Spongy moth, Lymantria dispar
Order: Lepidoptera
Family: Erebidae
Description
Male moths have a wingspan of about 3.8 cm (1.5 inches), while females have a wingspan of up to 8.9 cm (3.5 inches) and are more heavy-bodied. The forewings of males are usually brown with dark, wavy markings, and males hold their wings in an orientation that forms a triangular body shape when at rest. Although female moths do not fly, they have white wings with distinct markings that include rows of spots along the wing margins and a dark wavy band running along the center of each forewing. Males have feather-like antennae, while females have threadlike antennae.
The caterpillars measure 3.8-5 cm (1.5-2 inches) long when mature and have a yellow head capsule with five blue and six red or orange tubercles arranged in rows on the back. The hairs on spongy moth caterpillars are not as dense as the hairs on other species of caterpillar, such as the banded woollybear (Pyrrharctia isabella). Egg masses are about 3.8 cm (1.5 inches) long light brown to yellow, hard, and covered in pale brown hairs deposited by the adult mother. The dark brown pupae differ in size depending on sex, with female pupae being larger. The pupae of spongy moth are covered in small hairs, lack a silken cocoon, and resemble a teardrop.
In Colorado, the moths of several closely related species may be captured in monitoring traps and misidentified as male spongy moths. These include the Douglas-fir tussock moth (Orgyia pseudotsugata), western tent caterpillar (Malacosoma californicum), and army cutworm (Euxoa auxiliaris). Caterpillars of the western tent caterpillar and fall webworm (Hyphantria cunea) may be misidentified as spongy moth caterpillars.
Quick Facts
- The spongy moth (Lymantria dispar), formerly referred to as the gypsy moth, is an invasive species in North America that was introduced in the late 1800s. This species is native to Europe and Asia.
- This pest can feed on the foliage of over 300 tree and shrub species. Defoliation increases host plant susceptibility to secondary diseases or other pests. Since 1924, over 81 million acres of forest have been defoliated in the United States.
- The spongy moth is a federally regulated pest, which entails the establishment of quarantine zones and restrictions on moving materials that may contain egg masses.
- Monitoring involves the use of traps baited with a sex pheromone for capturing male moths.
- While this pest has been reported in Colorado in the past, eradication efforts have been successful in preventing populations from becoming established.
Life history and habits
This pest overwinters as eggs laid on the surface of trees, fence posts, outdoor furniture, cars, rock walls, firewood, or other hard structures. After hatching in the spring, newly emerged caterpillars disperse from the egg mass and ascend suitable host trees where they begin feeding on new foliage. The caterpillars produce silk as they crawl, which functions as a safety line and facilitates dispersal to new locations when caterpillars drop from branches and are carried on wind currents, a process known as “ballooning”. This dispersal mechanism allows young caterpillars to move up to half a mile from the egg mass and is a relatively slow process that limits its rate of spread across the United States. Young caterpillars tend to remain in the lower canopy or forest understory throughout the day, while mature larvae are highly mobile. Although oak (Quercus spp.) is the preferred host, spongy moth caterpillars can feed on a wide variety of trees and shrubs including but not restricted to maple (Acer spp.), birch (Betula spp.), poplar (Populus spp.), apple (Malus spp.), hawthorn (Crataegus spp.), and willow (Salix spp.). Spongy moth caterpillars develop through four or five instars and tend to feed at night, while resting in cool, shaded sites during the day. However, they may remain active continuously at high densities and mature more rapidly when compared with growth rates at lower densities. When mature, caterpillars migrate to protected sites under bark flaps, in crevices, under branches, or on the ground before entering the pupal stage. High densities of spongy moth caterpillars can cause individuals to pupate on exposed portions of the trunk or the foliage of non-host trees.
After emerging from the puparium, female moths emit pheromones that attract male moths as they fly in a zigzag pattern in search of mates. Female moths produce pheromones that attract male moths and die shortly after laying eggs. Eggs of the spongy moth tend to be laid near the mother’s pupation site. Low densities of spongy moth can exist for many years in regions of North America where this pest is established, with outbreaks more likely to occur when late larval survival is high. However, the timing of outbreaks is difficult to predict. One generation is produced annually.
Injury
Feeding of caterpillars can lead to severe defoliation, dieback of branches, and increases the host tree’s susceptibility to attack by other organisms. Although healthy trees can recover from complete defoliation, multiple years of heavy defoliation can result in high mortality of oaks. Trees stressed by drought may fail to replace lost foliage. Some individuals are sensitive to skin and hairs shed by molting caterpillars, which can irritate the respiratory tract and cause rashes. High densities of caterpillars produce copious amounts of frass (excrement) that can accumulate on outdoor surfaces and pollute water sources.
