Noticing Winter Moth in MA? You’re Not the Only One.

Overview
If you’ve been managing insect pests of trees & shrubs in eastern and coastal Massachusetts for the last 3 or 4 decades, you’re likely familiar with Operophtera brumata, also known as winter moth, a springtime geometrid (inchworm) defoliator of oak, maple, and apple but also blueberry, basswood, cherry, ash, white elm, crabapples and others. Tiny, green caterpillars wriggle into the expanding buds of these host plants, damaging flowers that turn to fruit for agricultural crops and leaving shade trees and ornamentals with holes in their leaves. At the height of their outbreak in the late 1990s (on Cape Ann) and early 2000s elsewhere in the state, this equated to persistent, annual defoliation. That is, until 2018 when Dr. Joseph Elkinton Professor in the Department of Environmental Conservation at UMass Amherst, now emeritus, and cooperators converted the winter moth to a non-pest. This was accomplished with a successful biological control program using a parasitic fly, Cyzenis albicans, which is host-specific to the winter moth. 

With that historical review in mind, it’s no surprise that small, light brown moths with fringed wings seen flying about Massachusetts landscapes on cold days from mid-late November through January can easily put observers on alert. Arborists and pest managers in eastern Massachusetts report a mix of springtime inchworm defoliators most growing seasons. Talk of these defoliators has been consistent in recent years, but since November 2025 UMass Extension and other regional groups monitoring tree & shrub insects have received emails, questions, and reports via iNaturalist about suspicious little brown moths flying in Massachusetts and coastal locations in New England. So what’s going on with the winter moth population?

2025 Observations
Winter moth (Operophtera brumata) and native look-alikes Bruce spanworm (Operophtera bruceata) and fall cankerworm (Alsophila pometaria) all have winged, light brown and variously marked adult male moths that can be seen flying roughly from November through January. This unusually timed activity (for an insect) along with their attraction to nighttime lights can make them very noticeable. For all three species, the adult females are virtually wingless and do not look like a typical moth. They may be found crawling along the ground and at the base of trees. While some geographic trends about the location of the population of each of these species in Massachusetts, particularly for winter moth and Bruce spanworm, are known in the literature - identification to species is not a simple process. Often, this requires a skilled taxonomist who specializes in the identification of moths and has the knowledge required to dissect the males of each species. Even then, adult males of the two species are extremely difficult to distinguish, and DNA analysis is the only known accurate method for differentiating these two species.

Arborists have reported flights of moths in Brookline, Wellesley, Newton, and Sturbridge, MA and in Martha’s Vineyard, MA as well as Cumberland, RI in November & December of 2025. In the mainland MA and RI observations, groups of 30-50 moths were reported. On Martha’s Vineyard, in a 3.3 mile stretch, over 525 moths were counted. Over 1,000 reports of possible winter moths, Bruce spanworm, or fall cankerworm have been made via iNaturalist thus far this season across the region. These reports are based on individual’s observations, not trapping data, and represent an increase in reports from the winters of 2023 (n = 463) and 2024 (n = 696).

Geographic Trends
Historically, eastern and coastal MA have experienced the majority of winter moth activity. This is well documented in data collected by Dr. Elkinton and collaborators (Elkinton et al., 2015); see Figure 2 on page 7 in that publication). The central part of Massachusetts is where winter moth and Bruce spanworm are capable of hybridization, and western Massachusetts is primarily home to Bruce spanworm. 

