Most tree pest activity occurs during the warmer months. Oak bark beetles, for example, start flying in April, while lanternfly nymphs and magnolia scale nymphs both start feeding in May or June. Most other local insects follow similar activity patterns.
But there’s at least one notable exception: the winter moth.
Winter moth was once one of southern New England’s most destructive early-season defoliators. Although biological control has greatly reduced its populations across much of Massachusetts, localized outbreaks can still threaten trees and fruit crops in parts of New England.
Read on to learn more about this insect and the best ways to protect your trees.
The Basics: What Is Winter Moth?
To protect your trees from any insect pest, you’ll need to learn a little about its biology, life cycle, and feeding habits.
Native and Introduced Range
Winter moth (Operophtera brumata) is a destructive invasive pest that has become established throughout much of the northeastern United States, including Massachusetts and Rhode Island, as well as parts of Canada and the Pacific Northwest.
While winter moth can cause outbreaks in its European homeland, populations are usually kept in check by an established community of predators, parasitoids, and pathogens.
Importantly, it’s not the moth itself that damages trees; it’s the caterpillars that cause trouble. They feed voraciously on leaves and buds, and they are capable of stripping the foliage from trees very quickly.
The Winter Moth’s Life Cycle
One of the things that makes winter moth unusual is its life cycle.
Adult moths emerge in late fall and early winter, between November and January, when most other insects are inactive. The wingless females climb tree trunks to lay eggs on the tree’s bark, where the eggs remain throughout the winter.
Like many other insect eggs, the young winter moth caterpillars begin hatching in the spring. But they typically do so much earlier than most other tree-feeding pests do. In fact, they sometimes begin hatching before the trees leaf out.
The tiny caterpillars immediately begin feeding on developing buds, flowers, and young leaves. This early-season feeding can significantly reduce a tree’s ability to produce healthy foliage, leading to considerable stress — particularly when attacks occur in consecutive years.
By late May or early June, mature caterpillars drop to the ground, burrow into the soil, and pupate.
Preferred Hosts for Winter Moth
Part of the challenge winter moths present is their caterpillars’ opportunistic feeding habits. They feed on a remarkably broad range of deciduous trees, shrubs, fruit crops, and ornamental plants.
However, some species are attacked more frequently or suffer more significant damage than others, making them especially important to monitor.
These trees include:
- Oaks (Quercus spp.): Oaks are among winter moth’s preferred hosts. Caterpillars feed on expanding buds and young leaves, and repeated defoliation can weaken trees over time.
- Maples (Acer spp.): Red (A. rubrum), sugar (A. saccharum), and other maple species are commonly attacked, particularly as new leaves emerge in spring.
- Apples (Malus spp.): Winter moth can damage both ornamental crabapples and fruit-producing apple trees by feeding on developing leaves, flower buds, and blossoms.
- Elms (Ulmus spp.): American elm (U. americana) and several of its closest relatives may suffer substantial foliage loss during heavy outbreaks.
- Blueberries (Vaccinium spp.): Commercial and backyard blueberry plants alike may suffer reduced flowering and fruit production when caterpillars feed on buds and blossoms.
- Cherries and Plums (Prunus spp.): Both ornamental and fruiting species can experience damage to newly emerging foliage and flowers.
Winter Moth Threat Level: How Concerned Should You Be?
Although healthy trees can usually recover from an occasional defoliation (whether caused by winter moth or some other pest), repeated infestations can cause significant stress. This, in turn, can leave trees more susceptible to drought, disease, and other insect pests.
And unfortunately, trees that are already suffering stress from poor growing conditions or existing disease issues will have a much harder time recovering. So, while it is usually a good idea to contact a plant health care expert to assess any tree under attack from winter moth, it is especially important to do so for trees that:
- Have sustained structural damage during storms.
- Are growing in compacted, poorly drained, or otherwise unsuitable soil.
- Are already affected by a bacterial or fungal pathogen.
- Have been planted or transplanted within the last two years.
- Are growing in unsuitable locations, such as excessively shaded, exposed, or drought-prone sites.
Additionally, it’s always wise to act quickly for any trees that have significant value to you or your property.
Signs of Winter Moth Activity
Winter moth damage typically becomes noticeable in early spring, just as trees begin producing new leaves. Because the caterpillars hatch around the same time buds begin to open, many of the first symptoms appear before the canopy has fully leafed out.
