Eastern Tent Caterpillar

Classification

Common name: Eastern tent caterpillar

Scientific name: Malacosoma americanum (Fabricius, 1793)

Family: Lasiocampidae (tent caterpillars, lappet moths)

Order: Lepidoptera (moths and butterflies)

Summary

Eastern tent caterpillars occasionally outbreak and defoliate deciduous forest and ornamental trees. While this can concern land- and homeowners, healthy trees can withstand the defoliation and should recover without intervention.

Description

Eastern tent caterpillars are perhaps best known for the web tents they build collectively (Figure 1). These tents occur in the crotches and forks of branches in spring and early summer, and should not be confused with fall webworm tents, which occur at branch tips in the late summer and fall.

Figure 1. Eastern tent caterpillar tent. Note that it is in the fork of the branches, not at the tip as in fall webworm. Photograph by kavityman via iNaturalist, used under a CC BY-NC 4.0 license.

Caterpillars of all sizes are covered in defensive hairs. Young caterpillars are dark with two thin yellow stripes along the back (Figure 2). Mature caterpillars are about two inches long. They have a median white stripe on the back flanked by two yellow stripes; the rest of the body is mottled blue, yellow, and black (Figure 3). They may be confused for forest tent caterpillars, which have white dots down the back instead of a solid line and do not build silk tents.

Figure 2. Young eastern tent caterpillars. Photograph by Jim Moore via BugGuide, used under a CC BY-ND-NC 1.0 license.

Figure 3. A mature eastern tent caterpillar. Photograph by Thomas Schultz via iNaturalist, used under a CC BY-NC 4.0 license.

Cocoons white to yellowish-white and are spun in dry, protected places.

Adults are fluffy, tan to light brown with two white, oblique stripes on the wings (Figure 4).

Figure 4. Adult eastern tent caterpillar moth. Photograph by Michael H. King via iNaturalist, used under a CC BY-NC 4.0 license.

Eastern tent caterpillar eggs are black masses that wrap around small twigs of appropriate host plants (Figure 5). They overwinter as eggs.

Figure 5. Eastern tent caterpillar egg mass (left), compared to black knot fungus (right), which were found on the same black cherry tree. Note how the egg mass is smooth and shiny while the fungus is rough and knotty. Photograph by MC Barnhart via iNaturalist, used under a CC BY-NC 4.0 license.

Distribution

Eastern tent caterpillars occur throughout eastern North America, from the northeastern United States and adjacent Canada, south to Florida, and west to the Dakotas and Texas (Figure 6). Sporadic records occurs somewhat further west into Colorado and Montana as well.

Figure 6. Range of eastern tent caterpillars in North America. Map via Butterflies and Moths of North America (BAMONA 2023).

Life History and Behavior

Eastern tent caterpillars have one generation per year. Caterpillars hatch out in early spring. Caterpillars of all sizes use the web tents for protection from predators and parasitoids and feed on leaves away from the tent. When caterpillars mature in early summer, they migrate away from the tent to find a protected place to spin a cocoon and pupate. This can be cause for concern for homeowners when they encounter large numbers of wandering caterpillars. However, mature caterpillars in this stage do not feed and are no longer a threat to plants.

Adults emerge in late June and July and lay egg masses that contain 150-350 eggs. The eggs overwinter.

Host Plants and Economic Damage

Eastern tent caterpillars preferentially feed on rosaceous trees, including cherry, crabapple, and apple. The eastern tent caterpillar occasionally feed on other deciduous shrubs and trees. When caterpillars are abundant, they can completely defoliate an infested tree. Because they hatch early, they may consume all of the fresh buds and leaves, which gives the appearance that a tree did not leaf out.

Historically, eastern tent caterpillars were considered economically important pests that warranted widespread control efforts. However, the caterpillars are a native species that are attacked by a plethora of natural enemies, including birds and various insect predators and parasitoids. So outbreaks are periodic and short-lived and damage is generally considered aesthetic rather than a health threat to most trees.

Medical and Veterinary Concern

Eastern tent caterpillars are covered in long defensive hairs. It is recommended to wear gloves when dealing with them, but even ungloved it is usually safe to handle them gently. Just make sure to wash your hands after handling caterpillars in order to avoid getting hairs into the eyes or mouth, where they may be irritating. There are no recorded instances of children swallowing eastern tent caterpillars like there are for hickory tussock moth caterpillars, but it is likely that swallowing one would cause a similar reaction.

Accidental consumption of wandering caterpillars has been linked to mare reproductive loss syndrome in horses, so horse owners and breeders should be cognizant of the host trees around horse pastures (since the caterpillars prefer cherries and apples) and outbreak status of caterpillars.

Prevention and Control

Healthy deciduous trees can withstand total defoliation multiple years in a row without long-term detrimental effects, so it is not recommended to attempt to control caterpillars in natural forest settings. For ornamental trees, one option is to keep trees healthy and do nothing.

If control is warranted, caterpillar density can be reduced by pruning out egg masses during the winter before they hatch and manually removing silk tents using a stick or other implement and disposing of them. CATERPILLAR TENTS SHOULD NEVER BE BURNED WITH FIRE. Besides potential danger to nearby structures, the fire will cause more damage to the tree than the caterpillars could.

If manual removal is not possible, young caterpillars can be controlled using Bt or horticultural oil. Older caterpillars can be controlled using broad spectrum insecticides – there are many products labeled for caterpillar control on ornamental trees – but this should only be done as a last resort given the considerations mentioned above.

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

References

(BAMONA) Butterflies and Moths of North America. 2023. Eastern tent caterpillar moth.  Accessed 15 February 2023.

Sebastian, M., M. G. Gantz, T. Tobin, J. D. Harkins, J. M. Bosken, C. Hughes, L. R. Harrison, W. V. Bernard, D. L. Richter, and T. D. Fitzgerald. 2003. The Mare Reproductive Loss Syndrome and the eastern tent caterpillar: a toxicokinetic/statistical analysis with clinical, epidemiologic, and mechanistic implications. Veterinary Therapeutics 4(4): 324 –339.