Browntail-Moth Caterpillar (Euproctis chrysorrhoea)
Brown-tail moth caterpillars emerging from the egg mass

Fall nests in which the caterpillars overwinter.  Fernald and Kirkland (1903) found a mean of 290 caterpillars per web but up to as many as 802, showing the colonies originating from separate egg masses may fuse.  Images by permission of Paul Cecil
Many species of social caterpillars are urticating, and the caterpillar of the brown-tail moth is no exception.  The aggregated caterpillars, constituting a large mass of potential food, would be particularly attractive to bird predators were it not for the fact that they are rendered unpalatable not only by the long copious hairs that invest their bodies but by short poisonous setae as well.  When the caterpillar first invaded New England, residents of infested areas experienced mild to severe rashes.  Initially, the cause of the problem was not recognized and there was concern among affected individuals that a new contagious disease had arrived.  The cause of the trouble was soon traced to the shed and wind-disseminated hairs of the caterpillars.  To quote a resident of Somerville:  

“While cutting the limbs from the infested trees my husband was badly poisoned, his eyes in particular being affected. In cleaning the window screens in the house I was also badly poisoned; the skin of my body was as much inflamed as if a mustard paste had been applied. The poisoning did not seem to come entirely from contact, but the air seemed to be filled with something which caused the itching and burning sensation. We were affected during the whole two months that the insect was in the larval stage. In 1897, while the house was being painted, the painters all suffered more or less. Their distress was so great that they were obliged to stop their work and bathe the face, hands and aims with soda water to get relief. We could not hang out the clothes, as the hairs would cling to them, especially to the flannels, and when worn they would badly irritate the skin.”
The setae that cause the irritation appear in the third and subsequent instars. These hairs are very small, approximately 0.1 mm in length, sharp pointed and barbed. They occur on the upper surface of first and second abdominal segments. They are easily detached and the loose hairs can become airborne.  They lodge in the silk of the cocoon as it is spun by the caterpillar and can adhere to the emerging adult which then disseminates them.

It was originally thought that the irritation caused by the hair was purely mechanical but it is now known to be due to chemical toxins as well.  The offending setae are hollow and when attached to the caterpillar have at their bases canals that lead to poison glands in the epidermis of the caterpillar, the liquid of which floods the interior of the seta.  When they dislodge and penetrate the skin of the victim, the setae spill their contents, causing the rash.  Analysis has shown that the irritants produced by the caterpillar include histamine and a number of enzymes.  While in most people the combined effect of the mechanical and chemical assault causes only a mild rash, some individuals experience a much more severe reaction  Both immediate and delayed-type hypersensitivity reactions have been reported and at least one individual has died from an immune system reaction to the insect.
“In the summer of 1896 I first noticed the brown-tail moth on my premises. The caterpillars did not do any particular damage that year, though the millers came out thick. In 1897 the caterpillars came out in such numbers they destroyed everything. I did not take any special pains to get rid of them until after they had eaten up everything. We had no fruit that year, and we have had none in 1898. While feeding, the caterpillars would devour leaves, fruit, buds and all. In 1896 only one small tree was attacked the next year they attacked the cherry, elm, pear and apple trees. They came into the house; the walks and fences were also covered with them. For three or four days I went out every morning and swept them off the planks. Last winter I cut off all the tents, so that we did not have any this year. Generally speaking, my neighbors took care of their trees, though there were a few who did not.”
Accidental Introduction to New England

The brown-tail caterpillar was accidentally imported to Somerville, Massachusetts, most likely on roses, in the late 1800’s.  The insect came to the attention of the public when it achieved outbreak proportions in 1897.  To quote a local resident:
A laboratory colony of young caterpillars of the brown-tail moth showing the silk nest, the egg mass (arrow), and the larvae skeletonizing the leaves of apple.
Female brown-tail moth.  The brown hairs on the abdomen are shed when the moth oviposits, covering the egg mass.  Photo credit Wikimedia Commons by Joaquim Alves Gaspar.
Brown-tail moth oviposting.  The eggs are hidden under the brown, felt-like covering.
Dermatitis from exposure to the urticating setae of the brown-tail moth caterpillars.  (Photo: Wikimedia commons)
Setal types found on the caterpillars of the brown-tail moth caterpillar.  The short setae in illustration 3 are urticating, causing a rash on contact.  Plate from Fernald and Kirkland (1903)
The setae causing irritation are found on the four pads that occur on the dorsal surface of the first and second abdominal segements (arrows)

The caterpillars construct cocoons during the latter half of June.  The cocoons are often spun in leaves at the ends of branches.  The moths emerge from the cocoons in July and mate and oviposit soon thereafter.  The stage for larval sociality is set by the adult which deposits its eggs in an egg mass which it covers with the brown hairs of its abdomen.  The egg masses typically contain 200-400 eggs which hatch in about 3 weeks.


The leaves that the caterpillars encounter when they eclose from their eggs in late July or early August are typically aged and tough.  The caterpillars are able to feed only on the softer parts of the leaf, leaving the tough veins untouched.  This type of feeding habit is referred to as skeletonizing.  Though undocumented, it is likely that a pheromone defines the limits of the foraging arena and serves to hold the colony together.  The caterpillars stop feeding in September after having advanced to the second or third instar.  Caterpillars lie sequestered and quiescent within their nests until the following spring when they resume feeding.

First instar caterpillars skeletonizing the leaf of an apple (Malus) tree.  Side by side feeding is a common habit of social caterpillars.
The later instars that occur in the spring are no longer constrained to the skeletonizing habit and consume whole leaves leading to the rapid defoliation of colonized trees.
The small caterpillars spin silk as they feed and rest under the material when they are not feeding.  These early webs are added to and eventually become nests on which the colonies aggregate.
Fully developed spring nest.  Although unstudied, the caterpillars are likely use the nest to facilitate thermoregulation as shown for other social caterpillars. Photo credit: Jan Samanek, State Phytosanitary Administration, Bugwood.org
As the caterpillars grow, they continue to enlarge the nest and it becomes the center of their activity.  As nearby leaves are consumed, the caterpillars move farther from the nest to find food but always return to the structure after feeding.  Efficient movement between the nest and distant feeding sites is facilitated by trail marking. Although the frequently used pathways leading from the tent to feeding sites are covered with silk, studies suggest that a trail pheromone rather than the physical properties of the silk guides the caterpillars as they move along the trails.

Fernald C. H. and A. H. Kirkland. 1903. A report on the life history and habits of the imported brown-tail moth. Massachusetts.  State Board of Agriculture.
Hossler E. W. 2010. Caterpillars and moths: Part I. Dermatologic manifestations of encounters with Lepidoptera. J. Am. Acad. Dermatol. 62 (1): 1–10
Hossler E. W. 2010. Caterpillars and moths: Part II. Dermatologic manifestations of encounters with Lepidoptera. J. Am. Acad. Dermatol. 62 (1): 13–28
Schaefer P. W. 1986. Bibliography of the browntail moth, Euproctis chrysorrhoea (L.) (Lepidoptera: Lymantriidae) and its natural enemies. Agric. Exp. Sta. Univ. Delaware. 66 pp.