Caterpillars come in many colors, sizes, and patterns. They are insect larvae of lepidopteran moths and butterflies. Some have bristles or hairs along their body, called setae, which may be urticant, that is, they may irritate or poison a predator. They either do so by breaking off in the predator or curious human, or by release into the air which causes irritation in the sinuses or even asthma attacks. Sometimes the poison is in the insect rather than the hairs. Sometimes there is no poison at all. The following are species I’ve seen and photographed in the NYC Watershed.
Danaus plexippus: The iconic monarch butterfly caterpillar munches on milkweed for five instars before forming their chrysalis. Compounds in the milkweed make them poisonous if eaten
Euchaetes egle: The milkweed tiger moth feeds on milkweed like monarchs do and also ingests chemical compounds which render them poisonous to predators. Don’t eat one.
Urticant with Stinging Setae
Arctia caja: Hollow tubes of irritating compounds line the body of the garden tiger moth caterpillar.
Lymantria dispar: The gypsy moth caterpillar is an invasive defoliator, primarily of deciduous trees. Their setae can cause irritation.
Orgyia leucostigma: The setae of the white-marked tussock moth caterpillar are urticant.
Poisonous and Urticant with Stinging Setae
Lophocampa caryae: The hickory tussock moth caterpillar can be poisonous to predators and their barbed hairs can cause rashes. Steer clear of this one.
Fuzzy but harmless
Hypercompe scribonia: Giant leopard moth caterpillars are very fuzzy like woolly bears with non-urticant setae.
Spilosoma virginica: The non-urticant yellow bear caterpillar will become the Virginia tiger moth.
Polygonia comma: The white spines of the eastern comma caterpillar, which will become the eastern comma butterfly, look mean but are purported to be innocuous.
Malacosoma disstria: Although the forest tent caterpillar neither bites nor stings, they do defoliate deciduous hardwoods which is why the moth is named for the caterpillar and not the other way around.
Eutrapela clemataria: This brown inchworm will become the large curve-toothed geometer moth.
Hyles gallii: Bedstraw hawk-moth caterpillars have no setae at all and the “horn” on the tail is harmless.
Sphinx kalmiae: The brightly colored fawn sphinx moth caterpillar also has a harmless “horn”.
Eudryas unio: This colorful caterpillar becomes the Pearly wood-nymph moth. Interestingly, this moth does not eat at all.
Not a caterpillar
Diprion similis: Introduced pine sawfly larvae aren’t technically caterpillars since they don’t become either butterflies or moths, but they are caterpillar-like. They are also invasive pine defoliators.