What are the Poisonous Caterpillars

Caterpillars are the larval form of butterflies and moths. Butterflies and moths undergo complete metamorphosis, meaning they mature from an egg, to larvae, pupae, then emerge from the cocoon as an adult. The relatively short-lived larval stage of butterflies and moths is the time we are concerned with caterpillars that sting and may be poisonous.

Not all caterpillars are poisonous and sting. In nature, warning coloration is the tendency for brightly colored animals to be poisonous, spiny, aggressive, or otherwise unpalatable to predators. Some of the poisonous caterpillars warn of their poison with colors of bright green, bright orange, and unusual fuzz about their bodies. Unfortunately, the unique visual features that warns animals of their potential danger, invites curious children to pick them up and attempt to pet them. The sting from some caterpillars can be quite painful and elicit allergic reactions in some people.

All About Stinging Caterpillars

Stinging caterpillars are considered an occasional invader; their season is short lived and known areas of stinging caterpillars should be avoided if possible.

Stinging caterpillars come in many colors and patterns. Several species have developed defense mechanisms such as horns, spikes, thick hair that either covers their body, or hair that is arranged in patterns. Stinging caterpillars do not sting in the same manner as bees or yellowjackets. Stinging caterpillars have spines which are connected to glands which contain poison. They do not intentionally insert their spines into people; the sting is not deliberate, only the result of incidental contact with our skin. When the spines are brushed against our skin, either from the caterpillar falling from a tree, or brushing against the leaf it was feeding upon, the spines break and release toxins. The poison from the caterpillar may enter our system by penetrating the skin, but sometimes the toxin simply “spills” onto our skin.

There are many caterpillars with hairs or spines and bright colors on their bodies, but not all of them contain venom. Some species even mimic the stinging caterpillars in appearance so they will be left alone. Identification is important because touching the wrong caterpillar can be painful. Some caterpillars are camouflaged; their body color may help them blend into the environment. In addition, some caterpillars remain motionless and resemble small twigs, which helps them to protect themselves from predators, parasites, and humans.

Stinging caterpillars are very diverse in their growth and development. As previously stated, the caterpillar is the second (larval) stage of butterfly or moth metamorphosis. How long this larval stage lasts depends upon the species, weather conditions, temperature, availability of food, and other conditions. Each caterpillar goes through four to six growth stages, or instars, where they shed their skin to grow and complete development. It is during this growth phase that they may sting people.

Caterpillar Identification

Stinging caterpillars are very diverse in their growth and development. Some prefer to live in clusters, and some prefer to live alone. Caterpillars feed on foliage such as shrubs, trees, grass, ornamentals, vegetable plants or weeds, or they can bore into plants or trees. Some caterpillars have a wide range of leaves they will eat, while others will eat only one. Some species has one generation per year, while others have multiple generations per year. As you assess the risk posed to your family by stinging caterpillars, accurate identification is important. With a proper caterpillar identification, you can know which tree to avoid, the expected time frame of the caterpillar invasion, and the intensity of the toxin. Species identification is important for making the best management decisions.

Puss Caterpillar

• Species: Megalopyge opercularis
• Size: Up to 1 inch long (1 5/8 inches with hairs)
• Color: Fine, long, gray-brown hair
• Behavior: Two generations spring/early summer and late summer/fall
• Hosts: Apple, elm, hackberry, maple, oak, pecan, sycamore, etc.
• Key Characteristics: Hair peaks like a roof and tapers at the back like a tail; most painful and dangerous.

Saddleback Caterpillar

• Species: Sibine stimulea
• Size: Up to 1 inch long
• Color: Brown and green
• Behavior: Solitary; late summer and fall
• Hosts: Apple, basswood, cherry, dogwood, elm, maple, oak, plum, palms, hollies, viburnum, Brazilian pepper, hibiscus, etc.
• Key Characteristics: Appears to be wearing a saddle, horns with spines at anterior and posterior; severe sting.

Spiny Oak Slug

• Species: Euclea delphinii
• Size: Up to 3/4 inch long
• Color: Yellow- green
• Behavior: Late summer and fall
• Hosts: Beech, cherry, maple, oak, redbud, sycamore, willow
• Key Characteristics: Three pairs of horn-like spines with black bristles at the front and two at the rear.

