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Puss Caterpillar (Flannel Moth larvae)
Megalopyge opercularis

by Jerry Cates

The puss caterpillar is one of more than 50 types of caterpillars found in the U. S. that are capable of stinging. Of these, the puss caterpillar is considered to be near the top of the list in terms of the severity of its sting. 

The larval stage of this insect is a small, wooly, pussycat-appearing caterpillar covered with rows of long spines that look like fine, soft, cuddly hairs. Because they appear to be small, innocuous pieces of fluff, children and adults alike are led to pick them up and play with them, usually with immediate, extremely painful results. The spines, hidden among the hairs, are hollow, and release a toxin of unknown chemical composition that produces a skin lesion. The lesion usually begins as a halo of reddened skin surrounding the sting site. This is soon followed by the appearance of a number of small cysts that gradually develop into a matrix of blood-filled lesions where the mass of spines punctured the skin. This clinical presentation is characteristic enough, generally, to permit physicians to make a diagnosis of the cause even in the absence of the caterpillar.

These caterpillars have been called by several names, including puss caterpillar, wooly worms, wooly slugs, opossum bugs, tree asps, and, in Mexico, el perrito (little dog). The larvae pupate inside the skin of the caterpillar itself, and spend the winter attached to a tree limb, trunk, or brick/masonry post. When you see a "dead" puss caterpillar, you are probably looking at its pupa, which is still very much alive. The adult flannel moth emerges from the pupa in the spring, mates, lays its eggs on the foliage of a nearby shrub or tree, and dies. The larvae hatch from the eggs and voila! Your experience with puss caterpillars begins afresh. 

Although M. opercularis is fairly common throughout the south central and southeastern U.S., it rarely is present in large enough numbers to present a serious risk to humans. Historical records show that large outbreaks occurred in San Antonio, Texas in 1923 and in Galveston, Texas in 1954. I found these specimens at a client's site where aggressive treatments had been performed to eliminate red imported fire ants (Solenopsis invicta). S. invicta is a major predator of M. opercularis, and it is likely that the absence of the former allowed the latter to thrive. Judging from reports received in 2003, however, serious outbreaks are now more common than previously thought. Whether this is due to underreporting in past years, or to recent surges in infestations, remains to be determined.

The larvae I found were feeding on the leaves of several dwarf yaupon holly (Ilex vomitoria nana) bushes at the affected site. I. vomitoria is a favorite host for this insect, but it will also feed on citrus, oak, elm, hackberry, sycamore, and several other plants. A thorough inspection of the site confirmed that several shrubs, all of the same species, were infested, but no other plants were being attacked. Using organic pesticides,  the infestation was quickly brought under control.  

The specimens presently pictured here were retained in our laboratory, in a special enclosure that prevented their escape. I do not advise others to keep these insects, alive or dead, where they may be touched by others. The risk of accidental stings is too great. One evening shortly after placing these in the lab, for example, I suddenly felt an excruciating sharp pain in my left eye, and later concluded that a single spine from a puss caterpillar I had stepped on earlier in the day had been transferred from my boot to my hand while undressing. Rubbing my eye transferred the spine from hand to eye. I felt the effects of this error for more than a few hours... It is hard to imagine the pain that would be caused if a child picked one of these up, got numerous spines on their hands, and rubbed both eyes vigorously.

This experience proves the caterpillar does not have to be alive to "sting" you. The hollow spines, filled with toxin, will continue to cause painful stings hours long after the insect has been killed.

FIRST AID: See comments on first aid on Puss Caterpillar Encounter Reports.

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