Poisonous Caterpillar Identification: 13 Species to Avoid

A caterpillar that can hurt you almost always carries branched spines, stiff bristle tufts or a thick coat of hair; smooth, bare caterpillars such as hornworms are not on any stinging list we found. Strictly these caterpillars are venomous rather than poisonous: the University of Kentucky explains that their hollow, quill-like hairs connect to poison sacs, and when the hairs are touched they pierce the skin and release the poison.

Three practical rules follow:

• Treat any spiny or hairy caterpillar as one not to touch. Some hairy species are harmless, but you cannot reliably tell which by eye, and the puss caterpillar hides its short spines under soft-looking hair (University of Kentucky).
• Check the plant. Most stinging species below feed on a short list of trees such as oak, willow, cherry and redbud, so the host plant narrows the choice quickly.
• Look without handling. Photograph it where it is, including the plant, rather than picking it up.

The University of Kentucky's entomology fact sheet describes nine species. Sizes are for mature caterpillars where the sheet gives them.

• Puss caterpillar (*Megalopyge opercularis*, southern flannel moth): about 1 inch, covered in a dense woolly coat of soft brown hair that hides many short poisonous spines. Its stings are often more severe than those of other caterpillars. Found on apple, elm, maple, hackberry, oak and sycamore. Texas A&M describes it as teardrop-shaped, like a tuft of cotton or fur, yellow to reddish-brown and 1 to 1 1/2 inches long, and notes it is called an "asp" in Texas.
• Saddleback caterpillar (*Sibine stimulea*): brown at the front and rear and green in the middle with a purple spot, with two prominent horns at both the front and rear. Can cause severe irritation. On basswood, chestnut, cherry, oak, plum and sometimes corn.
• Io moth caterpillar (*Automeris io*): about 2 inches, light green to yellow with a narrow red line along each side and branched, black-tipped yellow spines. Feeds on many plants including corn, roses, willow, oak and apple.
• Buck moth caterpillar (*Hemileuca maia*): about 2 inches, brown to purplish-black with many yellow spots and branched black spines that may have red or black tips. On oak or willow from spring to mid-summer.
• Black-wave flannel moth (*Megalopyge crispata*): similar to the puss caterpillar; young ones have long white wispy hairs. Late summer on hardwoods; usually not as severe as the puss caterpillar.
• White flannel moth (*Norape ovina*): about 1 1/4 inches, yellow with a wide black stripe down the back bordered with red. Later in summer on redbud, honey locust, hackberry, mimosa and beech.
• Hag moth caterpillar (*Phobetron pithecium*): brown with nine pairs of fleshy lobes, all carrying stinging hairs; usually seen on the undersides of leaves of oak, chestnut, dogwood, sassafras and ash.
• Stinging rose caterpillar (*Parasa indetermina*): yellow to red and spiny with black and blue stripes and spiny yellow horns at the front, rear and centre. On redbud, oak, hickory, bayberry, wild cherry and sycamore.
• Spiny oak slug (*Euclea delphinii*): pale yellow-green with four dark patches of spines towards the rear and many spiny yellow or red lobes. Less venomous than the saddleback.

• Oak processionary moth: grey body, dark head and very long white hairs that stand out against much shorter, almost undetectable irritating hairs. The caterpillars move nose to tail in processions in late spring and early summer and build white silk nests on oak trunks and branches. The short hairs contain a protein called thaumetopoein that causes itchy rashes, eye irritation, sore throats and breathing difficulties in people and animals; Forest Research says the risk is highest in May and June. It is established in most of Greater London and some surrounding counties. Do not touch or approach nests or caterpillars, do not try to remove them yourself, and report sightings with a photo through TreeAlert.
• Brown-tail: black with long brown hairs, two red spots on top and a line of white marks along each side; August to May, in coastal scrub. The Wildlife Trusts say the hairs cause skin irritation.
• Yellow-tail: black with long greyish-white hairs, a pair of red lines on top and a row of white blotches either side; August to June. Its hairs are also irritating (Wildlife Trusts).

*Lonomia obliqua*, a caterpillar of southern Brazil, Uruguay, Paraguay and Argentina, is the one with recorded deaths. Wikipedia describes it as about 4.1 to 6.7 cm long, green to brown, with rows of bumps topped by whorls of easily detached spines. Its venom stops blood clotting and can lead to bleeding. A 2021 study it cites counted 42,264 recorded cases between 2007 and 2017, of which 248 were severe and five fatal. An antiserum made by the Butantan Institute in São Paulo reverses the clotting disorder.

Outside that region you will not meet it, but it is the answer to "which caterpillar is deadliest".

The University of Kentucky says reactions range from mild itching to severe pain, swelling, blistering, skin inflammation and even intestinal upset. For the puss caterpillar, Texas A&M adds that intense, throbbing pain starts immediately or within five minutes, red spots can appear, and headache, nausea and vomiting can follow; the pain usually eases within an hour. Rarely there is shock or breathing distress.

The University of Kentucky notes that very young, elderly or unwell people are more likely to have severe reactions.

The steps below are the ones the University of Kentucky and Texas A&M give. They are first aid, not a judgement that you are fine.

• Lift the spines. Press adhesive or clear tape on the spot and peel it off to pull out broken spines (University of Kentucky; Texas A&M suggests it for stout-spined species).
• Wash the area thoroughly with soap and water to remove venom (University of Kentucky).
• Cool it. An ice pack, or baking soda, may reduce pain (University of Kentucky). Texas A&M adds oral antihistamines for itching and burning.
• Get medical care at once for generalised itching, difficulty breathing, any eye injury, or any severe reaction (Texas A&M; University of Kentucky), and sooner for a young child or an older person.

In the United States, Poison Help on 1-800-222-1222 is free, confidential and open 24 hours a day and can tell you what to do. Elsewhere, call your local poison centre or emergency number.

People often worry about the horn on a hornworm. The Woodland Trust says the tail horns of hawk-moth caterpillars are soft and harmless, and may act as a decoy for predators. The caterpillars to leave alone are the spiny and hairy ones above; the hawk moth caterpillar page covers the horned ones, and the woolly bear page covers the familiar black and rust hairy caterpillar.

You do not need to touch a caterpillar to identify it. Photograph it from above and from the side where it sits, with the leaf or bark in frame, since the host plant is often the fastest clue. Photograph it and InsectLens identifies it; then check the result against the lists above before anyone handles it. If a child has already touched one, follow the first-aid steps and call Poison Help rather than waiting for the name.

Research and drafting for this page used AI assistance. Every figure, date and quotation was then checked line by line against the sources listed at the bottom of the page, each of which was opened while writing. Nothing here is a diagnosis. If a bite, sting or contact with an insect is followed by trouble breathing, swelling of the face or throat, spreading redness, fever or any symptom that worries you, get medical care; in the United States Poison Help (1-800-222-1222) is free, confidential and open 24 hours a day.