Order: Lepidoptera
Family: Erebidae (formerly the Lymantriidae)
Lymantria dispar
Overview
Spongy moth (Lymantria dispar) was accidentally introduced into the US when it escaped the home of E. Leopold Trouvelot in Medford, MA in the late 1860’s. Trouvelot had intentionally brought spongy moth to his home in Massachusetts, from France, to study the insect with an interest in using the caterpillars for silk production. Since then, the spongy moth has spread throughout the Northeast and well beyond. It can be a serious defoliator of trees and a nuisance when population outbreaks occur. Homeowners rarely appreciate sharing their property with excessive numbers of caterpillars, as well as the copious amount of excrement (frass) they produce during high population years. (Frass may be experienced like a gentle rainfall from the canopy of infested trees.) In addition to favored host plants being stripped of their leaves, the inconvenience of co-existing with the caterpillars during population outbreaks can be very frustrating to some.
Host Plants
The preferred hosts of the spongy moth are alder (Alnus spp.), basswood (Tilia spp.), certain species of birch (Betula spp.), especially the black birch (B. lenta), gray birch (B. populifolia), paper birch (B. papyrifera), and river birch (B. nigra), hawthorn (Crataegus spp.), larch (Larix spp.), oak (Quercus spp.), poplar (Populus spp.), willow (Salix spp.), and witch hazel (Hamamelis spp.). Other hosts of the insect include but are not limited to apple (Malus spp.), blackgum (Nyssa sylvatica), cherry (Prunus spp.), cottonwood (Populus spp.), elm (Ulmus spp.), hemlock (Tsuga spp.), hickory (Carya spp.), hornbeam (Carpinus spp.), maple (Acer spp.), ninebark (Physocarpus spp.), pine (Pinus spp.), sassafras (Sassafras spp.), southern white cedar (Chamaecyparis thyoides), spruce (Picea spp.), and yellow birch (Betula alleghaniensis).
Identification/Life Cycle
Spongy moth overwinters as a brown/tan and fuzzy egg mass in a cluster of 500 or more eggs. Eggs typically hatch in the spring during the first week in May in Massachusetts, but variations in climate and spring weather can either accelerate or delay egg hatching (or between 80-100 GDD’s, base 50F, for the start of egg hatch). For example, in warm springs, eggs may hatch by mid-to-late April. Once hatched, the tiny, hairy caterpillars may remain in the lower forest canopy or, when in high populations, migrate upwards to the tree tops, where each one then spins down on a long silken thread. The tiny caterpillars hang in the air waiting for a strong wind to break the thread and carry them to a new location. This process of dispersal is known as “ballooning” and is somewhat common in caterpillar species where the adult females cannot fly. It is the only silk that this species produces. Spongy moth caterpillars do not make silken webs or tents. (However, they are sometimes active at the same time as the eastern tent caterpillar, which does. This can be a source of confusion.) This type of dispersal helps young larvae relocate to more favorable hosts, such as oaks, while factors such as food quality (species composition) and the availability of suitable areas to hide during the day (such as in rough oak bark) may affect spongy moth dispersal patterns. Of course, ballooning is not a precise way of dispersing, and during very high population years early instar spongy moth caterpillars can be found in locations that are not on host plants, including caught in swimming pools or on fencing. Population sizes of this pest can change dramatically from one year to the next for a variety of reasons, especially natural enemies discussed below.
Once the caterpillars settle on a new host, they begin feeding on the foliage. Small to moderate sized populations will often feed at night and come down out of the trees during daylight hours to avoid predators and parasites. Caterpillars in high populations usually stay in the trees around the clock due to intense competition for foliage. While spongy moth caterpillars are only 1/16th of an inch in length when they hatch, they may exceed 3 inches in length by the time they pupate, usually about six weeks later. The caterpillars have hairy bodies; along the length of their backs, they have five pairs of blue dots followed by six pairs of red dots. The caterpillar stage typically lasts until about the third week in June in Massachusetts, whereupon they pupate. Adults start to appear by late June/early July. Neither the male nor female adult moths feed. Therefore, by July, most of their defoliation (feeding damage) is complete for the year.
Adult male spongy moths are brown with black markings and have highly feathered antennae. Female spongy moths are white with black markings and have straight, threadlike black antennae; female European spongy moths (the species in Massachusetts) do not fly. Spongy moth caterpillars have numerous hairs on their bodies, as do the adults. A small percentage of the population reports experiencing allergy-type reactions to these hairs, particularly during population outbreak events. Symptoms range from itchy skin to sinus allergies with itchy eyes and a runny nose. This can be more of an issue for sensitive individuals, such as children, and case studies in schools and communities experiencing spongy moth outbreaks have been documented, such as “An Epidemiologic Study of [Spongy] Moth Rash” (1984). For most people, spongy moth does not cause allergic reactions as readily as certain other hairy caterpillars (ex. browntail moth; Euproctis chrysorrhoea) and their medical relevance is typically not significant.
