Puss and Saddleback Caterpillars: Insects That Sting Humans

Some attractive caterpillars live in North America. Their colours and decorations are often very impressive. Some of these caterpillars sting, however, including puss and saddleback caterpillars. Observers should examine these insects without touching them or while wearing heavy-duty gloves for protection.

The effects of caterpillar venom range from mild and short-lived to serious enough to require emergency medical attention. In some cases, a severe allergic reaction or respiratory distress may develop. The symptoms depend on the insect species involved, the amount of venom that a person absorbs, and the person’s individual susceptibility to harm from the venom.

The words venomous and poisonous are often used interchangeably. Technically, however, they mean different things. A quick way to remember this is the idea that venomous animals inject and poisonous ones secrete.

  • A venomous animal must bite, stab, or sting its victim in order to inject its venom.
  • We have to bite or eat a poisonous animal or touch it without being bitten, stabbed, or stung in order to be poisoned.

For example, venomous snakes inject a venom into our body through hollow fangs as they bite us. The skin secretion of poison dart frogs harms us by a more passive process. When we come into contact with a frog’s secretion, it may be absorbed into our body through cuts in our skin or through our mucous membranes.

The harmful caterpillars that I describe in this article sting humans with spines. Therefore they are technically venomous. They are often referred to as poisonous, however. In everyday life, this doesn’t really matter. The unpleasant effects of the caterpillar chemicals on our body are the same whatever terminology we use.

Caterpillars are the larval forms of moths and butterflies. These insects belong to the order Lepidoptera. The insect’s life cycle contains four stages: egg, larva (caterpillar), pupa, and adult. The caterpillars look very different from the adults and feed on different food. The pupa often appears to be an inactive stage, but this is far from true. Inside the outer covering of the pupa, the caterpillar’s body undergoes a dramatic change as it turns into a moth or butterfly. This change is known as complete metamorphosis.

Caterpillars molt (lose their outer covering) as they grow larger. The outer covering of an insect is called the exoskeleton. Unlike us, insects don’t have an internal skeleton. When an insect’s exoskeleton is lost as a result of molting, a larger and softer exoskeleton is revealed. This expands and hardens, giving the insect room to grow.

The different stages between the molts of a caterpillar are called instars. The caterpillars in the different instars may be significantly different from one another with respect to both size and appearance. The photos in this article show the mature caterpillars.

Stinging caterpillars don’t deliberately reach out to humans and inject venom as a snake might do. Instead, their spines pierce our skin when we touch the insects. The spines are hollow and are connected to a venom sac (or, as it’s more often called, a poison sac). Venom is found within the spines and enters our body as the spines penetrate our skin.

Some spines become detached from the caterpillar’s body when we touch the insect. This is why a common first aid tip for a sting is to place sticky cellophane or Scotch tape over the wound. When the tape is peeled off, it should carry spines with it.

The chemicals in the venom of most stinging caterpillars haven’t been identified. The effects of the venom are well known, however.

The puss caterpillar (Megalopyge opercularis)is also known as the asp caterpillar. Both names sound very appropriate to me. As the first name might suggest, the caterpillar appears to have long, soft fur. Some people—including young children—may be very tempted to stroke the “fur”. Touching the caterpillar is not a good idea, however, because venomous spines are hiding amongst the soft fur. Each instar of the caterpillar is hairier than the one before. The last instars have so much hair that their actual body can’t be seen.

The caterpillar has a teardrop shape and is variable in colour. It may be grey, yellow, or red brown. It has a tail-like structure at its posterior end. The insect is small and is only around 1.2 inches to 1.4 inches long. Despite this seemingly insignificant size, it’s potentially very harmful.

I’ve never been stung by a puss caterpillar, and I’m glad that this is the case. The sting is said to be very painful. The caterpillar is considered to be one of the most dangerous in the United States and sometimes the most dangerous of all.

The caterpillars feed on the leaves of various trees and shrubs and often stay out of the range of humans. They sometimes fall to the ground, however. Occasionally, they are unusually numerous. At these times they may defoliate trees and present a greater threat to humans.

The adult form of the puss caterpillar is known as the southern flannel moth. Like the larva, it has an attractive and furry appearance. Also like the caterpillar, it’s small in size and reaches a length of 1.0 to 1.5 inches.

The hairs and bristles of insects are technically known as setae. Although setae may look like human hair, they have a different structure and are made in a different way. Our hairs are solid and are made of a protein called keratin. Insect setae are hollow and are made of a carbohydrate called chitin. Chitin is the main component of an insect’s exoskeleton.

The chemical makeup of puss caterpillar venom is unknown. Researchers have discovered at least one dangerous component in the venom of a closely related species, however. They say that the venom of Megalopyge (or Lagoa) crispata contains a particular protein that can act as an enzyme and trigger harmful reactions.

