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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.

To sting a stinger: Saddleback caterpillar, Acharia stimulae, and brachonid wasp, Cotesia


Front, rear, and along the flanks, poison glands in the skin of the caterpillar prepare a potent toxin delivered by urticating hairs.


Like other stinging caterpillars saddlebacks are sometimes found in clusters on a leaf.

A close encounter of the first kind with the saddleback caterpillar is usually a memorable one, more likely to be experienced through the sense of touch rather than the sense of sight. Protecting the front and rear flanks of the garishly beautiful caterpillar are projections festooned with nasty spines. Like the stinging spines borne by the larvae of Io moths we met in last week’s episode, these spines, or urticating hairs in entomological lingo, contain toxins released upon contact with a would-be predator or unlucky human. The toxins cause a mild to severe burning sensation reminiscent of a wasp’s sting. In some cases, a very uncomfortable and persistent rash may develop at the point of contact.

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.

In a scene straight out of Aliens, wasp larvae emerge from their saddleback host and spin white cocoons in which to pupate.

While mowing one day, I received a memorable sting on my ankle from a saddleback dining on small violets in my lawn. With such a potent defense, one might think the saddleback has gained immunity from attack by enemies. Unfortunately for the saddleback, this is not the case. Recently while hiking the Appalachian Trail I happened upon a most unfortunate saddleback caterpillar reliving a scene from the 1986 movie Aliens. Festooning the back of the motionless caterpillar were dozens of tiny legless larvae. Like the Alien, fully developed wasp larvae drilled their way through the skin of their victim and writhed on its back as they spun cocoons made of white silk. Observant gardeners have likely seen small white objects like these on the back of hornworms on tomatoes and identified them as 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 on the foliage of plants. Upon encountering a suitable host, they jump aboard and rapidly deliver many stings using an appendage called the ovipositor. Once inside the caterpillar, eggs develop and hatch, and then the wasp larvae feed on the tissues of its host. However, to survive successfully, the tiny wasp larvae 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 Cotesia injects a special virus known as a polydnavirus into the caterpillar. The polydnavirus disables the caterpillar’s immune system, paving the way for her young to develop without interference. Once development is complete, wasp larvae move near the surface of the caterpillar, burrow through its skin, and spin a cocoon on the exterior of their host. Stinging and being stung, part of the circle of life in a bug’s world.


Watch this half-speed video as a parasitic wasp “stings” the saddleback and deposits eggs.


We thank 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 textbook “Medical and Veterinary Entomology” by Gary Mullen and Lance Durden, 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 site: