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Spongy moth chronicles

Observations on spongy moth activity in the Cary forests throughout the 2024 season.

Photo by Camya Robinson

michael fargione
Manager of Field Research & Outdoor Programs

Entries

May 1: Caterpillars Raining DownMay 8: Settling Down After Moving On
May 16: Can I See Your ID?May 22: “Have you ever seen the rain, coming down on a sunny day?”
May 30: A Party for the AgesJune 12: The Beginning of the End?


During the summer of 2023, spongy moth caterpillars caused severe defoliation of oak trees in parts of Southeastern New York. Questions and concerns about the likelihood and impacts of additional damage this coming year have prompted us to share our observations on spongy moth activity in the Cary forests during the 2024 season. Our 2000-acre research campus in Millbrook, New York is about 70% forested, and dominated by oak, hickory, maple, hemlock, and white pine trees. Major outbreaks have been observed periodically at Cary with peak emergences in 1980, 1992, and 2023. It remains to be seen if the spongy moth population will increase more this year or if it will collapse due to pathogens and stress from food shortages.

Background

The spongy moth (Lymantria dispar dispar) is an introduced insect accidentally released in Massachusetts in 1869.  Formerly known as gypsy moths, spongy moths have slowly spread across the Northeast. Larval stages (caterpillars) feed on a wide variety of trees, but oaks are the preferred food source. During periodic outbreaks, the insect can completely defoliate large areas of forests, and may severely damage shade and ornamental woody plants. Caterpillar droppings (frass) can significantly deter outdoor enjoyment during severe outbreaks.

Spongy moth populations rise and fall based on disease and predator pressures, with outbreaks occurring about every 10 years in New York. Cary Institute scientists have been studying the spongy moth since the early 1980s, and have unraveled some of the interrelationships between the moth, mouse populations, acorn abundance, and moth pathogens.  

During 2022, spongy moth density in parts of Southeastern NY caused noticeable defoliation. Severe and more widespread damage occurred during 2023. Abundant egg masses were visible in our area during the winter of 2023–2024. It remains to be seen if this insect’s population will continue to build and cause more severe defoliation this year, or if disease will cause it to collapse.

Spongy moth observations from the Cary forests

May 1, 2024: Caterpillars Raining Down

 We have been keeping an eye on the spongy moth egg masses overwintering at Cary Institute so that we could note when things start happening this spring. The time has arrived! Spongy moth caterpillars (first instar larvae) were seen hatching at the beginning of the week of April 29. The translucent bodies of the newly hatched insects quickly turned black while they remained clustered around the egg mass from which they emerged. Within a day, larvae were seen ballooning from their original hatching site, catching the wind with the silken threads they produced. I first noticed these migrating juveniles on my car. Later, they appeared on the sides of buildings. Now, every forest walk in outbreak areas may leave you with a hitchhiking caterpillar or two.

spongy moth hatch
Newly hatched first instar caterpillars. Credit: Mike Fargione/Cary Institute

The insects hatched in synchrony with the appearance of leaves on several oak species, their favorite foods. These small larvae will try to make their way to the upper portions of their host tree, where they will begin feeding on the tender new leaves. Feeding damage at first appears as small holes in the leaf tissue. Later, as they molt into larger instars, their feeding will remove sections of the leaf until pieces fall to the ground or they consume all but the veins. Spongy moths will remain as caterpillars for about seven weeks, going through five (males) or six (females) molts before pupating and then turning into adult moths.

First instar caterpillars on oak leaves, their favorite food. Credit: Mike Fargione/Cary Institute


May 8, 2024: Settling Down After Moving On

Phenology: Hatching of egg masses is complete; 1st Instar larvae have ballooned to food sources and have begun feeding. Molt into the 2nd instar larval stage is imminent.

The word is out! Spongy moths were the talked-about nature phenomenon in our region last week as people became aware of the emergence of this invasive forest pest. Local news outlets, social media, and blogs took notice and sounded the alarm. Egg hatch and larval dispersal caught peoples’ attention, as did reports of skin irritation from contact with the insects’ tiny hairs. I caught myself taking a second look at my poison ivy-caused rash, just in case it might have been the result of this burgeoning pestilence.

