Over the weekend, my good friend Ron Rothhaas (President, Arbor Doctor) sent me a nice picture of a colony of Yellownecked Caterpillars (Datana ministra) rearing their ugly heads, and other ends, in a park in Hamilton County. Coincidentally, at about the same time that he must have been taking his picture, I was taking pictures of Walnut Caterpillars (D. integerrima) on their namesake host in a park in Butler County. 'Tis the season!
Both yellownecked and walnut caterpillars feed in groups, or "colonies," throughout their development. Both also practice an interesting defense behavior when disturbed: the entire colony will rear their front and tail ends in unison presumably to confuse predators. Thus, both types of caterpillars have great entertainment value!
Caterpillar colonies may include 10-30 individuals which is why their defoliation is often focused on a single branch or a group of adjoining branches. However, it's also why multiple colonies can quickly defoliate small trees.
As their common name implies, walnut caterpillars favor walnut trees, but they will also feed on hickory and pecan trees and will occasionally infest apple, birch, honeylocust, oak, and willow. Yellownecked caterpillars will feed on a wider variety of trees and shrubs including: beech; boxwoods; crabapples and other ornamental fruit trees; elms; hickories; honeylocust; maples; and oaks.
Both species of caterpillars pass through distinct "color phases" during their development meaning they change their colors and markings as they mature. The changes can sometimes present a challenge with their identification.
Walnut caterpillars have at least two generations per year. They practice an unusual molting behavior. When molting, they group together on trunks, branches, or twigs and all of the caterpillars molt at the same time leaving behind a mass of hairy exoskeletons that looks like a patch of fur. Finding these furry patches on or near denuded walnut leaves is good way to identify the culprit behind the defoliation.
Yellownecked caterpillars have one generation per year in Ohio; however, adult moths continue to lay eggs for about a month so early instar caterpillars may be found at the same time as late instars giving the impression there is more than one generation.
Although the damage caused by both types of caterpillars can be devastating to small trees in landscapes, they are seldom considered serious pests in woodlands. The caterpillars can be easily managed on small landscape trees using a two-step control method. Step one involves positioning the colonies on the ground. Step two consists of performing the "caterpillar stomp."
This control method is approved for Integrated Pest Management (IPM) programs. So far, no walnut or yellownecked caterpillars have become resistant to this control method.