Monitoring
Monitoring for spongy moth in Colorado is often conducted with pheromone baited delta traps deployed in high-risk areas. However, it is worth noting that caterpillars and eggs are more likely to be observed by homeowners. Capturing caterpillars is possible with a burlap barrier band trap, which can be made at home by following these protocols. In such instances, taking photos or preserving specimens of the suspected spongy moth is highly recommended. Spongy moth sightings can be reported here.
Prevention
Adequately watering plants during periods of drought as well as properly pruning and fertilizing trees and shrubs is recommended. Maintaining a weed- and grass-free diameter of four feet around the base of trees will limit competition for resources. Firewood should not be transported across counties that harbor this pest.
Biological control
When infestations are light, natural enemies such as wasps, flies, beetles, spiders, ants, birds, mammals, and certain pathogens can effectively suppress spongy moth populations. Some examples of parasitoid wasps include Aleiodes indiscretus, which attacks spongy moth caterpillars, and Pimpla disparis, which attacks pupae. Pathogens of spongy moth include the nucleopolyhedrosis virus and Bacillus thuringiensis kurstaki (Btk), which are naturally occurring disease agents of caterpillars. In western states such as Colorado, eradication programs involve multiple applications of Bt within a one-mile radius of the trapping site. This insect pathogen is most effective against young caterpillars and requires adequate timing to provide effective control. Applications of Btk are particularly useful in integrated pest management programs since it has a relatively low environmental impact.
Chemical control
Homeowners can apply certain broad-spectrum insecticides to the tree crown or ground to target larvae. However, these insecticides also act on non-target organisms and can lead to outbreaks of secondary pests.
References
Alalouni, U., M. Schadler, and R. Brandi. 2013. Natural enemies and environmental factors affecting the population dynamics of the gypsy moth. Journal of Applied Entomology. Available https://onlinelibrary.wiley.com/doi/full/10.1111/jen.12072
Camper, M., and W. Cranshaw. (n.d.). Gypsy Moth in Colorado – Identification of Insects and Damage of Similar Appearance. Colorado Exotic Insect Detection and Identification Fact Sheet Series. Available https://static.colostate.edu/client-files/csfs/documents/Gypsy_Moth.pdf
Childs, R. 2016. Gypsy Moth (Lymantria dispar). University of Massachusetts Amherst. Available https://ag.umass.edu/landscape/fact-sheets/spongy-moth#:~:text=Host%20Plants%3A&text=Conifers%2C%20such%20as%20pine%20and,can%20highly%20stress%20host%20trees.
Elkinton, J., and A. Liebhold. 1990. Population Dynamics of Gypsy Moth in North America. Annual Review Entomology. 35: 571-596. Available https://www.annualreviews.org/doi/pdf/10.1146/annurev.en.35.010190.003035
Liebhold, A., J. Elkinton, D. Williams, and R. Muzika. 2000. What causes outbreaks of the gypsy moth in North America? Population Ecology. 42:257-266. Available https://esj-journals.onlinelibrary.wiley.com/doi/abs/10.1007/PL00012004
Liebhold, A., J. Halverson, and G. Elmes. 1992. Gypsy moth invasion in North America: a quantitative analysis. Journal of Biogeography. 19:513-520. Available: https://www.jstor.org/stable/2845770?seq=5
McCullough, D. 2022. Spongy Moth Life Cycle. Michigan State University: Extension. Available https://www.canr.msu.edu/resources/spongy-moth-life-cycle
Mull, A., and L. Spears. 2022. Spong Moth. Utah State University – Extension. Available https://extension.usu.edu/pests/research/spongy-moth.pdf
Rindos, M., and Liebhold, A. 2023. The spongy moth, Lymantria dispar. Current Biology Magazine. 33: 665-668. Available https://www.cell.com/current-biology/pdf/S0960-9822(23)00371-8.pdf
Stafford, K. 2017. The Gypsy Moth. The Connecticut Agricultural Experiment Station. Available https://portal.ct.gov/-/media/CAES/DOCUMENTS/Publications/Fact_Sheets/Entomology/GypsyMothFactSheetpdf.pdf
Webb, R., R. Peiffer, R. Fuester, K. Thorpe, L. Calabrese, and J. McLaughlin. 1992. An Evaluation of the Residual Activity of Traditional, Safe, and Biological Insecticides Against the Gypsy Moth. Journal of Arboriculture. 24(5):286-293.