Spongy Moth
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Spring & Summer 2022 Caterpillar Outbreak
In the summer of 2021, elevated populations of spongy moth caterpillars caused noticeable leaf damage across New York State. Populations were especially high in Essex, Warren, Saratoga, Schenectady, Livingston, Steuben, Seneca, Chemung, and Schuyler counties.
Photo of a spongy moth caterpillar
by Karla Salp, Washington State
Department of Agriculture, Bugwood.org
About the Insect
See our spongy moth factsheet (PDF) for quick facts about this species.
The spongy moth (Lymantria dispar dispar) is a non-native insect from France. In New York, spongy moth caterpillars are known to feed on the leaves of a large variety of trees such as oak, maple, apple, crabapple, hickory, basswood, aspen, willow, birch, pine, spruce, hemlock, and more. Oak is their preferred species. Spongy moths have "naturalized" in our forest communities and so they will always be around.
Cyclical Outbreaks
Spongy moth populations rise and fall in cycles of roughly 10-15 years. Populations vary during the cycle, from years with few caterpillars and very minor damage, to years with large numbers of caterpillars and very noticeable leaf damage and tree defoliation. These population cycles are driven by predator-prey interactions, specifically the interactions between spongy moth, small mammals, and acorn production. Acorn production is cyclical and varies by tree species, but there are years when crop failures align for all species and no acorns are produced. This decreases the food source for small mammals, which are predators of spongy moth. Low numbers of mammals can allow spongy moth populations to rise, reaching outbreak levels. However, acorn crop failures do not lead to an immediate boom in spongy moth populations. Instead, it may take about 3 years for the small mammal population to decrease enough to allow for a spongy moth outbreak.
Outbreaks are usually ended by natural causes such as disease and predators.
U.S. Introduction and Locations
Spongy moths were accidentally introduced in 1869 when they were brought to the U.S. in the hope that they could breed with silkworms to create a hardier variety of silkworm and develop a silk industry in the US. Even though they failed as a textile producer, some of the spongy moths escaped and established their first U.S. population in Medford, Massachusetts.
According the U.S. Forest Service (leaves DEC website), the current "invasion front" stretches from North Carolina across to Minnesota and includes: Maine, New Hampshire, Vermont, New York, Massachusetts, Rhode Island, Connecticut, Pennsylvania, Delaware, Maryland, Michigan, Wisconsin, Minnesota, Illinois, Indiana, Ohio, West Virginia, Virginia, and North Carolina.
Damage Caused by Spongy Moths
When outbreaks occur and populations are high (every 10-15 years in NY), thousands of acres of trees can be damaged. Although spongy moths do not pose a major threat to New York's forests, they are not native and their populations can reach high, destructive (outbreak) levels.
Spongy moth caterpillars eat young, tender leaves in the spring. Deciduous trees (trees that lose their leaves each fall) can regrow a new set of leaves by July and can usually withstand 2-3 successive years of defoliation (removal of leaves) without being killed. However, defoliation does reduce the vigor and resistance of the tree, and it becomes more susceptible to pests and diseases.
Tree death can occur when other stresses such as disease or other insect outbreaks attack trees in the same year. When populations of spongy moths are very high, or when oak and other preferred trees are limited, they will even eat evergreen species including pine, spruce and hemlock. Evergreens do not regrow leaves as easily as deciduous trees and can die as a result of complete defoliation.
Spongy Moth Identification
The larval stage of the spongy moth is a caterpillar that hatches in the spring from eggs laid the summer before. It hatches about the time oak buds start to open. Young caterpillars need to eat tender new leaves. As a caterpillar grows, it can eat older, tougher leaves.
Spongy moth adults and eggs
The caterpillars grow to about 2.5 inches in length. They have five pairs of raised blue spots followed by six pairs of raised red spots along its back. The hairs on their backs can cause mild to moderate skin irritation in some people. Female moths are white with brown markings. Males are brownish. Females do not fly. Egg masses are light brown in color and appear as fuzzy patches on tree trunks, branches, firewood, or in a sheltered spot, even on lawn furniture. Each egg mass contains 600-700 eggs.
Tent caterpillars look similar to spongy moths and also feed in the spring. See the Caterpillar Comparison Chart to determine which type you have.
How to Control Spongy Moths
Please note that all of these options may protect individual trees or small areas, but they will not erase a local spongy moth population now or in future years. In large forested areas, manual removal is often not practical during an outbreak.
DEC and its partners typically do not manage spongy moth, and treatments are usually limited to ecologically or culturally significant forests. At this time, DEC does not provide funding for controlling spongy moths on private property.
Caterpillars are always being naturally controlled by birds, rodents, parasites, and diseases. Extremes in temperature can also reduce population numbers.
Squishing and Scraping
When populations are low or when you have just a few trees you want to protect, spongy moth caterpillars and adults can be killed by squishing them. Egg masses can be destroyed by scraping them off trees or other structures and dropping them in a container of detergent.
Bands, Barriers, and Traps
Example of a barrier band,
caterpillars visible below the band
In late April, sticky/barrier bands may be placed around the tree's trunk to catch caterpillars when they hatch and crawl. These bands can be bought or made at home using common household materials. View detailed instructions on how to make and use your own trap on the University of Wisconsin website (leaves DEC website). If you choose to use a barrier band, please check it often in case unintended wildlife pass through, and replace as necessary after rain events.