After its initial detection in eastern Massachusetts, winter moth quickly spread westward reaching the Fitchburg area, before its spread then slowed considerably (Andersen et al. 2022). Since then, occasional winter moth individuals have been detected in traps as far west as Shelburne Falls, MA. However, the combination of colder winter temperatures and increased hybridization with Bruce spanworm in the central and western parts of the state, appear to be restricting the establishment of large populations in this region and, likely as a result, defoliation has not been reported there.  Despite being “winter active”, studies from Maine and Massachusetts (Chandler et al., 2023) have shown that winter moth males do not fly when night temperatures are below 32°F/0°C, and that early-season snowfall can prematurely terminate flight for the season. The snowfall we received during the first week of December 2025 in central and western MA is a great example of why winter moth has had limited success establishing large populations in central and western Massachusetts compared to coastal areas of the state. In addition to the limits imposed by freezing temperatures, early season snow can also prevent the flight of winter moth if it melts and then freezes again, creating a layer of ice that emerging males and females cannot crawl through. In Europe, the flight of winter moth males is thought to be finely timed to occur after the departure of potential avian predators but before the onset of early season snow. In coastal areas of New England where it primarily rained and did not snow earlier in December 2025, flight of adults continued. Aerial survey from the MA Department of Conservation and Recreation continues to support this trend, with no significant defoliation from winter moth detected in central or western Massachusetts in surveys conducted in 2022-2024. Even in eastern MA locations where winter moth populations were historically high, defoliation via aerial survey was not documented in 2022-2024. However, in 2023, fall cankerworm defoliation was detected in the above statewide survey.

Currently, Dr. Jeremy Andersen, an Assistant Professor in the Department of Environmental Conservation at UMass Amherst, reports that western areas in Rhode Island are experiencing a true winter moth outbreak. These areas are not close to the sites where Dr. Elkinton and collaborators released C. albicans in eastern Rhode Island prior to 2021. Andersen reports thousands of moths caught in traps, indicative of a heightened population. This is very different from the dozens to even hundreds reported in parts of Massachusetts. Efforts to promote Cyzenis albicans, the biological control agent of winter moth, at these outbreak sites in Rhode Island is currently underway in collaboration with Allana Russell from the Rhode Island Department of Environmental Management.

Cyzenis albicans: A Biological Control Success Story
In 2018, the Elkinton Lab declared major progress in the reduction of the winter moth population in Massachusetts. This was after 14 years spent releasing thousands of parasitic flies (Cyzenis albicans) at 44 sites across New England. At 38 of these sites, establishment of the fly was documented. Additionally, data collected from sites in Massachusetts reflected that as Cyzenis albicans densities increased, winter moth (Operophtera brumata) densities decreased at the same locations (Elkinton and Roehrig, 2023; see Figure 9.3 on page 244 of that publication). These trends mirrored the successes of the biological control of winter moth in Nova Scotia, Canada in the 1960’s - the model for reducing winter moth using this parasitic fly. Defoliation by winter moth often exceeded 10,000 acres per year between 2003 and 2015 but dropped to zero after that (see Figure 3 from Elkinton et al. 2021 on page 8 of that publication).

As such, it’s not surprising that the population dynamics of winter moth in Massachusetts post Cyzenis albicans establishment appear to be also mirroring those reported in Nova Scotia by Roland and Embree, 1995 (see Figure 1 on page 480 in that publication). Populations of winter moth with C. albicans are known to fluctuate every 8-9 years in Nova Scotia and every 9-10 years in the UK. Bruce spanworm and fall cankerworm also experience natural population increases and subsequent decreases. The increase in wintertime flying moths in November and December 2025 in Massachusetts appears to be following this trend.

What Might Spring 2026 Look Like?
Springtime inchworm defoliators are an annual phenomenon on trees and shrubs in Massachusetts, and are a sign of a healthy, functioning ecosystem. This can include a mix of native fall and spring cankerworm caterpillar feeding too. As for a serious introduced pest, the winter moth, while defoliation by these caterpillars may be noticed in eastern, coastal, and island locations in Massachusetts, the extent of the defoliation one might expect in 2026 is not anticipated to reach levels that occurred before 2018.

It is important to understand that populations of winter moth following the establishment of C. albicans vary in density from place to place. In most locations in eastern Massachusetts and Rhode Island, it is almost impossible to collect more than a handful of winter moth caterpillars in May when they are active (Elkinton, Personal Communication). At other locations, such as south-coastal Maine, it is easy to collect hundreds of them and leaf tattering is notable on deciduous trees at these sites, even though C. albicans is present. 

