Spongy Moth Outbreaks: Potential New Ecological Impacts of an Old Pest

These oak trees in New York’s Hudson Valley died following a drought and an outbreak of spongy moths that defoliated the trees multiple years in a row. Photo by Loren Merrill.

In early June 2023, I was driving down the Taconic Parkway through New York’s Hudson Valley when I noticed an abrupt change to the roadside scenery. The lush forest canopy that had been my traveling companion for the previous three hours disappeared. Many trees were entirely naked, while others bore the tattered remains of their once-leafy attire. It was almost as though I had driven back in time to early April, before bud-break. Mile after mile, I passed by barren forests, astonished by the extent of defoliation.

That evening, I searched online for information about what I had seen. After a little digging, I found a New York State Department of Environmental Conservation post that identified the culprit: the invasive spongy moth, Lymantria dispar dispar, formerly called the “gypsy moth.” And the Hudson Valley was not alone; spongy moth outbreaks were creating pockmarks across the northeastern landscape.

The spongy moth (so called because the egg masses it deposits on tree trunks, buildings, and vehicles are spongy to the touch) arrived in the United States in Medford, Massachusetts, in 1869. The first documented outbreak occurred 20 years later. Thus started a cycle of booms and busts that has since radiated from Medford throughout New England, down the eastern seaboard to the Carolinas, up into southeastern Canada, and across to Minnesota and Iowa.

Spongy Moth Effects on Forests

Despite a reputation for consuming anything and everything in their path, spongy moths are really oak specialists. The spongy moth has an extremely alkaline gut that helps to break down the high tannin levels in oak leaves. When oaks leaves are in short supply (typically because they’ve been denuded by other spongy moth caterpillars), the caterpillars will feed on just about any other woody vegetation available (including conifers), but they quickly become nutritionally stressed.

Newly hatched (first instar) spongy moth caterpillars emerge from egg masses. The large size and number of egg masses indicate the adult moths were in good condition when they produced the eggs. Photo by Mike Fargione / Cary Institute of Ecosystem Studies.

A defoliation event usually lasts for a few weeks, at which point the caterpillars have either begun pupating, or died. Most trees that have been denuded will produce a new batch of leaves by early summer, although there is some evidence that defoliation of the canopy can have a short-term impact on the local forest community, as increased sun exposure on the forest floor can facilitate growth of the understory, alter soil moisture levels and temperature, and warm vernal pools.

The bigger impact, however, is linked to acorn production; oaks that have been defoliated are unlikely to produce an acorn crop that year. Acorns are an important food source for many animals, including numerous insects, a host of small mammals, a handful of birds that includes woodpeckers, jays, and wild turkeys, and medium to large mammals such as porcupines, raccoons, deer, and bears. Many of these species can suffer population declines following a spongy moth outbreak.

Anatomy of an Outbreak

For many people in the Northeast, the recent spate of spongy moth outbreaks seemingly came out of nowhere; the last major spongy moth outbreak in the Hudson Valley, for example, happened about 30 years ago. According to Clive Jones, scientist emeritus at Cary Institute of Ecosystem Studies who has studied the impacts of spongy moths for decades, this 30-year gap between outbreaks was unusual; formerly, outbreaks occurred every 8 to 12 years in the Northeast. During the multidecade quiet spell, spongy moths were present in our forests, but in most areas, they never crossed the population threshold that would have catapulted them into outbreak status.

An old spongy moth egg mass is riddled with holes where the caterpillars emerged in spring. Smaller holes at the edge of the mass may be from parasitoids laying their eggs in the moth eggs. Egg masses that are soft to the touch are generally old (eggs have hatched), and those that are firm generally are unhatched. Photo by Loren Merrill.

Without the eye-catching, headline-grabbing effects of an outbreak – vast swaths of denuded forestland, the air full of parachuting caterpillars, and the patter of caterpillar frass (poop) falling from the trees – the moths faded from our collective consciousness. The recent outbreaks have thrust them back into the spotlight, and researchers are working to better identify the factors that shape their population cycles – and to understand the moth’s impact on our trees and forests.

Spongy moth population cycles depend on the interplay between a suite of biotic (living) and abiotic (nonliving, or physical) factors. The key biotic players that keep spongy moth cycles in check are a fungal pathogen (Entomophaga maimaiga) – introduced from Japan in 1910 and 1911 to help control the moth, which infects the larval (caterpillar) stage; a viral pathogen (nuclear polyhedrosis virus, or NPV) that also infects the caterpillar; white-footed mice and deer mice that eat the moth pupae; and a handful of wasp and fly parasitoids – some native and some introduced – that attack the egg masses or caterpillars. The important abiotic variables that limit population growth are low precipitation and high temperatures in spring. Outbreaks typically occur when the moth’s enemies are at low densities and high-quality food is readily available to larvae.

For example, a dry and/or hot spring keeps the fungus in check. A poor acorn crop the previous year reduces mouse populations. A lapse of several years since the previous outbreak drives parasitoid levels almost to zero. Moreover, a few years without a spongy moth outbreak allows oak trees to recover, resulting in a bountiful supply of the caterpillar’s preferred food. With favorable conditions in place, the moth population can increase rapidly from one year to the next and can overwhelm the few enemies that remain. Once started, an outbreak usually lasts for 2 to 3 years in a particular area, although some outbreaks end within one year, and others may persist for 4 to 5 years.