Common signs of winter moth activity include:
- Small Holes in Young Leaves: Newly emerged leaves often develop tiny holes as the young caterpillars begin feeding.
- Tattered or Distorted Leaves: Feeding that begins while leaves are still folded inside the buds may leave expanded foliage riddled with holes, ragged edges, or missing tissue.
- Partially or Completely Defoliated Trees: Heavy infestations can strip much or all of a tree’s foliage, sometimes by late spring.
- Damaged Buds & Flowers: Caterpillars commonly feed on swelling buds, flowers, and developing shoots before leaves have fully expanded.
- Fine Silken Threads: Young caterpillars produce silk that allows them to disperse on air currents, a behavior known as ballooning.
- Tiny Inchworms: Newly hatched winter moth caterpillars are extremely small and initially dark in color. As they grow, they become pale green inchworms with faint white stripes along their sides.
If you notice any of these signs, it’s important to contact an ISA-certified arborist promptly to have an evaluation carried out.
Support & Treatment Options for Winter Moth in New England
Winter moth infestations don’t always require treatment. Healthy, established trees can often survive a single season of defoliation and produce a second flush of leaves later in the year.
However, high-value, young, or newly transplanted trees, as well as those that suffer from repeated defoliations, may require professional intervention.
Potential management strategies include:
- Monitor During Bud Break: Because winter moth caterpillars hatch around the time that the new buds will begin to open, early spring is the best time to inspect susceptible trees for feeding damage.
- Support Tree Health: Simply ensuring that your trees are healthy can help. This includes properly watering them during dry periods, placing a layer of mulch beneath the canopy, and avoiding unnecessary root disturbance.
- Apply Targeted Treatments When Appropriate: Products containing Bacillus thuringiensis var. kurstaki (Btk) or spinosad may be used to manage winter moth caterpillars. Btk is most effective against young larvae, while spinosad can control a broader range of caterpillar sizes.
- Use Dormant Oils Selectively: Horticultural dormant oils may help reduce overwintering eggs on smaller trees, shrubs, and fruit crops. However, they are generally less practical and less effective for large landscape trees, where complete coverage is difficult to achieve.
It’s also worth noting that, in Massachusetts and much of southern New England, a beneficial parasitoid fly (Cyzenis albicans) has become established and is helping suppress winter moth populations.
If you notice defoliation, feeding on newly emerging leaves, or green inchworms in your landscape, having your trees evaluated early in the season can help protect their long-term health.
An Approach That’s No Longer Recommended
You may have seen some sources recommend wrapping tree trunks with sticky bands to trap wingless females as they climb to lay eggs. But while this was once a common management practice, it is no longer recommended as a control strategy.
Sticky bands often have limited effectiveness, may trap beneficial insects, and don’t significantly reduce winter moth populations.
Protect Your Trees from Winter Moths
Although biological control has dramatically reduced winter moth populations across much of Massachusetts, localized infestations can still occur, and individual trees may suffer significant damage. That’s why it’s important to proactively support your trees and get professional help when necessary.
If you’re concerned about winter moths — or you’ve already seen the green caterpillars on your property — contact Tree Tech Inc.
We’ll have one of our ISA-certified arborists perform an on-site evaluation of your trees, identify whether winter moth is responsible for the damage, and recommend the most effective treatment or management strategy for your landscape.
Sources and References
- UMass Extension – “Winter Moth Identification & Management”
Detailed extension fact sheet on winter moth (Operophtera brumata) identification, life cycle, host plants, look-alike species, and egg, caterpillar, and biological control management.
https://www.umass.edu/agriculture-food-environment/landscape/fact-sheets/winter-moth-identification-management - URI Biocontrol Lab – “Winter Moth”
University of Rhode Island Cooperative Extension overview of winter moth biology, damage, and the Cyzenis albicans parasitoid biocontrol program monitored across Rhode Island, Massachusetts, and Connecticut.
https://web.uri.edu/biocontrol/projects/winter-moth/ - Maine Forest Service – “Winter Moth”
State forestry agency profile covering winter moth’s spread into coastal Maine, host range, life cycle, its resemblance to the native Bruce spanworm, and control options.
https://www.maine.gov/dacf/mfs/forest_health/insects/winter_moth.htm - Mass Audubon – “Winter Moths”
Conservation organization guide describing winter moth life cycle, defoliation impact on New England trees, and homeowner management options including biological control.
https://www.massaudubon.org/nature-wildlife/insects-arachnids/winter-moths