Stinging Caterpillars

With over 1 million described species of insects in the world, and perhaps 9 or 10 more million kinds still to put names on, it stands to reason that insects fit just about every environmental niche possible, and have a bewildering variety of life styles and appearances. Since most of the species on Earth today have been around for millions of years, it is obvious that each of them has found some mechanism that allows them to survive.

Some of the survival techniques employed include colors or shapes that camouflage the insect in its environment, looking for all the world like a dead leaf or the bark of a tree. It may be walking sticks that look like twigs, and the hope of this technique is that the bug simply cannot be seen by predators who otherwise would dine on it.

Other insects, however, show the opposite. They exhibit colors that make them obvious to all who glance their way. Their reds, yellows, pink, or orange colors make them stand out against the backdrop of their environment, and you might wonder why it is they haven’t long ago been a meal for some hungry bird or lizard. In many cases, as it turns out, these bright colors are “warning” colors, and they advertise to potential predators that THIS particular bug should not be messed with, for it can fight back.

Stink bugs, such as the Harlequin Bug, have foul smelling and tasting body fluids, and their bright orange and black bodies warn predators away. The striped Monarch Butterfly larva is telling predators that its body is filled with toxic alkaloids, gained from feeding on the milkweed plants that contain these chemicals. Wasps and bees, with their bright yellow and orange colors, advertise their ability to sting any animal foolish enough to try to swallow them.

One group of insects that we might never think of as harmful, though, is caterpillars. When we think of butterflies and moths we think of beautiful flying insects, whose larvae are fuzzy or colorful, but certainly not dangerous. However, this group of insects includes a great many that are either toxic, distasteful, or very well known for their ability to sting. Unlike the scorpions or the wasps, the sting does not come from a “stinger” at the end of the body, but instead it comes from the many hairs located along the body of the caterpillar. These spiny hairs are called “urticating” hairs, and in many cases they are attached to small poison glands in the skin of the caterpillar.

In some cases, should you contact the caterpillar with your skin, the effect would just be an irritation, somewhat like touching the plant stinging nettle. However, with a few families of the caterpillars, and with sensitive people, the effect can be dramatic. Some of the families of butterflies and moths that have caterpillars with urticating hairs are:

  • Nymphalidae – butterflies such as the large blue Morpho butterflies in South America and the Mourning Cloak butterfly in North America.
  • Saturniidae – the big silk moths, whose caterpillars are large and covered with spines.
  • Arctiidae – the tiger moth caterpillars that often are spiny or hairy.
  • Lymantriidae – some of the Tussock Moth caterpillars

However, the king of the stingers, without a doubt, is the moth family called Megalopygidae, or the Flannel Moths. This is a family of somewhat small moths, and in the United States is represented by only a handful of different kinds. In the American Tropics, though, there are many dozens of different species. The caterpillars may be extraordinary, with long, colorful hairs completely covering their bodies and looking for all the world like anything but a caterpillar. In the U.S. the larvae also are fully clothed with hairs, and the most notorious of these is called the Puss Caterpillar.

The adult Puss Caterpillar is a moth with the scientific name of Megalopyge opercularis, and it is a nondescript, hairy brown caterpillar about 1 inch long. The adult is a white moth with a wing span of around 1 inch as well, and it is found in the eastern states of the United States. This particular caterpillar has made the news many times, for in some years its numbers increase to high levels, making the likelihood of human contact very high as well. There have been several years, in the San Antonio, Texas area, when public schools were closed due to the high numbers of caterpillars in the schoolyard trees.

In Texas in 1958 there were more than 2,100 stings reported. In 1960, in South Carolina, populations of caterpillars were at a peak, and more stings were reported that year alone than in the 25 previous years combined. The year 2001 appears to have been another banner year for stinging caterpillars, as University Extension Service personnel were reporting dramatically higher numbers of incidents than usual.

A recent encounter, by a landscape company employee, was described, after he was simply “brushed” on his leg by a Puss Caterpillar. The contact occurred at around four in the afternoon, and within 15 minutes that entire leg was in severe pain. Within another hour or two the leg had swollen to almost twice its normal size. After another 6 hours he began to have drainage of fluids from the skin at the site where the caterpillar has touched him, and it took a full 24 hours for the leg finally to get back close to normal.

One account by a researcher in South America told of picking up the caterpillar of one of the moths in this family. As any good researcher would do, when faced with an organism that he has found for the first time, this person crushed the caterpillar in his hand “just to see what would happen”. According to the researcher’s notes, his hand and arm were virtually paralyzed for almost a month.

The Puss Caterpillar gains this awesome protection from its urticating hairs, and each time the caterpillar molts, or sheds its skin, it becomes slightly larger and armed with a greater number of hairs. Even the shed skin, or a dead caterpillar, is capable of causing stings to human skin. The nature of the venom is currently not well understood, and treatment of people who have reacted violently to the venom is generally in the area of first aid – ice the area, take pain medication, elevation of the area affected, and removal of any lingering spines with adhesive tape applied and removed several times.

Two more of the commonly encountered caterpillars that may cause stinging are the Saddleback Caterpillar – Sibine stimulea – a green and brown caterpillar with tufts of hairs front and back, and the IO moth caterpillar – Hemileuca io – a green caterpillar with rows of hairs along its body. Both of these species may feed on a variety of landscape shrubs or trees, or on blackberry, and the chances for human contact are increased.

The good news is that, while it may be painful, the stings are rarely as dramatic as the one we described earlier. Different people react in different ways, and most people will certainly find it to be uncomfortable, but also quite temporary.