The flannel moth family (Megalopygidae) is widely known for the intense pain caused by contact with one of its life stages, leading to frequent questions about whether the adult moth stings. These insects have a life cycle that includes an immature stage notorious for its potent defensive mechanism. The family contains species found across the United States, particularly in the South and Southwest, which are responsible for numerous painful envenomations each year.
Clarifying the Threat: The Moth Versus the Larva
The adult flannel moth does not sting, bite, or pose any physical threat to humans. Adult moths are chunky-bodied creatures covered in long, dense, fuzzy hair on their bodies, legs, and wings, giving them a soft, fluffy appearance. These moths are typically yellowish, whitish, or brownish, and lack the venom delivery system. If you encounter an adult flannel moth, it is harmless, though its presence indicates that its offspring, the larval stage, may be nearby.
The source of the danger lies exclusively with the larval form, the caterpillar, which is responsible for the insect’s painful reputation. This caterpillar is not capable of actively stinging like a bee or wasp, but its defensive structures deliver venom upon touch. The adult moth and the larva represent two distinct forms, and only the larva possesses the specialized mechanism for causing injury.
Identifying the Danger: The Puss Caterpillar
The larva responsible for the most severe reactions is the Puss Caterpillar, the immature stage of the Southern Flannel Moth (Megalopyge opercularis). This caterpillar is considered the most venomous in the United States, found from New Jersey to Florida and west to Texas and Arkansas. It is a herbivore that feeds on the leaves of various trees and shrubs, including oak, elm, sycamore, and citrus trees.
The caterpillar’s appearance is misleading, as its body is entirely covered by a thick, dense coat of long, silky hairs, making it resemble a small tuft of cotton or a Persian cat. This dense “fur” gives the caterpillar its name but conceals the danger. Hidden beneath the outer layer of hair are brittle, venomous spines (setae), which are the delivery mechanism for the toxin.
These spines are short, stout, and connected to an underlying poison gland at their base. When a person brushes against the caterpillar, slight pressure causes the delicate, hollow spines to break off in the skin. The broken spines then release venom from the poison gland, causing an immediate, intensely painful reaction. The caterpillars are typically teardrop-shaped, grow to about 1 to 1.5 inches long, and become more potent as they mature.
Symptoms, Immediate Care, and Recovery
Contact with the puss caterpillar’s venomous spines causes immediate, severe burning pain, often described as similar to a broken bone or blunt-force trauma. A characteristic reaction is the development of a raised, itchy rash of red blotches or welts, sometimes appearing in a grid-like pattern where the spines made contact. The pain frequently radiates away from the contact site, potentially traveling up the entire limb.
Systemic Reactions
In addition to localized pain and swelling, systemic symptoms can develop, including:
- Headache
- Nausea
- Vomiting
- Abdominal distress
The first step for immediate care is to remove the embedded venomous spines. This is best accomplished by gently pressing and lifting a piece of adhesive tape (such as cellophane or medical tape) repeatedly over the affected area to extract the hairs.
After removing the spines, the area should be gently washed with soap and water to prevent secondary infection. Applying a cold compress or an ice pack wrapped in a thin cloth can help reduce localized pain and swelling. Over-the-counter pain relievers (such as acetaminophen or NSAIDs) and oral antihistamines for itching may also provide relief.
For most individuals, the pain subsides within an hour, and localized irritation resolves without complications within a day or two. However, professional medical attention is necessary if symptoms worsen, persist beyond 48 to 72 hours, or if signs of a severe systemic reaction occur. Severe reactions include difficulty breathing, widespread hives, facial swelling, or symptoms suggestive of anaphylaxis, requiring immediate emergency treatment.