Iowa is home to several species of stinging caterpillars that can cause painful skin reactions on contact. None are life-threatening to most people, but a brush against the wrong caterpillar while gardening or climbing a tree can leave you with hours of burning, swelling, or blistering. The species you’re most likely to encounter feed on common Iowa trees like oaks, willows, and elms, and they’re active from late spring through early fall.
How Stinging Caterpillars Hurt You
Stinging caterpillars don’t bite. Instead, they carry hollow, quill-like hairs connected to small poison sacs. When you touch or brush against these hairs, they pierce your skin and release venom. Some species have obvious spines you can spot from a distance. Others, like flannel moth caterpillars, hide their venomous spines beneath a coat of soft-looking fur, which is what makes them especially deceptive.
Reactions range from mild itching and redness to intense pain, swelling, blistering, and rashes that can last for days. In rare cases, people experience nausea, headache, fever, vomiting, muscle spasms, or swollen glands. Severe allergic reactions (anaphylaxis) are possible but rare.
Io Moth Caterpillar
The Io moth caterpillar is one of the most common stinging species across the Midwest. It’s about two inches long, bright green to yellow-green, with a distinctive red-and-white stripe running along each side. The entire body is covered in branched, yellow-green spines tipped in black. Those spines deliver a sting often compared to a wasp’s.
This caterpillar feeds on an unusually wide range of plants, which is why you’re likely to encounter it in yards and gardens. Host plants include oak, willow, elm, ash, apple, locust, linden, clover, roses, and even corn. It’s active from late spring through summer.
Saddleback Caterpillar
The saddleback is easy to identify once you know what to look for. It’s brown at both ends, bright green in the middle, with a purple or brown oval spot in the center of the green section that resembles a saddle. Two prominent spiny horns stick out from the front and rear. Contact with its spines can cause severe irritation, and the sting tends to be more intense than many other species. It’s a type of slug caterpillar, meaning it’s short, stocky, and creeps along leaves slowly with its legs hidden underneath.
Puss Caterpillar
The puss caterpillar is widely considered the most painful stinging caterpillar in North America. It’s only about an inch long and covered in a thick coat of soft, wavy brown hair that makes it look like a tiny toupee or a fuzzy pet. That appearance is exactly the problem: children and adults are tempted to touch it. Hidden beneath the soft outer hair are rows of short, sharp venomous spines that cause severe, immediate pain radiating outward from the sting site.
The closely related black-wave flannel moth caterpillar looks similar in its final stage but appears quite different when younger, bearing long, white, wispy hairs. Both should be avoided.
Buck Moth Caterpillar
Buck moth caterpillars are dark brown to purplish-black with rows of yellow spots. Branched black spines with red or black tips cover the body, making them look unmistakably dangerous. They grow to about two inches and are commonly found on oak and willow trees from spring through midsummer. In Iowa, oaks are abundant in parks, yards, and wooded areas, so these caterpillars turn up regularly.
Other Stinging Species
A few less common stinging caterpillars also live in the region:
- Hag moth caterpillar: Brown and bizarre-looking, resembling a dried leaf. It has nine pairs of fleshy lobes covered in stinging hairs.
- White flannel moth caterpillar: Yellow with a wide black stripe down the back bordered in red, reaching about 1¼ inches. Raised tufts of stinging hairs line the stripe and sides of the body.
- Stinging rose caterpillar: A brightly colored slug caterpillar with prominent spines, found on rose bushes and woody plants.
- American dagger moth caterpillar: A fuzzy, pale yellow-green caterpillar up to two inches long, often found on the ground near host trees from July through late October. Its bristly black hair tufts can irritate skin on contact.
When and Where You’ll Find Them
Most stinging caterpillars in Iowa are active from late May through September, with some species like the American dagger moth persisting into October. They feed on the leaves of hardwood trees, so you’re most likely to encounter them on or beneath oaks, willows, elms, maples, and fruit trees. They also turn up on shrubs, garden plants, and even corn.
Peak encounters happen in mid to late summer when caterpillars reach full size and begin wandering away from their host plants to pupate. This is when they end up on deck railings, lawn furniture, the sides of buildings, and the ground where you might kneel or sit. Caterpillars that fall from trees onto your neck or arms while you’re mowing, raking, or walking underneath are another common scenario.
What To Do if You Get Stung
If a caterpillar stings you, the first step is removing any spines still embedded in your skin. Press a piece of adhesive tape (duct tape or packing tape works well) firmly over the affected area and peel it off. Repeat several times. This pulls out the tiny hairs you can’t see or grab with tweezers, and it noticeably reduces the irritation.
After removing the spines, wash the area thoroughly with soap and water. Ice packs help with swelling and pain. Over-the-counter antihistamines can reduce itching, and hydrocortisone cream helps calm the rash. Most stings resolve within a few hours to a couple of days, though some species like the puss caterpillar can cause pain that lasts longer.
If a caterpillar touches your eyes, flush them immediately with plenty of water. Seek medical attention if you experience difficulty breathing, widespread swelling, or if you inhaled caterpillar hairs. These situations are uncommon, but they require prompt care.
Avoiding Contact
Wear gloves and long sleeves when pruning trees, picking fruit, or handling firewood during summer and early fall. Before leaning against a tree trunk or sitting on outdoor furniture beneath trees, take a quick look. Teach children not to handle fuzzy or spiny caterpillars, no matter how interesting they look. The most dangerous-looking caterpillars (covered in obvious spines) are actually easier to avoid than the flannel moth species, which look soft and harmless.
If you find a large number of stinging caterpillars on a tree in your yard, you can knock them into a bucket of soapy water using a stick, or simply wait. Most species only feed for a few weeks before pupating, and they rarely cause lasting damage to healthy trees. Spraying pesticides is generally unnecessary for a handful of caterpillars, and it kills beneficial species too.