Caterpillars are often perceived as harmless, yet some species possess defense mechanisms that can cause significant irritation upon contact. While “sting” is commonly associated with insects like bees or wasps that inject venom through a specialized appendage, caterpillars do not possess such stingers. Instead, their defense relies on specialized hairs that can cause discomfort or other reactions in humans.
How Caterpillars Defend Themselves
Many caterpillars employ a passive defense using urticating hairs. These hairs deter predators through irritation, unlike a bee’s stinger. Urticating hairs can be barbed, hollow, or contain irritating chemicals or toxins. When a caterpillar is touched or disturbed, these hairs can break off and embed themselves in the skin, releasing their irritating contents.
The effect of these hairs ranges from mechanical irritation to toxic reactions. Some urticating hairs have a weakened ring at their base, allowing them to easily detach and embed in the skin. Others are connected to venom glands, delivering venom upon penetration. These toxins cause reactions like itching, redness, swelling, and a burning sensation, collectively known as caterpillar dermatitis or lepidopterism.
Common Irritating Caterpillars
Several caterpillar species are known for causing skin irritation. One is the Puss Moth Caterpillar (Megalopyge opercularis), also known as the asp caterpillar. Covered in tawny to grayish hairs, appearing fluffy. Beneath these hairs are venomous spines that, upon contact, cause instantaneous and severe pain, which may radiate to regional lymph nodes. Reactions can include redness, swelling, and pustules. Puss moth caterpillars are found in the southern United States.
The Saddleback Caterpillar (Acharia stimulea) is known for its irritating bristles. It is distinctively marked with a bright green body, a prominent brown, saddle-like spot, and horn-like projections. Its venomous spines can cause a painful “sting” leading to redness, swelling, and welts lasting up to a week. Saddleback caterpillars are common in Eastern North America.
The Io Moth Caterpillar (Automeris io) is a pale yellow to green caterpillar with red legs and rows of spiny, branched hairs. Contact with these spines results in immediate itching and stinging of skin, though reactions usually resolve quickly.
The Buck Moth Caterpillar (Hemileuca maia) is a large, dark caterpillar with a reddish head, small white spots, and bristly spines. These spines are connected to venom glands, causing immediate pain that can radiate, with localized itching, swelling, and redness. Welts can remain visible for several days to a week. Buck moth caterpillars are distributed from Maine to Florida and as far west as East Texas.
What to Do If You Encounter One
If you come into contact with an irritating caterpillar, prompt action minimizes discomfort. First, avoid rubbing the affected area, as this can embed the hairs deeper into the skin. Instead, carefully remove the caterpillar using tweezers or a stick, without touching it directly. To remove any remaining urticating hairs, apply adhesive tape (such as duct tape) to the affected skin and then pull it off. Repeat with fresh tape until most hairs are removed.
After removing the hairs, wash the area thoroughly with soap and water. Applying a cold compress or ice pack reduces swelling and pain. Over-the-counter remedies like baking soda paste, aloe vera, or hydrocortisone cream soothe itching and inflammation. Oral antihistamines may be taken for widespread or significant itching.
Seek medical attention if symptoms are severe, cover a large area, worsen, or show signs of a serious allergic reaction like difficulty breathing, widespread rash, severe pain, or dizziness. To prevent encounters, avoid touching unfamiliar caterpillars, especially those that are brightly colored, spiny, or fuzzy. When gardening or working in areas where these caterpillars might be present, wearing protective clothing like long sleeves, pants, and gloves prevents accidental contact.