While most adult moths are not truly poisonous, many moth species, particularly in their caterpillar stage, possess defense mechanisms that can cause adverse reactions upon contact. The primary concern for human interaction with these insects usually stems from their larval form. These defensive strategies protect them from predators but can also affect people who accidentally touch them or come into contact with their shed hairs.
How Moths and Caterpillars Can Be Toxic
Moths and caterpillars employ various biological mechanisms to deter predators, some of which can be harmful to humans. Many caterpillars, and some adult moths, have urticating hairs or spines. These are barbed hairs that can easily break off from the insect’s body and embed in skin, leading to irritation, rashes, or even respiratory problems if inhaled. The hairs often contain irritating chemicals that are released upon contact.
Some species also possess chemical irritants within their bodies or on their surfaces. These chemicals, such as histamine-like compounds, are released when the caterpillar is touched or ingested. The release of these irritants triggers an immune response in humans, causing symptoms like itching, redness, and swelling.
Another defense mechanism involves dietary toxin sequestration. Certain caterpillars feed on toxic plants and then store these plant-derived toxins within their own bodies. This makes the caterpillars unpalatable or harmful to predators, and if accidentally ingested or handled by humans, these sequestered toxins can cause adverse effects. While very few adult moths are truly poisonous if ingested, some retain these sequestered toxins from their larval stage, providing a defense if consumed.
Identifying Toxic Species
Appearance often signals danger in caterpillars, with bright colors, prominent hairs, or spines serving as warnings. One example is the Puss Moth Caterpillar (Megalopyge opercularis), also known as the asp caterpillar, which is covered in dense, soft, fur-like hairs that conceal venomous spines. Contact with these hidden spines can cause intense burning pain and a rash. This caterpillar is found in the southern United States.
Another notable species is the Saddleback Caterpillar (Acharia stimulea), which has a distinctive appearance with a green body, brown ends, and a prominent brown, saddle-like spot in the middle. Its body is covered with stinging spines that can deliver a painful sting, causing immediate pain, swelling, and redness.
The Io Moth Caterpillar (Automeris io) is bright green with red and white stripes along its sides and is covered in branched, venomous spines. Contact with these spines can result in a sensation similar to a nettle sting.
The Buck Moth Caterpillar (Hemileuca maia) is typically black with white dots and is covered in hollow, stinging spines connected to venom glands. These spines can cause immediate burning, itching, and swelling. The Gypsy Moth Caterpillar (Lymantria dispar) possesses tiny, hair-like bristles (setae) that can dislodge and become airborne. These hairs contain histamines and can cause itchy rashes, welts, and even respiratory irritation upon contact or inhalation.
Symptoms and First Aid
Contact with toxic caterpillars or moths can lead to a range of symptoms. Common reactions include pain, itching, redness, swelling, hives, and a burning sensation at the site of contact. If hairs or toxins are inhaled, respiratory irritation, such as wheezing, coughing, or shortness of breath, can occur. In rare instances, systemic symptoms like nausea, headache, dizziness, or muscle spasms have been reported.
If a caterpillar has touched the skin, it is important to remove any remaining hairs or spines without using bare hands. This can be effectively done by gently applying adhesive tape (such as Scotch tape or duct tape) to the affected area and then pulling it off to lift embedded hairs. Repeat this process with fresh pieces of tape as needed.
After removing the hairs, wash the affected area thoroughly with soap and water. Applying cold compresses or ice packs can help reduce pain and swelling. Over-the-counter anti-itch creams, such as hydrocortisone cream or calamine lotion, provide relief. Oral antihistamines can help with itching.
If symptoms are severe, cover a large area, or do not improve after a few days, it is advisable to seek medical attention. Medical consultation is also important if difficulty breathing, eye involvement, or signs of a more severe allergic reaction occur. Avoiding direct contact with unfamiliar caterpillars is the best preventative measure.