One common stinging caterpillar you may encounter this time of year is the saddleback caterpillar. I have been "stung" many times by these critters when working in corn research plots in Maryland in the past. They are also found in the landscape. The following is an article on the subject from the University of Kentucky.
Caterpillars are tasty morsels for predators so many species rely on camouflage to stay off the menu. Those that can protect themselves, stinging caterpillars for example, stay in plain sight and advertise with bright or distinctive markings. They are covered with sharp, brittle bristles that break off and stick in the skin like so many cactus spines. Unfortunately, that’s not all, the bristles are hollow and contain an irritating substance that produces a very unpleasant skin reaction.
Saddlebacks are relatively common and are among the most venomous species in North America. A full grown caterpillar is brownish red, about 1.25 inches long, and has several distinctive characteristics which include two fleshy horns studded with spines on each end, shorter armed bumps along both sides. The back carries a bright green "blanket" trimmed in white, with brown-to-purple central spot or "saddle", also trimmed in white. Saddlebacks occur on many plants including apple, basswood, cherry, chestnut, dogwood, elm, maple, oak, plum, and even corn. They are most abundant at this time of year.
A brush with a saddleback results in immediate pain followed by swelling and blistering if the spines are not removed. The irritation should be gone in 8 hours or less if the spines are removed. They can be stripped cellophane or adhesive duct tape. Wash the area with soap and water and apply an ice pack to relieve the irritation. Contact a physician immediately if the irritation does not subside of if other symptoms appear.
More information on stinging caterpillars is available in www.ca.uky.edu/entomology/entfacts/entfactpdf/ef003.pdf
Saddleback caterpillar. Photo by Herbert A. "Joe" Pase III, Texas Forest Service, Bugwood.org
Reprinted from "Caterpillars That Pack a Punch" By Lee Townsend in the current edition of the Kentucky Pest News from the University of Kentucky http://www.uky.edu/Ag/kpn/kpn_09/pn_090811.html