Hag Moth Caterpillar

• Species: Phobetron pithecium
• Size: Up to 1 inch long
• Color: Covered in short, brown hair
• Behavior: Two generations spring/early summer and late summer/fall
• Hosts: Oak, Rose, sassafras, alder, dogwoods, hickories, and various woody trees and ornamental shrubs
• Key Characteristics: 9 pairs of fleshy appendages on body.

Buck Moth Caterpillar

• Species: Hemileuca maia
• Size: Up to 2 3/8 inch long
• Color: Red-brown to black head, body gray-black
• Behavior: May-June
• Hosts: Oak
• Key Characteristics: Body has yellow dots and short tufts of spines on back with large, long bristled red-black spines either side; young larvae are black.

Io Moth Caterpillar

• Species: Automeris io
• Size: Up to 2 ¼ – 2 ½ inch long
• Color: Yellow- green, red legs
• Behavior: Summer and fall
• Hosts: Apple, black locust, cherry, dogwood, elm, hackberry, hickory, maple, oak, sycamore, willow, palms, hibiscus, wax myrtle, etc.
• Key Characteristics: raised tubercles with green spines on each segment on the back.

What To Do if You Get Stung by a Caterpillar

Most caterpillar stings occur when brushing against a bush or tree with a feeding caterpillar, or the caterpillar falls from a tree and landing on a person. If the barbs make contact with your skin, the toxins may cause a burning sensation, produce a red swollen area, and for people allergic to insect stings the possibility of anaphylactic shock and death. The severity of symptoms varies by caterpillar species, the age and general health of the victim, and whether there is an allergic or anaphylactic reaction.

Wash the area with soap and water to remove any toxin that may be on the surface of the skin. If any barbs or broken spines are visible, remove them. Symptoms may be reduced through the application of an over-the-counter insect bite and sting products. Treatment of sting areas with ice packs or ammonia may give some relief from pain. However, for severe reactions, victims should promptly seek medical attention.

Preventing Stinging Caterpillars

It is not advisable or reasonable to expect to eliminate all caterpillars from your yard. Knowing the time of year they emerge, the species of caterpillar, and which trees and shrubs they prefer can help you to avoid interactions with them. Teaching small children, who are naturally curious, to not pick up these often beautiful and unique creatures will prevent many negative contacts. Teach children, that if a caterpillar is on your pant leg or shirt sleeve, brush it off with a stick rather than with their hands.

Know the trees in your yard that harbor stinging caterpillars. When planting new trees, choose trees and foliage that does not sustain stinging caterpillars. To prevent stings, keep trees trimmed back from the sidewalks and other areas where caterpillars may be encountered. If necessary for safety, remove or relocate trees where stinging caterpillars pose an unreasonable risk.

Pest Control Treatments for Stinging Caterpillars

*Products listed are not endorsements, just examples.

Generally, stinging caterpillars do not pose such a risk that preventative or contact sprays should be used to eliminate them from your yard. Most caterpillar infestations are usually short lived and should be left undisturbed, unless they are causing a problem. However, these infested areas should be noted and avoided. Most infestations will die out on their own in a short amount of time.

However, in some situations, such as serious infestations on a playground or recreation area, treatments may be warranted. As caterpillars die from an insecticide treatment they will drop off of the foliage. Dead and dying caterpillars can still sting so the area must be closed following a treatment and the dead caterpillars cleaned up prior to allowing people back in the area.

Pesticides must be used in accordance with their EPA-approved label directions. Applicators must be certified to apply pesticides and should always wear protective equipment during applications. Do not apply these materials when buildings are occupied, and never apply them where they might wash into drains or sewers.

If you must treat for stinging caterpillars, there are a few options that are low toxicity for people and pets while being very effective. Thuricide, or products containing Bacillus thuringiensis var. kurstaki. stops caterpillar feeding immediately and the caterpillars die within several days. Some thuricide products are OMRI Certified Organic. If the caterpillars are small, you may see a caterpillar reduction with an insecticidal soap. These options are very effective, but only last a few days and you must completely cover all areas of the plant (not easy to do on a 50 foot oak tree).

Tree injections are a good option for shade trees, particularly for trees over play grounds or other areas where people frequent. By injecting insecticide directly into the tree, the risk of surface residue, drift, and runoff are eliminated. Treeage® by Arborjet® works very well on caterpillars and will prevent caterpillars for one to two years. This will also allow treatments during summer months when children are not present and prevent closures to infested areas during the school year.