The moth population typically begins to crash when the caterpillars run out of food (the adults do not eat). Once they have denuded the oak trees, the caterpillars either succumb to starvation or forage on other woody species that have a lower nutritional value for the spongy moth. Food limitation leads to stressed caterpillars, which are much more susceptible to NPV. The virus is always present in the moth population, but it only causes high rates of mortality once caterpillars become stressed. This stress typically coincides with very high caterpillar densities, so NPV spreads rapidly, as does the fungus, Entomophaga maimaiga, which increases in the environment as more caterpillars become infected and spread the fungal spores. In the second and especially third years of an outbreak, the moth’s parasitoids have had a chance to rebound after a year or two of their moth prey being superabundant. The combination of food limitation and the moth’s enemies brings about the population crash, effectively ending an outbreak.

The Old Paradigm

This male spongy moth (left) is distinguishable from the female (right) by its darker color and brushlike antennae. Male spongy moths, like other moths, use these sensitive antennae to detect pheromones and locate females, which do not fly. Photo by Mike Fargione / Cary Institute of Ecosystem Studies.

Does the end of an outbreak also mean the impacts to the trees and forest are over? This is a question that interests Charles Canham, another scientist emeritus at Cary Institute whose work on the spongy moth dates to the 1980s. One of Canham’s first projects upon his arrival at Cary Institute in 1984 was to help set up plots on the property as part of a classic vegetation study. The area was a few years removed from a major spongy moth outbreak in 1981 and 1982 but in the process of setting up their vegetation plots, researchers “could already see the legacy of that defoliation” event, said Canham. At one location on the property, where there had been an understory of hemlock saplings, all the young hemlock trees had died, their needles consumed by desperately hungry spongy moth caterpillars. Hemlocks spend considerable energy to create their needles, which generally last on the tree for three years, and the shade-growing hemlock saplings did not have the carbohydrate reserves to survive what was likely two seasons of defoliation.

Other than the young hemlocks, there was “shockingly little” mortality in the forest, said Canham. He and his colleagues took tree core samples from hundreds of trees to study the effects of defoliation, examining annual tree rings to determine the growing conditions in a given year. Wider tree rings indicate good growing conditions (typically wet years), and narrower rings reflect poor growing conditions (often drought). When Canham and his colleagues examined the tree cores, they found a sharp decline in tree ring width during the years of the spongy moth outbreak. This was convincing evidence that the trees were being stressed by the spongy moth. But by 1984, Canham said, the trees had recovered.

“In general, most trees will withstand two years of defoliation and come back without a problem,” Jones said. But trees facing other stressors – those affected by disease or severe water limitations – are more susceptible to dying following a spongy moth outbreak.

New Worries

The current spongy moth outbreaks are presenting a different – and concerning – picture. “The interesting difference is in the amount of mortality we’ve observed in oaks,” said Jones. That mortality now exceeds 50 percent in some places hit by the spongy moth. What explains the difference in tree mortality from the previous outbreaks to the current ones? Canham and others think it might be drought.

The summer of 2022 was dry throughout much of the Northeast, resulting in drought conditions for many areas, including the Hudson Valley. “The stress from the drought that preceding summer basically tipped the [trees’] carbohydrate reserves,” said Canham, bringing the trees close to some critical energy reserve threshold, beyond which they could not recover. He said a tree’s carbohydrate reserves serve four key functions: they help trees recover from damage or stress; the sugars in the cells help to keep the cells from freezing; they provide the energy in springtime to produce new leaves; and they power the growth of new wood. Two years of defoliation following the drought may have pushed many oaks beyond their carbohydrate reserve tipping point.

A dime provides a sense of the size of these third through sixth instar spongy moth caterpillars. Photo by Mike Fargione / Cary Institute of Ecosystem Studies.

Another Cary Institute scientist, Evan Gora, has joined the team investigating the high rate of oak mortality. Gora is a forest ecologist who studies, among other things, the causes and ecological consequences of tree death, especially big trees. He and postdoctoral researcher Ian McGregor have begun mapping areas affected by spongy moths using drones with very high-resolution cameras.

“We can map up to a couple hundred hectares (ha) in about an hour, with somewhere between 1.5 and 2 cm resolution,” Gora said. “We take photos at very high density, and that allows us to see the 3-dimensional structure of the forest.” They stitch those photos together to create one large image, and then use software McGregor designed to identify where trees are being defoliated on the landscape. “The idea is to test where mortality is happening,” said Gora, “and see if we’re seeing mortality specifically in places that” experienced more intense drought conditions.

Does the recent high-mortality event portend a long-term shift in the spongy moth’s impact on our forests, and on oak trees in particular? That’s a difficult question to answer.

“What worries me is trying to apply science developed 40 years ago in a different climate and a different time to outbreaks now,” Canham said. Scientists studying whether and how the dynamics between spongy moths and oaks may be changing have to contend with not only high levels of uncertainty regarding climate forecasts (especially at the state and county level), but also a spate of new pathogens, including oak wilt (a fungal pathogen), sudden oak death (a water mold), anthracnose (fungal), and bacterial leaf scorch. Canham is also worried about climate change or other stressors throwing off the existing balance between oaks and some of their natural enemies, which could lead to additional oak mortality. The loss of these important canopy trees and their acorns may have far-reaching consequences for our northeastern forests.

According to Jones, who has helped generate spongy moth forecasts for parts of the Hudson Valley, “This outbreak is almost certainly over in the Northeast.” But big questions remain regarding the long-term impacts of the most recent spongy moth outbreak and what future outbreaks will look like.