Gypsy moth
Click for larger image Spongy moth larva or caterpillar (Lepidoptera). Key identifiers: spines, beige head, prominent blue dots followed by prominent red dots. CT Ag Experiment Station Archives, CAES, Bugwood.org

The spongy moth (formerly gypsy moth), Lymantria dispar, is the most important tree-defoliating insect in the eastern U.S. and is slowly expanding its range to include Missouri and Arkansas. Spongy moth caterpillars have very large appetites and are capable of feeding on 500 species of trees and shrubs. The caterpillars defoliate trees at an alarming rate and are best controlled when their populations are at low levels. Spongy moth caterpillars do not build tents; if you observe tents or webs in your trees they were probably constructed by other defoliating insects such as eastern tent caterpillar or fall webworm. The adult moths are active during daylight hours and the adult male may be observed as an active brown moth flying about in a zig-zag pattern. The large, offwhite female moth doesn't fly but may be observed crawling on the ground or clinging to the bark of trees. The name 'spongy moth' was adopted as the new common name for the species in 2022 by the Entomological Society of America, as the foremer name was a derogatory term for Romani people. 

Symptoms and Diagnosis

Spongy moth caterpillars feed on leaves of their preferred host plants, most species of oak. As they increase in size, they are capable of defoliating entire trees. Older caterpillars will feed on the foliage of trees that younger caterpillars avoid. Caterpillars can attain a size of 2 inches and are hairy with a beige head. Prominent blue dots followed by red dots are distinguishable along the back. Spongy moths can be serious pests of oak trees and will readily feed on birch, willow, hawthorn, fruit trees, and many shrubs. The caterpillars are best controlled when their populations are at low levels.

Life Cycle

Egg masses are laid during July on the underside of branches, on tree trunks, firewood, or in other shady spots. They may also be deposited on recreational vehicles, which facilitates the spread of spongy moth when they are moved to another site. The egg masses overwinter and caterpillars emerge from egg masses beginning the following April. Caterpillars climb up to the tops of the trees and begin to feed by chewing small pinholes in the tender, young leaves. As the caterpillars get older, they begin to feed at night. At dawn they crawl down the tree and rest in the leaf litter, returning to the treetop at dusk. Caterpillars defoliate trees for 6–8 weeks and pupate for 7–14 days in leaf litter in late-June to mid-July. Adult moths emerge from pupation and are present from July into August. The female spongy moth is offwhite and does not fly. The smaller, male moth is brown and is active during daylight hours.

Integrated Pest Management Strategies

1. Maintain plant health. Young healthy trees can withstand one to three defoliations with minimal damage. Older trees may not be able to withstand more than one defoliation.

2. Wrap tree trunks with burlap bands or sticky bands in early June to trap the older spongy moth caterpillar as it treks from the canopy to hiding places on the ground. Remove trapped caterpillars daily. Sticky bands have to be replaced periodically.

3. Bacillus thuringiensis kurstaki (Btk) is a biological insecticide that kills caterpillars. Spray Btk on the leaves of the tree at the time the Spiraea x vanhouttei is flowering. This control is best used on low populations and on small caterpillars.

4. Spray the leaves with an insecticide. Treat the leaves in early June. Pesticides registered for use include acephate (Orthene), carbaryl (Sevin), malathion, naled, permethrin, phosmet (Imidan), and pyrethrins.

5. Inspect the underside of vehicles, especially recreational vehicles, traveling from known spongy moth infested areas for egg masses. Destroy egg masses by removal or spray with Golden Natur'l Spray oil or other soybean spray oil to suffocate the eggs.

6. Plant resistant species of trees. This is practical if you are not adjacent to a large stand of oaks. Consider planting ash, tulip poplar, locust, sycamore, or silver maple.

Note: Pheromone traps put up in May and left until August or September are for monitoring the spongy moth population. Traps are not used to eradicate spongy moth.

Organic Strategies

Strategies 1, 2, 3, and 6 are strictly organic approaches. For an organic approach to Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate pyrethrin products.

More images:

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Oak trunk (Quercus) with egg masses of spongy moth (Lepidoptera). Eggs are usually laid in July or August and hatch in April. M. Zubrik, Forest Research Institute - Slovakia, Bugwood.org
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spongy moth larva or caterpillar (Lepidoptera). USFS - Region 8 - Southern Archive, USFS, Bugwood.org
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European white birch (Betula) with adult female spongy moth (Lepidoptera) on egg mass. H. Lemme, Bugwood.org