Damage
Oaks are preferred hosts, and often suffer the most defoliation during population outbreaks of the spongy moth. Apple, birch, poplar, willow, and witch hazel may also be preferred host plants for the spongy moth caterpillars; however, when broad-leaved host plants are defoliated, spongy moth caterpillars have been known to feed on conifers such as spruce and white pine. In the host list above, preferred hosts are noted using the ranking system given by Liebhold et al., 1995 (Suitability of North American tree species to the gypsy moth: a summary of field and laboratory tests).
Newly hatched caterpillars are small and may be found on the undersides of host plant leaves that are newly opening in the spring. The smallest caterpillars may feed on the leaves by creating small holes. As the caterpillars grow in size, they are capable of devouring the entire leaf - often leaving only larger leaf veins behind. Evidence of spongy moth caterpillar feeding can also include the presence of brown, rounded frass pellets and shredded leaf pieces beneath heavily infested host plants. During population outbreaks, frass and shredded leaves can rain down from the tree canopy. Entire trees may be completely defoliated during these outbreak events. Defoliation can be extensive state-wide or regionally. For example, in 2017, 923,000 acres of defoliation occurred across Massachusetts alone, according to MA Department of Conservation and Recreation aerial survey results.
While otherwise healthy trees (properly planted in the right location, no additional stressors, etc.) can usually withstand a single year of complete defoliation, repeated years of defoliation can cause branch dieback or even tree mortality. If an outbreak event is occurring and 2-3 years of defoliation are expected, high-value, ornamental host plants may require protection.
Management Strategies:
Due to the long history of the spongy moth in Massachusetts and the United States, many different management strategies for this insect exist. From banding trees to disrupt moving caterpillars during high population years, to classical biological control, now naturally occurring pathogens (a fungus and a virus), and chemical management options - a lot has been learned about the management of spongy moth over the last 155+ years. Some of this information is summarized for land managers below. However, undoubtedly, more strategies are known to the scientific literature and are not discussed here.
Cultural/Mechanical Management:
Spongy moth caterpillars may feed on at least 500 species of trees, shrubs, and vines. However, there are certain plants that they may avoid. These can be options for ornamental plantings where there is a history of cyclical spongy moth outbreaks. Plants that spongy moth mostly avoid include but are not limited to: American holly (Ilex opaca), arborvitae (Thuja spp.), ash (Fraxinus spp.), balsam fir (Abies balsamea), butternut (Juglans cinerea), black walnut (Juglans nigra), catalpa (Catalpa spp.), flowering dogwood (Cornus florida), locust (Robinia spp.), mountain laurel (Kalmia latifolia), red cedar (Juniperus virginiana), rhododendron (Rhododendron spp.), sycamore (Platanus spp.), and yellow poplar (Liriodendron tulipifera). However, some of the aforementioned plants (ex. ash and emerald ash borer) have other significant insect pests that may prohibit their successful use.
When using a sticky barrier on a susceptible tree or shrub to capture older, crawling spongy moth caterpillars, there are a few things to consider. 1) This will not prevent all caterpillars from accessing the tree to feed, nor is it a guarantee that no foliage will be eaten by them on the plant to which it has been applied. 2) This will not prevent the tiniest of caterpillars (newly hatched) from ballooning on the wind into tall trees and settling to feed. For example, this could mean that caterpillars will still have some access to the leaves following their dispersal in the spring. 3) Do not apply any sticky substances directly to the tree or shrub bark to avoid risk of injury to the plant. 4) Sticky bands will need to be monitored frequently throughout the growing season, particularly in June, to clean and replace them.
If the bands become covered in dead caterpillars, living ones can crawl over the dead and still access the leaves of the tree. It is recommended that bands be placed on trees (and sticky material on the bands) once the caterpillars are approximately an inch in length. This may be in early or mid-June, roughly. Bands should then be left up and changed frequently through July, until the caterpillars have pupated. This technique may work best with an engaged property owner that is willing and able to frequently check and help change the bands.
Biological Control/Natural Enemies:
Since spongy moth first arrived in North America, over 50 parasitoids of Lymantria dispar have been intentionally introduced during biological control efforts to suppress spongy moth populations. Only approximately 11 of those parasitoids are considered established. These include: Anastatus japonicus (egg parasitoid), Ooencyrtus kuvanae (egg parasitoid), Cotesia rnelanoscelus (larval parasitoid), Phobocampe disparis (larval parasitoid), Blepharipa pratensis (larval parasitoid), Compsilura concinnata (larval parasitoid), Exorista larvarum (larval parasitoid), Parasetigena silvestris (larval parasitoid), Monodontomerus aereus (pupal parasitoid), Brachymeria intermedia (pupal parasitoid), and Pimpla disparis (pupal parasitoid) (Kenis and Lopez Vaamonde, 1998). Many of these biocontrols are parasitic wasps as well as parasitic flies and attack different life stages of the spongy moth (eggs, larvae, or pupae).