We believe that it is variation from place to place in the abundance of various insect and mammalian predators (shrews, for example) feeding on winter moth pupae in the soil over the summer months that accounts for this variation. Together they cause at least 90% mortality of winter moth pupae. Broadley et al. 2022 describe the impact and importance of these soil-living predators and how they interact with C. albicans. That parasitoid only causes high mortality when winter moth populations reach high densities and it reduces those densities down to levels at which the predators in the soil can keep them in check. So both the predators and C. albicans are required to maintain winter moth densities below outbreak levels. 

While winter moth has not been eradicated from Massachusetts by the biological control agent (nor was this the expectation), the population, in most cases, will remain below levels that are significantly damaging to the overall health of trees and shrubs. In select scenarios where ornamental trees or shrubs may be impacted by winter moth caterpillar feeding in eastern, coastal, and island areas in Massachusetts integrated pest management options are available for individual trees and shrubs. If you need help identifying and managing a defoliating insect pest on a tree or a shrub, you can submit a sample to the UMass Plant Diagnostics Laboratory or contact a local trained arborist or landscape professional for assistance. Remember, accurate identification of the insect causing feeding damage that is observed reducing the overall health of a tree or a shrub is always recommended prior to making management decisions.

References

• Andersen, J.C., Havill, N.P., Boettner, G.H., Chandler, J.L., Caccone, A. and Elkinton, J.S., 2022. Real‐time geographic settling of a hybrid zone between the invasive winter moth (Operophtera brumata L.) and the native Bruce spanworm (O. bruceata Hulst). Molecular Ecology 31: 6617-6633. [PDF]
• Broadley, H.J., Boettner, G.H., Schneider, B. and Elkinton, J.S., 2022. Native generalist natural enemies and an introduced specialist parasitoid together control an invasive forest insect. Ecological Applications, p.e2697. [PDF]
• Chandler, J.L., Elkinton, J.S., Boettner, G.H. and Andersen, J.C., 2025. Differences in diel timing of flight may reduce hybridization between native and introduced geometrid moth congeners. Journal of Insect Behavior, 38(1), p.6. [PDF]
• Elkinton J, Boettner G, Broadley HJ, Reardon R & Weeks Jr. RD. 2018. Biological control of the winter moth in northeastern North America. USDA Forest Service, Forest Health Assessment and Applied Science Team-2018-03. 9pp. [PDF]
• Elkinton, J., G. Boettner, A. Liebhold and R. Gwiazdowski. 2015. Biology, spread and biological control of winter moth in the eastern United States. USDA Forest Service, FHTET-2014-07.[PDF]
• Elkinton, J.S. G. H. Boettner, H.J. Broadley. Biological control success against winter moth in the northeastern United States. Ch 20 In: Van Driesche, RG; Winston, RL; Perring, TM; Lopez, VM, editors. Contributions of classical biological control to the US food security, forestry, and biodiversity. FHAAST-2019-05. Morgantown, WV: US Department of Agriculture, Forest Service: 212- 223. [PDF]
• Elkinton J.S., Boettner G.H., Broadley H.J. 2021. Successful biological control of winter moth, Operophtera brumata, in the northeastern United States. Ecological Applications. 31: p.e02326. [PDF]
• Elkinton, J., Roehrig, A. 2023. Foliage Feeders. In: D. Allison, J., Paine, T.D., Slippers, B., Wingfield, M.J. (eds) Forest Entomology and Pathology. Springer, Cham. [PDF]
• Roland, Jens & Embree, D. (1995). Biological Control of the Winter Moth. Annual Review of Entomology. 40. 475-492. 10.1146/annurev.en.40.010195.002355. 

Authors
Tawny Simisky, Entomologist, UMass Extension
Dr. Joseph Elkinton, Emeritus Professor in the Department of Environmental Conservation, UMass Amherst
Dr. Jeremy Andersen, Assistant Professor in the Department of Environmental Conservation. UMass Amherst