During years when the spongy moth population is small, most newly hatched caterpillars crawl up into their host trees and begin feeding. In outbreak years like the current one, caterpillars may sense competition and possible food shortages to come. Or, they might recognize they hatched on a less-than-ideal food source. We don’t know why but when populations are large, many spongy moth caterpillars use silken thread to disperse from their hatch sites. The tiny caterpillars hang in the air waiting for sufficient wind to break their thread and carry them off to another location. This type of “ballooning” dispersal is common in moth species where the adult female cannot fly, including spongy moths.  Perhaps this is a way for offspring to compensate for the limited dispersal opportunities of their parent.  

Dispersing spongy moth caterpillars have no control over where the wind takes them. As we saw this week, many land in unsuitable places where their fates remain doubtful. Every possible surface (car, building, lawn furniture, posts, railings, and people) got a dose of these dispersing larvae; they even complicated activities like walking the dog. At the end of each walk, both dog and I had to be picked clean of the little black hitchhikers before the caterpillars crawled deeper and started me scratching. 

spongy moth larvae
Larvae on a lawn chair. Credit: Mike Fargione/Cary Institute

Egg hatch appears to have finished, and the subsequent wind dispersal of the tiny caterpillars has slowed or even stopped. Larvae that fail to find a suitable plant will soon be dead, but more than enough lucky ones found a new host tree and have begun feeding. Shot-hole damage is already common on many oak leaves.

Shothole damage on oak leaves. Credit: Mike Fargione/Cary Institute

Soft-bodied parts of caterpillars grow larger as they feed, but the insects cannot change the size of their head capsule, one of the limited portions of their body covered with a hard exoskeleton. To continue growing, these caterpillars must shed their skins (a process called molting) allowing the head capsule to be replaced with a larger one.  Some recently hatched caterpillars look ready to molt; you can spot this when their soft body grows wider than their heads and their bodies lighten in color to a characteristic greasy appearance. The next stage (2nd instars) has not yet been seen but should appear very soon. Larval stages are distinguished by a combination of head capsule size and color characteristics (see Identifying spongy moth early larval instars)

We have seen a number of news articles and social media posts that mistake other insects for spongy moths. Next week we will dive into the identification of this insect and its local look-alikes.



May 16, 2024: Can I See Your ID?

Phenology: Egg hatch appears to be completed. Both 1st and 2nd instar larvae are present and feeding in the tree canopies. Damaged leaves are visible, but the holes are still small.

This week I will compare spongy moth caterpillars to local look-alikes. I will also highlight the importance of native caterpillars as wildlife food, particularly for migrating songbirds who are arriving this time of year.

Right now, at least three species of hairy caterpillar larvae may be present and significantly damaging hardwood leaves in our region. All vary slightly in appearance depending on which instar you are looking at (early instars of all species are little, hairy, black caterpillars; older, larger life stages are easier to identify as different species). For an excellent visual, visit this Michigan State University Extension Bulletin

1st instar spongy moth (Lymantria dispar dispar) larvae have black heads and bodies. Irregularly shaped yellow dots may be visible on the backs of 2nd instars; these dots are transformed into thin broken yellow lines in older larvae. By the 4th instar, the head is yellow and black, while the back of the body sports pairs of blue and red spots. Spongy moth caterpillars do not gather in webbed nests, but crawl singly into the tree canopy to feed. They congregate in resting locations at the base of trees during the day (unless there are many, when they tend to stay up in the canopy).

1st and 2nd instar spongy moth caterpillars on a northern red oak (Quercus rubra) leaf with early feeding damage.
1st and 2nd instar spongy moth caterpillars on a chestnut oak (Quercus prinus) leaf with early feeding damage. Credit: Mike Fargione/Cary Institute

Eastern tent caterpillars (Malacosoma americanum) are native insects that make communal silken tents filled with many caterpillars and their droppings. These insects have a dark head and a narrow solid light-colored stripe down the center of their backs. Small blue dots may be visible on both sides. Eastern tent caterpillars are common this year in our area feeding on cherries, apples, and crabapples. This species prefers to feed on trees growing in full sun, presumably because leaves on such trees are more nutritious. Although distressing to see, healthy trees may withstand several years of total defoliation without long-term detrimental effects.

Eastern tent caterpillar silk nest on a black cherry tree (Prunus serotina).
Eastern tent caterpillar silk nest on a black cherry tree (Prunus serotina). Credit: Mike Fargione/Cary Institute
Eastern tent caterpillar larvae feeding on leaves of black cherry. Note how they eat everything but the mid-vein of the leaves.
Eastern tent caterpillar larvae feeding on leaves of black cherry. Note how they eat everything but the mid-vein of the leaves. Credit: Mike Fargione/Cary Institute

Forest tent caterpillars (Malacosoma disstria) are another species that may gather in large groups and can do significant damage to oaks and other native forest trees. They have bluish heads and sides and a central row of light-colored markings down their backs. Many references describe these markings as being in the shape of “keyholes”. Forest tent caterpillars do not make communal silken tents, but build silken mats under leaves and on the tree bark where they gather when not feeding. Like spongy moths, they feed on several tree species, particularly oaks and maples. 