In mid-June when caterpillars are larger, replace sticky/barrier bands with a burlap trap. View detailed instructions on how to do this on the University of Wisconsin website (leaves DEC website).
Insecticide Options
When considering whether to act or not, consider the extent, severity, and context of the outbreak first, as most management options generally can not impact regional spongy moth outbreaks. In certain cases it may be economically feasible to spray large areas. For example, maple syrup producers may be interested in aerial spraying since severe defoliations can reduce maple syrup production. If you are considering spraying, start planning in fall or winter as many of these services book up early. Various insecticides for spongy moths are available at garden centers.
Microbial insecticides are biopesticides made from naturally occurring bacteria, viruses, fungi, or protozoans that can be targeted to a specific pest. The most common of these is Bacillus thuringiensis (Bt), which occurs naturally in soil and on plants. The Bt subspecies kurstaki (Btk) is the most appropriate to use for LDD control.
Btk works best on young caterpillars, which become more resistant to treatment as they mature. When Btk is eaten, the caterpillar becomes paralyzed, stops feeding, and dies of starvation. Btk is harmless to people, animals, and plants, but does affect other young moth and butterfly larvae. Proper application will help limit exposure to non-LDD larvae.
Chemical insecticides are contact poisons. These chemicals can have an impact on a variety of beneficial, native insects (such as bees), as well as nesting birds and other wildlife so they should be used wisely. Spraying is not effective against spongy moth pupae or egg masses, and it is less effective once caterpillars reach 1 inch long.
Horticultural oil insecticides (aka dormant oils) are solutions refined from petroleum or plants and, when applied, smother insects or disrupt the protective coating around eggs. As with chemical insecticides, horticultural oils are non-selective but have the advantage of being relatively safe for humans and animals. The oils should be applied to egg masses in late March to early April before caterpillars emerge, and again in October to early November after adults have ceased activity.
In outbreak years, trees will appear
bare from the amount of leaves the
caterpillars eat.
Greater detail on insecticide treatment and timing can be found in the US Forest Service's guide to spongy moths (leaves DEC website). Professional pesticide applicators can be found online. In order to use restricted insecticides, applicators must be certified. You can check the certification status of a pesticide applicator on our website. We also recommend reviewing our tips for selecting a tree service.
You can use the egg sampling survey (PDF) in winter to determine if you will have a large infestation and may want to consider spraying.
How to Help Trees Recover From Defoliation
Most healthy trees can withstand a couple years of leaf loss from spongy moth caterpillars. Long-term damage depends on the type of tree as well as how much defoliation took place:
- Conifers - If a needle-bearing tree loses more than 50% of its needles, it probably won't recover. Check it for new needle growth in the months after the caterpillars are gone.
- Hardwoods - A healthy leaf-bearing tree will likely leaf out again as the caterpillars disappear in July/August, though leaves will probably be smaller than usual. If a tree loses ALL its leaves and does not grow any new ones in late summer, check it in the spring. If it still does not leaf out next spring, it has died.
If you have concerns about a dead or dying tree, or if think your tree could endanger a house if it were to fall, contact an arborist .
Losing lots of leaves in spring or summer can stress and weaken trees, which makes them vulnerable to pests, diseases, or competition from invasive plants that move into the now-sunny understory. If you have trees that show signs of recovery after the caterpillars have gone, check on them in the following months to watch for potential issues.
Urban or suburban trees that have been impacted by spongy moth caterpillar damage can benefit from some extra care such as:
- watering during dry or drought conditions,
- weeding around the trunk,
- mulching properly - just 1-2 inches deep and away from the trunk - IF you plan to mulch, and
- scraping off spongy moth egg masses in fall/winter (if applicable).
If you are a woodland owner who saw multiples acres impacted by the caterpillars, watch for new leaves in mid to late summer. If you are considering spraying, start planning in fall or winter as many of these services book up early.
Photos above, left to right, show the progress of a healthy tree growing new leaves after first being attacked by spongy moth caterpillars:
- June 11, 2021 - Tree is loaded with caterpillars which have begun to eat the tree's leaves (10-20% defoliation pictured).
- June 18 - Tree has obviously lost the majority of its leaves to caterpillar damage.
- June 28 - Tree begins to grow new, small leaves as caterpillars pupate and become moths.
- July 15 - New leaves have grown in size, tree is refoliated.
How to Predict Defoliation in Your Area
Egg masses appear fuzzy and tan
You can survey your woodlot or forest to determine the likelihood spongy moth defoliation for the next growing season. This can be done when egg masses are present, approximately August through April. The surveys typically take 20 minutes to 1 hour to complete, depending on population numbers.
If you do conduct sampling, please share your data with us. Your work is valuable to us for tracking annual populations across New York State.
Tools to help you survey:
- View a webinar from fall 2020 about surveying spongy moth egg masses in winter (leaves DEC website). NOTE: The first minute of the webinar contains technical difficulties. Skip to the one minute mark to begin.
- View the sampling protocol (PDF). This includes instructions and forms.
When completed, please mail or email your sampling data to:
NYSDEC
Division of Lands and Forests, Bureau of Invasive Species and Ecosystem Health
625 Broadway
Albany, NY 12233
Telephone: 518-402-9425
Fax: 518-402-9028
foresthealth@dec.ny.gov
