts alone.

• Municipalities should develop and update pest response plans and invest in surveillance.
• Nurseries and landscapers must source pest-free stock and follow best practices.
• Public education campaigns reduce risky behaviors like moving firewood or unknowingly transporting pests on vehicles and outdoor equipment.
• Support research and biological control programs working to establish natural enemies where safe and effective.

Economic and ecological consequences

Invasive pests reduce property values, increase municipal tree management costs, and require long-term investments in replanting. Ecologically, loss of dominant species (ash, hemlock, beech) changes understory composition, affects aquatic systems through altered shading, and reduces food availability for wildlife. Prevention and early action are far more cost-effective than large-scale removals and replacements.

Final recommendations

• Learn to recognize the specific signs of the pests described here and inspect trees twice a year.
• Report suspect pests to local forestry authorities or extension services for confirmation and coordinated response.
• Use professional arborists for diagnosis and treatment planning, especially for systemic insecticide injections.
• Do not move firewood or untreated wood. Buy local firewood and dispose of infested material responsibly.
• When replanting, choose diverse, native species to reduce vulnerability to future pests.
• Support community surveillance and management programs; individual vigilance combined with municipal action is the best defense.

Maintaining healthy trees in New Hampshire requires vigilance, stewardship, and informed action. By recognizing the major invasive threats, taking early and appropriate management steps, and working with neighbors and agencies, landowners can reduce the spread and long-term impact of these pests on the region’s forests and urban canopy.