8 Dangerous (Even Deadly!) Caterpillars to Avoid at All Costs

Earth is home to over 20,000 caterpillar species, found in every corner of the globe except Antarctica. While most are harmless, some species are just as dangerous as venomous snakes. Their defenses go beyond irritating hairs, and some even cause severe—and sometimes deadly—side effects due to their toxins. Some of these even reside in the U.S., and you’ve likely encountered them without even realizing it.

Giant Silk Moth Caterpillar

Lonomia obliqua, not to be confused with the harmless domestic silk moth, Bombyx mori, is a species of Saturniidae or “giant silk moth.” While its adult moth form is harmless, its larval stage makes it one of the deadliest caterpillars in the world. It’s easily identified by the tubercles that cover its body, which are topped with spines that detach when the caterpillar feels threatened. Its coloring ranges from various shades of striped browns to green, helping it camouflage in the forests of South America.

This caterpillar can cause a world of pain with its two-inch-long body, and is responsible for several deaths annually. It has several names among locals, including the “Killer Brazilian Caterpillar” and the “Assassin Caterpillar.” It wasn’t always known that this caterpillar produced enough toxin to kill a human, leaving many deaths a mystery. All it takes is one accidental brush of this caterpillar for it to release its spines, which act as hypodermic needles. These contain an anticoagulant that causes disseminated intravascular coagulation and hemorrhaging. In short, it causes the human body to bleed internally, with no way of stemming the flow.

Bag Shelter Caterpillar

The bag shelter caterpillar, or Ochrogaster lunifer, is a member of the Thaumetopoeidae family and is native to Australia. Don’t let its appearance fool you—this “fluffy” looking caterpillar is covered in hundreds of irritating hairs. It’s also known as the “itchy grub” and the processionary caterpillar, and it certainly lives up to each one. The hairs on the caterpillar are urticating, which means they come off easily. The good news is they’re easy to spot, but the bad news is that their hairs can become airborne and cause irritation even without direct contact. It’s best to avoid these caterpillars, which can be identified by their black bodies and long, distinctive “hair.”

The most considerable inconvenience of brushing up against one of these caterpillars is the itchiness, which can last for days. Its hairs also contain an anti-coagulant toxin, which can be severe if ingested or inhaled. This can also cause issues if you come into contact with this caterpillar and rub your eye or another sensitive area on your body. After the itchiness subsides, people still report soreness at the contact site that feels similar to a “splinter.”

Spiny Oak Slug Moth Caterpillar

Some insect species display their colors as a warning, and this is also true for the spiny oak slug moth caterpillar, also known as Euclea delphinii. This caterpillar is part of the Limacodidae moth family and can be found throughout the Eastern and Southern U.S., as well as Central and South America. It thrives in heavily wooded areas and is often found on oak and hardwood trees during the summer and fall months. It’s easily identified by its neon coloring, which includes green, orange, yellow, and red. Similar to the giant silk moth, the spiny oak slug moth caterpillar also has tubercles that contain toxin-filled spines.

The caterpillar is relatively small but appears fierce, with long, cactus-like spikes. Little is known about the toxin released by the spines of this caterpillar. While some people experience mild irritation, others experience more severe reactions. The spines function as hollow needles, releasing the toxin as soon as they break off. In rare cases, the toxin may cause an allergic reaction.

Saddleback Caterpillar

The saddleback caterpillar, Acharia stimulea, lives up to its name. Its brown body is uniform except for one neon green “saddle” it wears, in between towering spines on either end. This caterpillar is also part of the Limacodidae family, and is found in Eastern North America and parts of Mexico. While this caterpillar is easy to recognize, its small size (less than two inches) makes it prone to being accidentally encountered.

The toxin of this caterpillar causes several severe reactions. Similar to other spined caterpillar species, these hollow needles cause the toxin to take effect instantaneously. Rather than an anti-coagulant, the saddleback caterpillar releases RF-amide peptides. Some of its side effects are similar to those of a bee sting, but far worse.

Io Moth Caterpillar

Io moth caterpillars can be found all over North America, from southern Canada to Texas. They’re relatively common but unfortunately, are also full of toxic spines. On the outside, these bright green caterpillars look completely harmless. They’re easily identified by the brown and cream-colored stripe that runs down the center, and are often found in deciduous forests and even suburban areas with abundant vegetation. However, this also means humans can easily, and unintentionally, come into contact with an io moth caterpillar.

The toxic spikes along this caterpillar’s body are connected to poison glands. Little is known about the venom, specifically, but it can cause allergic reactions in some, similar to a bee sting. While the venom is generally not life-threatening, people spending time outdoors during the summer and fall should take care to avoid these caterpillars.

Southern Flannel Moth Caterpillar

The southern flannel moth caterpillar, Megalopyge opercularis, is an asp, or puss caterpillar. This means it’s a venomous species, found in the southeastern U.S. These caterpillars are commonly found in Florida, but also live as far north as New Jersey and eastern Pennsylvania. They’re not picky about their host vegetation and are found on a variety of trees, including oaks, elms, maples, hollies, sycamores, hackberries, and even certain shrubs. This puts them in close contact with humans, which means bad news if you brush them accidentally.

You wouldn’t know by looking at them, but the southern flannel moth caterpillar has spines underneath its fuzzy “hair.” This gives the illusion that they’re non-venomous, which is not the case. While their fuzzy exterior makes them easy to recognize, their widespread habitat puts humans at even greater risk of encountering them. Their venom-filled spines cause severe irritation, but that’s not all.

Hag Moth Caterpillar

The hag moth caterpillar, Phobetron pithecium, looks like something out of a science fiction movie. This unusual caterpillar is part of the Limacodidae moth family, and is also known as the “monkey slug.” It has unusual limbs that resemble tentacles, and its entire body is covered with stinging hairs that can cause significant discomfort.

The caterpillar’s appendages are used strictly for defense, as its legs are located underneath its body. This makes the hag moth caterpillar an even more formidable opponent, with an impressive defense system. Its brown “hair” helps it to blend in with tree bark, making it challenging to notice while hiking or walking through the woods. If you come into contact with one, the effects of its sting can last for days. The best course of action if this happens is to attempt to remove most of the stinging hairs with strong tape.

Hickory Tussock Caterpillar

Similar to the hag moth caterpillar, the hickory tussock caterpillar, Lophocampa caryae, is covered in stinging hairs. It’s a member of the Erebidae moth family and can be found from Nova Scotia, extending down to Texas and the Carolinas. Warnings are posted about this caterpillar each year, as it appears harmless but packs a potent sting if touched. The caterpillar itself is quite pretty, with a fuzzy white appearance and black spots that run down its spine. However, this is a look-and-don’t-touch type of insect.

The hairs on this caterpillar contain irritating substances that can cause instant irritation. The hairs are barbed and get stuck in the skin, which can cause dermatitis. Once the hairs are embedded in the skin, irritation can persist for days if left untreated.