Collection of forest tent caterpillars.  This insect does not make a tent-like structure but instead spins a silken platform on which caterpillars gather when not feeding. Photo credit Bugwood.org
Collection of forest tent caterpillars. This insect does not make a tent-like structure but instead spins a silken platform on which caterpillars gather when not feeding. Credit: Bugwood.org

These are only three of the estimated 1,500+ Lepidoptera (butterfly and moth) species found in New York State. Only 20 or so species of eastern forest Lepidoptera reach densities that can result in the defoliation of large acreages. Most native forest-dwelling insects play important roles in those communities, notably as food for baby birds. Some ecologists believe that the spring migration of songbirds evolved because of the wealth of insects, particularly caterpillars, that appeared annually in temperate regions. The timing of bird migration seems linked with the timing of the emergence of new plant tissue and the many insects that feed on it. Most of our songbirds, regardless of what the adult eats, will feed their young on soft-bodied insects. 

Populations of native loopers, inchworms, and spanworms (Insect family Geometridae) are booming in Cary forests and serve as the primary food for many breeding songbirds.
Populations of native loopers, inchworms, and spanworms (Insect family Geometridae) are booming in Cary forests and serve as the primary food for many breeding songbirds. Credit: Barry Haydasz/Cary Institute

A quick look at the understory leaves of oaks in the Cary forest this week revealed a smorgasbord of inchworms and other caterpillars that serve as the primary food source for soon-to-hatch forest songbird babies. When we reduce the amount of vegetation available for insects (as when deer overbrowsing eliminates the natural understory) or we reduce the diversity of plant species (by killing off some with introduced plants, forest pests, and pathogens), we reduce the amount of insect food available for other wildlife. We can do much better to alleviate the impacts of too many deer and invasive pests and pathogens.  Most important is to stop the spread of new pests before they arrive. Visit Tree Smart Trade to learn about this issue and what you can do to help.

For more on the importance of caterpillars to birds, visit Nature's Best Hope: Conservation that Starts in Your Yard, a Cary lecture with ecologist Doug Tallamy.



May 22, 2024: “Have you ever seen the rain, coming down on a sunny day?”

Phenology: Egg hatch is finished. Most caterpillars have reached 2nd or just started 3rd instar stages, although smaller 1st instars are still readily found. The rate of leaf damage from feeding caterpillars is increasing and frass (caterpillar poop) and leaf parts (greenfall) are raining down. 

spongy moth instar
3rd instar spongy moth caterpillars feeding on an oak leaf in Cary Institute’s forest. Credit: Mike Fargione/Cary Institute

Creedence Clearwater Revival’s song “Have You Ever Seen The Rain?” has been stuck in my head. Supposedly, the title/chorus was a metaphor for going through difficult times. Right now, it is difficult to watch legions of caterpillars (mostly spongy moths, but also other species) consuming the leaves of woody plants in our forests and yards. A walk under an oak canopy becomes a gauntlet, zig-zagging past caterpillars suspended in mid-air on silken threads. Inevitably, caterpillars and leaf parts get on you as you walk. The spongy moth is a rather messy eater!

spongy moth caterpillars on hat
Two spongy moth caterpillars, a Geometrid ‘looper’, plus greenfall adorn a hat after a quick walk through a Cary oak forest. Credit: Mike Fargione/Cary Institute

 This year, the aforementioned song might also be taken as a metaphor for the caterpillar frass, which is also raining down around us on sunny (and cloudy) days. Stand under the forest canopy in an outbreak area and you can hear the frass hitting surfaces all around you. Stop, close your eyes, and listen; it sounds like a gentle rain.

spongy moth frass on leaf
Spongy moth frass (feces) from caterpillar feeding activities rains down and collects on a white oak (Quercus alba) leaf. Credit: Mike Fargione/Cary Institute

Park your car under a tree, even for just a few hours, and you will return to a coating of tiny frass pellets on its surface. A real shower creates a brown, messy goop that must be cleaned off the windshield before turning on the wipers. Forget to clean, and you have a mess that requires a hose to get you back on the road. 

