Dr. Blake Layton is an university extension service entomology specialist.

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Beware the Hair!
by Blake Layton, Ph. D.    

Gardeners normally think of caterpillars as pests that injure plants they are trying to grow, but some caterpillars can actually injure people, stinging or causing skin rashes. However, most caterpillars cannot sting because they are not equipped to do so. Although many caterpillars have spines or hairs on their body that look like they might sting or cause irritation, there is usually no venom associated with these spines or hairs, and they are usually not able to penetrate human skin. Stinging caterpillars have special, sharp spines or hairs that are linked to venom glands. Unlike bees and wasps, caterpillars do not actively sting; rather, the stings occur when an unsuspecting victim accidentally presses an area of skin against the caterpillar.

Severity of caterpillar stings varies considerably depending on species, degree of contact and individual sensitivity. Lightly brushing the back of your hand across an Io moth caterpillar (Automeris io) may only cause a slight prickling sensation, but leaning back against a puss caterpillar (Megalopyge opercularis) that has crawled inside your shirt will probably cause much greater pain. Eye or mucous membrane contact is usually more serious than skin contact.

You cannot tell whether a caterpillar can sting just by looking at it. Some of the most dangerous-looking caterpillars are actually quite harmless. The hickory horned devil caterpillar, also known as the royal walnut moth (Citheronia regalis), is a good example. These large caterpillars are sometimes found in late summer or fall, crawling about in search of a place to pupate. The orange and black spines located on the back may look dangerous, but they are just a bluff. The cecropia moth (Hyalophora cecropia) is another large caterpillar with wicked-looking, but harmless, spines.

  Mature hickory horned devil caterpillar's spines may look formidable, but they are harmless. (5 inches)

  The spines on this cecropia caterpillar (Hyalophora cecropia) are not as dangerous as they look. (4 inches)

 The horn on the rear of the tobacco hornworm (Manduca sexta) is soft and pliable and will not penetrate the skin. It is not a stinger. (3½ inches)

Gulf fritillary larvae (Agraulis vanillae) are covered with spines, but they do not sting. (1¾ inch)

Many people are intimidated by the “horn” on the rear of the tobacco hornworm (Manduca sexta), a large, green caterpillar that often defoliates backyard tomato plants. This horn is a distinguishing trait of most sphinx moth caterpillars. It may look a bit like a stinger, but it is flexible and harmless. The spine-covered gulf fritillary (Agraulis vanillae) feed on wild maypops (Passiflora incarnata) and other passionflower vines (Passiflora spp.) in butterfly gardens. They certainly look prickly, but they do not sting. The orange color is to warn potential predators that they are poisonous if eaten.    

Io moth caterpillars (Automeris io) look like they can sting and they can. They are heavily armed with sharp, venomous spines. Buck moth caterpillars (Hemileuca maia) are similarly armed. Intense contact with a buck moth caterpillar, such as inadvertently sitting on one while wearing shorts, can even leave a caterpillar shaped scar. Fortunately, buck moths have only one generation per year, but in some regions the wandering prepupal caterpillars can be quite numerous in spring to early summer.

The saddleback caterpillar (Acharia stimulea) is another stinging species. This unusual caterpillar belongs to the group called slug caterpillars, and most slug caterpillars have stinging spines. Crowned slugs (Isa textula) are only about ⅝ inches long, but can cause unpleasant stings if one drops down your shirt collar on a windy fall day. One of our strangest-looking stinging caterpillars is the monkey slug, also known as the hag moth (Probetron pithecium).


Io moth caterpillars (Automeris io) are well armed with stinging spines. (2½ inches) 

Buck moth caterpillars (Hemileuca maia) can cause painful stings and can even leave scars. (2½ inches) 

Saddleback caterpillars (Acharia stimulea) look like they can sting – and they can. (1 inch) 

Crowned slug caterpillars (Isa textula) have a border of stinging spines. (⅝ inch)

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One of our most harmless-looking caterpillars causes the most painful stings. Puss caterpillars (Megalopyge opercularis) may look cute and cuddly, but their stings can send people to the hospital. Victims usually report intense pain that radiates through the armpits and across the chest from stings on an arm, or through the groin area from stings on a leg. Even dead caterpillars or shed skins can cause stings. Fortunately, puss caterpillars are not common, but outbreaks occasionally occur on shrubs in home landscapes or public grounds.

Puss caterpillars (Megalopyge opercularis) may look pettable, but their sting can be extremely painful! (1 inch)
Hag moth caterpillars or monkey slugs (Probetron pithecium) have stinging spines at the tips of the tentacles. (1 inch) 

Eastern tent caterpillars (Malacosoma americanum) do not have stinging spines, but those hairs can cause an irritating rash. (1¾ inch)

Many hairy caterpillars can cause rashes or dermatitis when they come in close contact with the skin. Symptoms range from mild irritation to an intensely itching rash with reddened, inflamed skin.   

Eastern tent caterpillar (Malacosoma americanum) is one species capable of causing skin rashes. This is the caterpillar that builds tents in the forks of wild cherry trees in early spring. There is only one generation per year, but mature larvae sometimes end up dropping down a shirt collar while wandering in search of a pupation site. These hairs can also cause problems if consumed. Of course, people rarely eat caterpillars, but grazing horses sometimes do. Pregnant mares that inadvertently ingest wandering eastern tent caterpillars will often abort their foals, a phenomenon known as mare reproductive loss syndrome.

How do you avoid having unpleasant encounters with stinging caterpillars? Knowing which caterpillars are capable of stinging helps. Teach children to recognize the stinging species most common in your area and teach them to be wary of any spiny or hairy caterpillars. Be especially alert during outbreaks of stinging species. People who live in areas where buck moth caterpillars are common know to look before sitting when these caterpillars are out. Gardeners are more likely to encounter stinging caterpillars than most folks because they spend more time outside working in the yard and garden. One of the best defenses is to wear gloves and long sleeves when pruning, hauling limbs and doing similar chores, especially in the fall when many of these stinging species are most common.

 

A version of this article appeared in a State-by-State Gardening February 2009 print editon. Photos by Blake Layton.

 

Posted: 02/23/16   RSS | Print

 

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