Stinging caterpillars do not sting in the familiar manner of bees, yellowjackets, hornets, and wasps. Instead, stinging caterpillars bear specialized nettling or urticaceous setae or spines. These structures are hollow and contain toxins from poison-gland cells to which they are joined. These are primarily defensive structures for protection of caterpillars from predators and other enemies. The sting inflicted on humans is not from a deliberate attack by the caterpillar, but the result of contact, usually inadvertent, with toxin-bearing setae or spines. When brushed against, these structures break away, releasing toxins. In some cases, broken setae may penetrate the skin; in others, toxins spill out to spread on the surface of the skin.
Caterpillars are the immature, larval form of butterflies and moths. They are variable in form and coloration, and several of them have various defense mechanisms such as horns, spines, venomous spines, eversible glands, secretions and body hairs. In addition, their body color may help them blend into the environment (camouflage) or their ability to remain motionless and resemble small twigs helps them to protect themselves from predators, parasites and humans. Caterpillars can be foliage feeders on shrubs, trees, grass, ornamentals, vegetable plants or weeds, or they can be borers into plants or trees. Each caterpillar goes through four to six growth stages, or instars, where they shed their skin to grow and complete development. This larval form is the second stage in the life cycle of the butterfly or moth. The total process is called metamorphosis and consists of the egg, larva, pupa and adult. While there are some caterpillars that have hairs that cause minor itching, those that will be discussed here have venomous hairs or spines that can cause a burning sensation when touched, producing a red swollen area, and for people allergic to insect stings the possibility of anaphylactic shock and death.
Stinging caterpillars are very diverse in their growth and development. Some may live in clusters and disperse before pupating or they can be solitary. Some stinging caterpillars are host plant specific and only feed on one plant, such as the buck moth caterpillar that feeds only on oaks. But other caterpillar species can develop on a wide range of hosts. Some species have a single generation each year while others have multiple generations. Species identification is important for making the best management decisions. One of these species, the puss caterpillar, is sometimes called an asp.
Stinging caterpillars have a series of hollow, glass-like hairs or spines on the body that contain amino acid venom. When touched the spines break off in the skin and the venom inside runs down the spine into the open wound created by the spine. This is what produces the burning, stinging sensation, reddening and swelling. Some people are very sensitive to the venom and may require medical attention.
Effects of the rash produced by these caterpillars can be reduced through the application of 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.
There are many caterpillars with hairs or spines on their bodies, but not all of them contain venom. Some species mimic the stinging caterpillars in appearance so they will be left alone. Identification is important because touching the wrong caterpillar can be painful.
One stinging caterpillar report on neighboring trees, shrubs or other plant material in close proximity of the school campus will justify monitoring for activity.
Because stinging caterpillars are an occasional invader, the best option for this group of pests is to train grounds crew personnel and custodial personnel to keep eye on trees and shrubs for possible caterpillar development. Some of the caterpillars occur on a wide variety of trees, shrubs and other plants, including corn. Common tree hosts are apple, basswood, cherry, dogwood, elm, maple, oak and plum.
Puss caterpillars, one of the most toxic, feed on foliage of a variety of broadleaf trees and shrubs. Some common tree hosts are apple, elm, hackberry, maple, oak, pecan and sycamore.
Avoid handling any hairy caterpillars or material with which they have been in contact. Suitable protective clothing, including eyewear and gloves should always be worn when handling these insects. Remember, dead caterpillars can still cause painful stings. 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 either through predation or when all food sources are removed or exhausted. Only plant tree and shrub species that do not attract stinging caterpillars. Where stinging caterpillars present hazards to people, such as around residences or schools, infested shrubs and trees may be vacuumed, sprayed or dusted to reduce or eliminate the caterpillars.
Teachers, parents and staff should be educated about not handling these pests.
Knock caterpillars off the plant and crush where possible. Relocate or remove plants that are known hosts, if possible. Vacuum caterpillars off plants, and dump the vacuum container in a bucket of soapy water.
Release green lacewings, predator tachnid flies, stink bugs or parasitic wasps, e.g., Trichogramma minutum.
Where stinging caterpillars are numerous or present hazards to people, such as around residences, recreation areas or schools, infested shrubs and trees may be sprayed to eliminate or reduce the caterpillars.
Lightly dust the leaves and infested areas with products containing Bacillus thuringiensis var. kurstaki and/or Safer’s insecticidal soap if caterpillars are small. Contact insecticides containing acephate, carbaryl, pyrethroids (permethrin, bifenthrin, cypermethrin, lambda-cyhalothrin) or others possibly mixed with an adjuvant or liquid soap can be used on populations that need quick knockdown. Always read and follow the label. The label is the law. Pesticides must be used in accordance with federal, state and local regulations. Applicators must have proper credentialing to apply pesticides and should always wear personal protective equipment (PPE) as required by the pesticide label during applications. All labels and Safety Data Sheets (SDS) for the pesticide products authorized for use in the IPM program should be maintained on file.
Regularly monitor plants where stinging caterpillars have been located. If stinging caterpillars are discovered, decide which combination of the above tactics to use to implement your management plan.