Spongy moth frass and greenfall cover a car
Spongy moth frass and greenfall cover a car parked under an oak tree. Credit: Mike Fargione/Cary Institute

All that caterpillar poop is a big deal to us humans. We don wide-brimmed hats, long-sleeved shirts, and long pants to avoid being covered with it (remember to keep your mouth closed when you look up!).  It is also a very big deal to our forests. Disturbances - like pest outbreaks - can result in a loss of nutrients from forest ecosystems. About 20 years ago, Cary scientists investigated the impacts of spongy moth defoliation on forest nitrogen, an essential nutrient for tree growth. Trees take up available forms of nitrogen from the soil and carefully conserve this precious commodity. In years with normal (i.e. limited) insect damage, deciduous trees in the Cary forests reabsorb about 70% of the nitrogen in their leaves before dropping them in fall. In spongy moth defoliation years, only about 23% of the nitrogen in leaves gets reabsorbed back into the plant. Much of the leaf nitrogen goes to the forest floor in the frass rain, along with some nitrogen trapped in small bits of greenfall when the caterpillars cut leaves and drop them.  

Eventually, the insects die and the nitrogen in their bodies that came from the leaves in the trees also goes to the forest floor. A revelation from the Cary research was that although spongy moth defoliation is a major disturbance, it primarily causes nitrogen to be redistributed and not lost from the forest ecosystem. The nitrogen that falls to the forest floor gets taken up mostly by microbes or is trapped in organic materials in the soil.  Trees may recapture small amounts of that nitrogen in the year of defoliation, helping to fuel the new flush of leaf growth once spongy moths finish feeding; the rest of the nitrogen will be available to the trees in subsequent years. Very little nitrogen is actually lost from the forest.

A single year’s defoliation is not likely to kill a healthy deciduous tree but does reduce its store of essential nitrogen.  Next week we will look at how other stressors combined with spongy moth defoliation can dramatically alter forest communities.  



May 30, 2024: A Party for the Ages

Phenology: Caterpillar stages range from 3rd to 6th instars with most in the 4th and 5th instars. Caterpillar frass (poop) and greenfall (pieces of cut leaves) continue to rain down. Oak canopy defoliation ranges from 50-100% within the forest.

A wide range of spongy moth caterpillar sizes are now found in the Cary forests. Why are so many different stages present? 

instars of spongy moths
Multiple instars (3rd through 6th) of spongy moths are feeding in Cary forests at this time. Credit: Mike Fargione/Cary Institute

Many different caterpillar stages can be found due to variation in the timing of spongy moth hatch and differences in their growth and development rates. The warmer it is, the faster insect development progresses. Female spongy moths laid egg masses in a wide range of sites at the end of last summer, each with its own microclimate. This spring, egg masses in warmer sites hatched before egg masses in cooler sites.

Once hatched, some lucky caterpillars found themselves on oak trees and had easy access to newly unfurled, high-quality leaves – which fueled fast growth and development. The less fortunate hatched on trees of lower quality, less nutritious food, and still decided to dig in. They grew and developed more slowly. Others dispersed from unsuitable hatch sites, and by doing so, delayed starting to feed for a day or two.

Timing of hatch, food quality, and food availability all contribute to why we see the many different caterpillar stages all at the same time. So what does this mean for the forest, and us? Feeding and defoliation will occur over a longer period of time than if all the caterpillars had developed in perfect synchrony on the best quality food. It means we will have to tolerate seeing caterpillars for longer. Unless a disease starts to spread rapidly through the spongy moth population, defoliation won’t end until all the remaining caterpillars stop feeding and pupate, preparing them to turn into adults and mate.

For the first time since bud-break in spring, oak canopies actually got smaller during the last week, with losses to spongy moth caterpillars exceeding leaf development. Some oaks are now completely defoliated. Hungry caterpillars will migrate laterally across the branches of leafless trees to those nearby, or drop down from the canopy and look for food in the understory, or walk over the ground and climb up another tree. Expect to find them feeding on less desirable species as oak leaves disappear.

spongy moth eaten leaves
Looking up at the same Cary oak canopy from below. The loss of most leaves to spongy moth feeding becomes obvious when we compare photos taken just one week apart. Credit: Mike Fargione/Cary Institute

Check your ornamental woody plants for incoming migrations of caterpillars. It is probably too late to apply BTK sprays, but frequent hand-picking, or removal from burlap bands placed around trunks, can reduce damage if you have the time to do this daily. Wear gloves or use a brush to sweep them into warm soapy water. 