The effects of a puss caterpillar sting may include:

  • immediate pain, which may be intense and throbbing
  • pain that radiates up the arm
  • pain that lasts for several hours or days
  • a burning sensation
  • redness
  • swelling
  • blisters

Less commonly, symptoms may include:

  • headache
  • nausea
  • vomiting
  • muscle cramps
  • fever
  • swollen or painful lymph nodes
  • allergy symptoms, including hives and respiratory problems

A person may not experience all of the symptoms. In addition, the symptoms are worse in some people than in others.

Saddleback caterpillars (Acharia stimulea) are brown in colour with a green “saddle” in the middle of their back. The saddle has a brown spot in its middle section. The caterpillar has two horn-like structures at the front end of its body and two at the back. It also has smaller protuberances on its body. The dramatic appearance of the insect is believed to act as a warning of venom presence to would-be predators. The large light spots at the end of the body look somewhat like eyes and may also serve to discourage a predator attack.

The caterpillars belong to a family known as the Limacodidae. The larvae of this class are known as slug caterpillars due to their method of movement. The insects feed on the leaves of a wide variety of deciduous trees and other plants. They are sometimes found on corn. They aren’t considered to be a major agricultural pest, however. In the eastern United States, the caterpillars are able to survive in both the temperate climates in the north and in the warmer climates of the south.

According to the University of Florida, the effects of the saddleback caterpillar’s venom are second only to those of the venom from the puss caterpillar’s family (the Megalopygidae). Someone who is stung may experience only a burning or itching sensation. Some people report that their caterpillar sting produced worse pain than a bee or wasp sting, however. The pain often lasts for one to several hours and then gradually weakens, but it may last for days.

Some people experience such a severe response to a sting that medical attention is necessary. As in the case of the puss caterpillar sting, people may develop symptoms beyond the wound or experience an allergic response to the venom.

It’s best to admire the caterpillar without touching it in any way. I read a comment from one victim who said they used a stick to move a saddleback caterpillar into a better position for a photograph. The stick brushed against the person’s skin and caused significant pain. Clothing that comes into contact with the insect should be washed before being worn again.

Like bee and wasp stings, first aid and patience as the symptoms weaken and disappear may be all that’s required to deal with a caterpillar sting. Some situations may require an immediate visit to a doctor, however. Seven of these situations are listed below.

  • The existence of pain that is very intense
  • The existence of pain that doesn’t weaken or that intensifies over time
  • The development of severe blisters
  • The appearance of severe symptoms beyond the wound site
  • The appearance of symptoms that suggest that an allergic response is developing
  • A sting in or near an eye
  • A sting in the mouth

Anyone that is concerned about the effects of a caterpillar sting should visit a doctor, even if the conditions listed above don’t apply to them.

The National Poison Control Centre suggests that people perform the following first aid steps if they are stung by a caterpillar.

  • If the caterpillar is in contact with the body, remove the insect without touching it.
  • Place sticky tape over the wound.
  • Peel off the tape to remove caterpillar hair and spines.
  • Repeat the process several times if necessary. (The longer the spines are left in the wound, the larger the quantity of venom released.)
  • Wash the area with soap and water.
  • If the area is itchy, apply a paste of baking soda and water.
  • If the baking soda doesn’t help, apply a hydrocortisone cream.
  • If the hydrocortisone doesn’t help, apply an antibiotic cream.

With reference to the hydrocortisone and antibiotic creams, the Poison Control Center points out that “some people have skin reactions to these creams”.

The Merck Manual article referenced below (which was written by an emergency physician) offers similar suggestions for treating a puss caterpillar sting. It suggests that rubbing alcohol and calamine lotion be applied to the wound instead of the hydrocortisone and antibiotic creams, however. It also suggests that an ice pack be applied as a final step. These steps may help all types of caterpillar stings.

Most caterpillars are not dangerous. They are interesting creatures to observe, unless they are munching on garden plants or agricultural crops. It’s probably a good idea to avoid touching them unless the observer is certain that they are not venomous.

In areas where stinging caterpillars live, children should be taught to watch but never touch a caterpillar without an adult’s permission (or without the permission of a younger person who recognizes the caterpillars). They will probably understand this requirement if the dangers of the insects are described. Children who are still too young to understand or follow the instructions should be watched carefully during the time of year when the caterpillars appear.

Adults should be careful, too. The insects should be avoided whenever possible or admired from a distance. Many stings appear to be accidental, however. People report that the insects are sometimes found on the underside of the leaves of their garden plants. A gardener may not even see the insects until they have been stung. In these cases, first aid procedures should be useful as well as medical attention if it’s necessary.

Facts about stinging caterpillars from the University of Kentucky

Puss caterpillar and southern flannel moth facts from the University of Florida

Information about Megalopyge opercularis from the American Association for Clinical Chemistry

Saddleback caterpillar facts from the University of Florida

First aid for caterpillar stings from the National Poison Control Center

First aid recommendations for puss caterpillar stings from the Merck Manual