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, 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.
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. 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. 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. Population sizes of this pest can change dramatically from one year to the next.
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 1/16 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 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. For most people, spongy moth does not cause allergic reactions as readily as certain other hairy caterpillars (ex. browntail moth) and their medical relevance is typically not significant.
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. 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.
Monitor for spongy moth egg hatch between 80-100 Growing Degree Days (Base 50°F) in the spring. This typically occurs by mid-late April or early May, depending upon if the spring is warm or cool. Egg hatch may first be noticed by looking for the tiny, dark or light colored caterpillars seen resting on top of the egg masses themselves in the spring. Eventually, the tiny caterpillars will balloon and land on their newly opened host plant leaves and may be found there feeding. A first sign of spongy moth caterpillar feeding is to look for small holes or tatters in the new spring leaves. If the insect is not noticed until caterpillars are almost 3 inches long and have completed the majority of their feeding for the season, it may be too late for management efforts in the current year.
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) 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 approximately in early or mid-June. 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.
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, works extremely well in 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 spongy caterpillars hanging head-down on the trunks of host trees is a good indicator that this fungus has been effective. 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.
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.
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)
Chlorpyrifos (N)
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)
Tebufenozide (NL)
Bacillus thuringiensis Kurstaki (Btk) must be used on the younger larvae prior to the fourth instar stage to be effective. It is recommended that caterpillars be no larger than 3/4 inch in length if using Btk. Treatment of egg masses with soybean (horticultural) oil or similar products may only be effective for small plants in the nursery.
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.
When used in a nursery setting, chlorpyrifos is for quarantine use only.
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.