Avoid using sticky bands unless you cover them with cages to prevent the accidental capture of birds and small mammals as many try to eat trapped insects.  If you have important trees in your yard that are defoliated, your best option to help them at this stage is to ensure they get adequate water. If we get a drought during summer be sure to mulch and water them. 



June 12, 2024: The Beginning of the End?

Phenology: Caterpillars have reached 5th and 6th instar stages. The first pupae were found today and appeared since trees were last examined on June 8th. The rain of frass (poop) and greenfall (pieces of cut leaves) have stopped as oak canopy defoliation approaches 100%. Sick and dead caterpillars are common on tree trunks and understory vegetation. 

Dead and dying spongy moth caterpillars are showing up all across the Cary forests. Is this the beginning of the expected and hoped-for population crash? It is too early to know for sure, but it could well be.

Three factors are currently killing the caterpillars - let’s review them.

Many spongy moth caterpillars have been infected and killed by the fungal pathogen Entomophaga maimaga. This disease was first introduced to control spongy moths in 1910, but failed to establish. Reintroduced again in the 1980s, it is now widespread in the Northeastern US. When the weather is favorable (damp and warm but not hot for too many days), and the spongy moth population is moderate to high, the fungus can have multiple generations – killing many caterpillars. Fungus-killed caterpillars look like dried-out, head-down caterpillar mummies. Based on what we have seen, this is a very good year for the fungus and a bad year for the spongy moth.

We are also seeing many caterpillars killed by Nuclear Polyhedrosis Virus (NPV), another introduced pathogen that occurs throughout our region. When the spongy moth populations are low, NPV kills a few caterpillars each year; in most cases, infection is sub-lethal. During boom years, when populations are large, NPV can be the primary cause of death for late-instar caterpillars. This is because caterpillars suffer from increased stress as food and resting space become limited, reducing their immunity to NPV, and allowing a viral pandemic to run through the population.  NPV-killed caterpillars typically hang down as soggy, bad-smelling, inverted v’s. This year looks like a viral pandemic in progress.

dead spongy moth caterpillars
Examples of spongy moth caterpillars recently killed by the fungus Entomophaga maimagawo and the Nuclear Polyhedrosis Virus (NPV) on an oak tree in the Cary forest. The appearance of the dead caterpillars gives clues to the pathogen responsible for their death.

With oak defoliation approaching 100%, some of the caterpillar deaths we are seeing are likely caused by a shortage of high-quality food. When the best food runs out, caterpillars attempt to migrate across the canopy or drop into the understory searching for better fare. A diet of poorer-quality food slows caterpillars' growth and, if food is inadequate, they die from starvation. In the interim, limited food and/or poor quality food puts those same caterpillars under stress, which then makes them more susceptible to dying from the virus. Inadequate food is a lose/lose proposition for a caterpillar. So in this kind of situation if the virus does not get you, then starvation will.

Spongy moth caterpillars
Spongy moth caterpillars must seek alternate foods when preferred oak leaves are used up as seen here. Food shortages can lead to starvation and increased susceptibility to the Nuclear Polyhedrosis Virus.
spongy moth dead caterpillars on a Cary oak
Many dead caterpillars on a Cary oak. Most probably died from fungal or viral pathogens. Starvation during severe overcrowding can also kill them, particularly on isolated trees.

Dr. Clive Jones, a Senior Scientist at Cary Institute, thinks the magnitude of caterpillar deaths we are seeing will result in a significantly lower spongy moth population next year at Cary. Will the population crash to the point where it is not a problem in our area?  It might if sufficient caterpillars die before pupating – the causes of mortality described here only act on the caterpillar stage. However, other players can attack and kill spongy moth pupae and eggs and may further reduce the pest population. That is a story for next week!

spongy moth pupae
What can control spongy moth pupae like the ones seen here? Tune in next week to learn more.

Are you concerned about the spongy moth in your area?  Check out the Resources section.


Resources

Insect defoliation and nitrogen cycling in forests

Identifying spongy moth early larval instars

What you need to know about spongy moths: a Q&A with Ecologist Clive Jones

The Spongy Moth in Our Yards and Forests
A fascinating and comprehensive look at this pest’s life history, ecology, and forest impacts.

michael fargione
Manager of Field Research & Outdoor Programs

Michael Fargione is Manager of Field Research & Outdoor Programs at the Cary Institute. He manages the natural areas and coordinates access to properties and their resources. He also provides direction, support and helps deliver outdoor educational efforts geared toward the general public.

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