What Are Gypsy Moths and Winter Moths (Spongy Moths)?
What many people still call gypsy moths are now known as spongy moths, a name change adopted to reflect both accuracy and sensitivity. Regardless of the name, their impact across Northern New England has been significant. These insects are now widespread throughout New Hampshire and Maine, with population spikes occurring during certain environmental conditions.
Spongy moths were introduced to the United States in the late 1800s after an experiment involving cross breeding silk moths and these months went wrong, causing the European moths to escape into North America. Once established, they spread rapidly and became one of the most destructive defoliators of hardwood forests in the region.
Their life cycle moves quickly. Eggs hatch in May, producing larvae just a few millimeters long. Within roughly two months, they can grow to over two inches in length. By the time most homeowners notice them, defoliation has already begun.
As the caterpillars grow, they produce silk threads that allow them to disperse from tree to tree, expanding the infestation. By June, they transition into moths. Their egg masses, often shaped like a flattened oyster shell and roughly the size of a quarter, can be found on tree bark, stone walls, and outdoor structures waiting to hatch the following spring.
Signs of Infestation
Timing is everything when identifying a spongy moth outbreak. Caterpillars hatch in May, and by late June, heavy populations can strip a tree of nearly all its leaves.
One of the most striking signs of a high population is sound. In heavily infested forests, especially oak stands, you can actually hear the caterpillars feeding. Their droppings fall steadily, sounding like rain. Yes, they poop so much it sounds like rain! Their poop collects on decks, patios, and outdoor furniture. Spongy moth excrement looks like tiny, dark, soil-colored pellets or grains. Homeowners may also notice caterpillars dangling from silk threads, drifting through the air as they move between trees.
The most apparent sign of an infestation is the sudden defoliation of trees. Defoliation is the process of leaves being stripped or shedded off of a tree. Leaves start to appear patchy and then all at once seem to disappear overnight. Oaks are the primary target of spongy moths. In parts of North Conway and along the Kancamagus Highway, widespread oak mortality has followed repeated infestations.
The 2022–2023 Spongy Moth outbreaks across New Hampshire demonstrated how quickly damage can escalate when environmental conditions align. Large areas of the state, especially southern and central NH, saw widespread defoliation as caterpillars stripped leaves from oak trees. It’s important to note birch, maple, and other hardwood trees were also swept up as victims. Entire hillsides appeared brown by early summer as repeated feeding cycles weakened trees across forests, neighborhoods, and wooded properties. Devastation could be seen high up in the sky from planes as pictured here. By late 2023, populations began to decline in parts of the state as weather patterns shifted and natural controls rebounded.
Outbreaks like the ones in NH are why its so important to get populations under control. Once populations reach a certain threshold, treatment becomes more difficult and outbreaks more common.
Long-Term Impact on Trees
Not every tree responds the same way to defoliation. Genetics, age, species, site conditions, and overall health all play a major role in determining whether a tree can survive a heavy feeding event. Some trees have deeper energy reserves and stronger root systems, allowing them to tolerate short-term leaf loss and recover more effectively. Others, particularly older or poorly sited trees, may already be operating with limited reserves before defoliation even begins. Trees that are already stressed by compacted soils, poor drainage, root damage, or competition for water and nutrients are far less likely to recover.
If a tree is otherwise healthy, it may be able to produce a second flush of foliage later in the summer, partially restoring its ability to photosynthesize and rebuild stored energy. It is important to note that this regrowth comes at a cost. Producing new leaves requires significant energy, and repeated defoliation over consecutive years can quickly exhaust a tree’s reserves.
Climate change has increased this vulnerability across much of New England. Extended dry periods, higher summer temperatures, and irregular rainfall patterns place additional stress on trees, even in years without severe insect pressure. Drought stress limits water uptake and reduces photosynthetic efficiency, making it harder for trees to rebound after defoliation. As a result, infestations tend to cause more severe damage, and tree mortality becomes more widespread when environmental stress and insect pressure occur at the same time.
Treatment Options
Effective treatment depends on timing. Intervening early, while caterpillars are still small, is critical. Two commonly used treatments include Bacillus thuringiensis (Bt) and acelepyrine-based products, which disrupt calcium metabolic pathways in the insect. These treatments are highly targeted, producing minimal environmental impact while effectively controlling the caterpillars before large-scale defoliation occurs.
Applying treatments too late significantly reduces effectiveness, reinforcing the importance of early monitoring and professional evaluation.
Prevention
Long-term protection starts with tree health. As climate change causes more extreme weather patterns in our state and across the world, spongy moth outbreaks will become more common. Keep your tree well nurished and property maintained. A Well-nourished, properly maintained tree is far more resilient when faced with defoliation. You can learn more about how to keep trees healthy in the P.C. Hoag & Co. Plant Healthcare Guide.
Remember to monitor too. While we can not control the weather, regular monitoring allows infestations to be identified early, when intervention is most effective. Once insects are present in noticeable numbers, treatment decisions should be made quickly and based on the specific tree species, site conditions, and infestation intensity. Working with experienced arborists helps ensure that responses are timely, appropriate, and grounded in science rather than reaction.
