Spiny caterpillars can cause adverse reactions upon contact. While not all species are harmful, some possess specialized hairs or spines that deliver irritating or venomous substances.
How Spiny Caterpillars Can Cause Harm
Spiny caterpillars do not actively “sting” like a bee or wasp. Instead, their harm comes from specialized structures called urticating hairs or spines. These hairs are typically hollow and connected to glands containing irritating compounds. When brushed against, the delicate, barbed hairs easily break off and embed into the skin.
The embedded hairs cause both mechanical irritation and a chemical reaction. Many species release toxins upon contact, including histamine-like compounds, proteins, or peptides that trigger an inflammatory response. This defense mechanism is venomous, involving the injection of a substance, rather than poisonous (toxic if ingested). The reaction’s severity depends on the caterpillar species, contact amount, and individual sensitivity.
Common Spiny Caterpillars to Watch For
Several species of spiny caterpillars found in North America are known to cause reactions. One notable example is the Puss Caterpillar (Megalopyge opercularis), which is covered in long, silky hairs that conceal stiff, venomous spines underneath. It resembles a tuft of cotton or fur and is often found on shade trees like elm, oak, and sycamore in the southern United States.
Another commonly encountered stinging caterpillar is the Saddleback Caterpillar (Acharia stimulea). This caterpillar has a distinctive appearance with a bright green body and a prominent brown, saddle-shaped marking in its middle. Its spines are equipped with poison glands, and contact can cause immediate, intense pain. These caterpillars feed on a variety of plants, including maples, hibiscus, and oak.
The Io Moth Caterpillar (Automeris io) is another spiny species, typically light green with red and white stripes along its sides, and its body is covered with branching green spines. These caterpillars are found on numerous trees and shrubs, such as willow, maple, and elm. Their spines also contain toxins that can cause a painful, nettling sensation upon contact.
What Happens After Contact and What to Do
Contact with a harmful spiny caterpillar can lead to a range of symptoms. Immediate pain, often described as a burning or stinging sensation similar to a bee sting, is common. This can be followed by redness, itching, swelling, and the development of a rash or welts. In some cases, more severe reactions may occur, including nausea, headache, abdominal pain, muscle spasms, or even systemic symptoms like fever or difficulty breathing. Allergic reactions are possible, and symptoms should be monitored closely.
If contact occurs, remove the caterpillar without touching it, using a stick or tweezers. Carefully remove any embedded spines or hairs from the skin by repeatedly applying and peeling off adhesive tape (such as cellophane, duct, or medical tape) over the affected area. After removal, wash the area thoroughly with soap and water.
Applying an ice pack can help reduce pain and swelling. A baking soda paste or calamine lotion may soothe itching, and over-the-counter hydrocortisone cream or oral antihistamines can also provide symptom relief. Seek prompt medical attention if pain is intense, symptoms worsen, severe blistering develops, or signs of an allergic reaction appear (e.g., difficulty breathing, widespread rash).
Staying Safe Around Caterpillars
To prevent accidental contact with spiny caterpillars, observe them from a distance and avoid direct handling. Educating children not to touch unfamiliar caterpillars is a practical safety measure, as their natural curiosity can lead to painful encounters. When gardening or working in areas where these caterpillars might be present, wearing protective clothing such as long-sleeved shirts and gloves can provide a barrier against contact.
Regularly inspecting outdoor play areas and plants can help identify and avoid areas with stinging caterpillars. Understanding the life cycles of local caterpillar species can also help anticipate their presence during certain times of the year. If a caterpillar is found on clothing or skin, brush it off gently with a stick or other tool rather than with bare hands.