During wet springs, an entomopathogenic (i.e. insect killing) fungus known as Entomophaga maimaiga, can work very well at keeping this pest in low numbers. This fungus is now “naturally” common in Massachusetts. It may have been introduced into the United States from Japan in 1910 for spongy moth management, but the actual fungus we have today seems to be from a later introduction. Observing dead, desiccated spongy caterpillars hanging head-down on the trunks of host trees is a good indicator that this fungus has been effective. The conidiophores (that produce the spores) of the fungus may, at the right time, be visible stuck to the hairs of dead caterpillars. Check out this great video from the Cornell Mushroom Blog. Entomophaga maimaiga can be effective at killing spongy moth caterpillars, even when population densities of this species are low.
The nucleopolyhedrosis (NPV) virus is also a common cause for the collapse of spongy moth caterpillar populations. Both the fungus and virus overwinter in the soil in Massachusetts and require ample amounts of moisture in the spring to build up in spongy moth caterpillar populations. The NPV virus spreads more successfully when the population has grown, typically during an outbreak, as it is spread from individual to individual caterpillar. It works best under high density population conditions.
Natural enemies of spongy moth caterpillars include not only various parasitoid insects and pathogens, but also mammals and some birds. Certain ants and ground beetles will attack Lymantria dispar in different life stages (caterpillars or pupae) as predators. Mice will also feed on caterpillars and pupae.
Chemical Management:
Once spongy moth caterpillars have settled to feed, they can be successfully treated with Bacillus thuringiensis (kurstaki), commonly known as B.t.k. This is a naturally occurring bacterium that is specific to caterpillars that become moths or butterflies (Lepidoptera). It is relatively safe for beneficial organisms and other insects, with the exception of other Lepidopteran caterpillars. A commercial, Massachusetts licensed pesticide applicator will be needed to apply B.t.k to larger trees due to the necessary application equipment required. However, once the caterpillars are older, B.t.k is much less effective. To establish whether B.t.k will work or not, inspect the caterpillars on the host plant; younger Lymantria dispar caterpillars have a head capsule that is all black while the older ones have obvious yellow markings on the head - these larvae are less susceptible to B.t.k (or caterpillars that are over ¾ inch in length, approximately). In this case, other compounds, such as spinosad, may be necessary.
Other active ingredients labelled for use on Lymantria dispar include abamectin (NL), acephate (NL), acetamiprid (L), azadirachtin (NL), Bacillus thuringiensis subsp aizawai (larvae) (L), Bacillus thuringiensis subsp kurstaki (NL), bifenthrin (NL), carbaryl (L), chlorantraniliprole (larvae) (NL), Chromobacterium subtsugae (NL), cyantraniliprole (NL), cyfluthrin (larvae) (NL), deltamethrin (larvae) (L), emamectin benzoate (L), flonicamid+cyclaniliprole (N), gamma-cyhalothrin (larvae) (L), horticultural oil (larvae) (L), indoxacarb (L), insecticidal soap (NL), Isaria (paecilomyces) fumosoroseus (NL), lambda-cyhalothrin (larvae) (L), malathion (L), neem oil (NL), permethrin (L), pyrethrin+sulfur (NL), spinetoram+sulfoxaflor (N), spinosad (larvae) (NL), tau-fluvalinate (NL), and tebufenozide (NL).
Horticultural oils may manage Lymantria dispar eggs and very young larvae but thorough coverage is necessary. This may be difficult to achieve on egg masses hidden in protected areas and on branch undersides. Egg masses may be scraped into a can of soapy water, however, this is time consuming and labor intensive and only possible for egg masses within reach.
Traps for the adults offer no population reduction benefits for this pest. While traps and bands around trees may be useful for monitoring this insect, they do not offer effective management of Lymantria dispar, especially in high population years.
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), chlorantraniliprole (soil drench), cyantraniliprole (soil drench, soil injection), emamectin benzoate (injection), and neem oil (soil drench).
Soil application of Isaria (paecilomyces) fumosoroseus is labeled for use against caterpillars.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
Read and follow all label instructions for safety and proper use. If this information contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: September 27, 2024. To check current product registrations in Massachusetts, please visit the MA Department of Agricultural Resources Pesticide Product Registration page and click on "Search Pesticide Products Registered in Massachusetts - Kelly Solutions".
References
Kenis, M. A. R. C., and C. Lopez Vaamonde. "Classical biological control of the gypsy moth, Lymantria dispar (L.) in North America: prospects and new strategies." Proceedings, population dynamics, impact and integrated management of forest defoliating insects. USDA Forest Service General Technical Report NE-247 (1998): 213-221.
UMass Extension Insect Management Guide
Authors
Updated by Tawny Simisky and Paige Brown.