The spiny oak-slug moth, like other slug caterpillar moths, has a chunky, fuzzy body with wide, rounded wings. Forewings are brownish, often with orange patches or purplish shades, with a green patch in the middle area that is bordered with white. The size of the green patch varies greatly among populations of this species. Males and females look different. Males are smaller, have comblike antennae, and may have a larger green forewing patch. Females are larger, with more rounded wings, and their green patches may be generally smaller.
The sluglike caterpillars (“spiny oak-slugs”) are green, yellow, orange, and/or red. As with other slug caterpillars, the heads are small and hard to see. At the front, there are 3 pairs of large, hornlike spines with dark-tipped bristles, and at the rear, there are 2 more pairs of these hornlike, bristly spines. Additional clumps of spines run in rows along the back and sides. Sometimes, there are 2 pairs of red or dark spots on the back.
Similar species: The form of this species most commonly seen in Missouri (form paenulata) is the only wide-ranging slug caterpillar moth in our state with green wings. Other similar species are rare and restricted to eastern and southern Missouri. There are more than 20 species of slug caterpillar moths in Missouri. The weird-looking caterpillars in this family are more commonly noticed than the adult moths.
Wingspan: ¾–1¼ inches.
Habitat and Conservation
Many species of slug caterpillar moths may be widespread regionally yet only occur in localized concentrations, while being rather uncommon elsewhere. They are most usually seen in wooded areas, though adults are commonly attracted to lights at night.
The caterpillars feed on a variety of trees — reported on oaks, apples, maples, plums (and other Prunus species), willows, and more.
In Missouri this species is double-brooded, with the moths flying from early May to late August.
If you are stung by one of these caterpillars, keep an eye on it. Usually the symptoms go away or decrease within about a day, but if they are severe or persist, seek medical help. Some people may be extra sensitive or have an allergic reaction.
First aid ideas: Some experts suggest gently applying tape sticky-side-down to the affected area, then lifting it off to draw out the tiny hairs from your skin. A paste of baking soda and water can help relieve symptoms, as can hydrocortisone cream.
A favorite Gary Larson “Far Side” cartoon showed humorous examples of “How Nature says ‘Do Not Touch.’” The brightly colored spiky horns and stiff, scary spines on the spiny oak-slug caterpillar certainly do look daunting!
These caterpillars don’t really need to hide from their enemies. Would-be predators quickly learn to avoid them after sampling just one individual. This may explain why so many slug caterpillar moth species occur in locally abundant groups. As with a school of fish or a herd of antelope, a few unlucky individuals will indeed be killed. But in this case, the rest will be bypassed by the “educated” predators.