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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Stinging caterpillars - White flannel moth, Norape ovina, and Saddleback caterpillar, Archaria stimulea


Poison glands in the skin of the caterpillar prepare a potent toxin delivered by urticating hairs.


Over the past few weeks, I have received several reports of surprising encounters with stinging caterpillars, including the white flannel moth caterpillar and the saddleback caterpillar. Often this experience commences when someone brushes up against foliage of trees, shrubs, or herbaceous plants where caterpillars are feeding. A few weeks ago I received an email from a colleague who had a nasty surprise while removing some damaged foliage from a redbud. Moments after brushing up against some leaves, he received “some of the worst stings of my life.” Although the initial pain and redness lasted a few hours, the next day he additionally discovered “blisters of varying sizes on my right wrist where the caterpillars stung me, a small area about 1" by a half inch. I don't think I'll forget these critters any time soon.” Ouch! This close encounter of the stinging kind was delivered by larvae of the white flannel moth. 

White flannel moth caterpillars are sometimes found in large numbers feeding on leaves of trees.

These caterpillars have spines, a.k.a. urticating hairs in entomological lingo, which contain irritating toxins released upon contact with a would-be predator or unobservant human. The toxins cause a mild to severe burning sensation. In some cases, a very uncomfortable and persistent rash may develop at the point of contact. In addition to redbud, these stinging caterpillars are known to eat black locust, greenbrier, elm, and hackberry. They are sometimes found in large numbers on a single branch. My most recent encounter with a stinging caterpillar happened while mowing a shady area of my lawn covered with violets. After walking through the patch, I was greeted with memorable stings on my bare ankles. The sensation was so strongly reminiscent of a yellow jacket attack that I quickly skipped away. On closer inspection, I discovered several small saddleback caterpillars grazing on my violets. Protecting the front and rear flanks of the garishly beautiful caterpillar are projections festooned with nasty spines. One look at the saddleback completely explains its name. In the center of its back is a striking brown shield surround by a ring of white that closely resembles a saddle used to ride a horse.

Saddleback caterpillars eat a wide variety of plants in the forest and garden including oaks, elms, lindens, apples, plums, corn, blueberries, and grapes. With such a potent defense, one might think the saddleback has gained immunity from attack by enemies.

Dozens of wasp cocoons adorn the back of this saddleback caterpillar.

Unfortunately for the saddleback, this is not the case. Recently, I received a saddleback bedecked with tiny white objects on its back. Gardeners have likely seen small white objects like these on the backs of hornworms on their tomatoes, and identified them as the eggs of some mysterious enemy of the caterpillar. In reality, these are cocoons of small parasitic wasps in the genus Cotesia. Female Cotesia wasps hunt saddlebacks and other caterpillars in the foliage of plants. Upon encountering a suitable host, they jump aboard and rapidly deliver many stings using an ovipositor to lay their eggs in the unwitting caterpillar. However, to survive and complete its development, the tiny wasp larva must avoid death by the caterpillar’s vigilant immune system. This is where a little help from mother comes along. In addition to depositing eggs, mother injects a special virus known as a polydnavirus into the caterpillar. The polydnavirus disables the caterpillar’s immune system, paving the way for the young wasps to develop without interference. Once development is complete, the wasp larvae move to just beneath the skin of the caterpillar, burrow through, and spin a cocoon on the exterior surface of their host. After a few days in the laboratory, dozens of parasitic wasps emerged from my hapless saddleback and immediately attacked other saddleback caterpillars nearby. Stinging and being stung: part of the circle of life in a bug’s world.


We thank Bob Nixon for sharing his flannel moth tales and images, and Ellery Krause and Dan Gruner for bearing the stings of caterpillars and capturing the wasps that inspired this week’s episode of Bug of the Week. The wonderful books “Medical and Veterinary Entomology” by Gary Mullen and Lance Durden, and “Caterpillars of North America” by David Wagner, and the fascinating article “Effects of the polydnavirus of Cotesia congregata on the immune system and development of non-habitual hosts of the parasitoid” by N. Lovallo, B. A. McPheron, and D. L. Cox-Foster were used as references for this episode.

To learn more about saddlebacks and other stinging caterpillars, please visit the following web sites: