Curtis Young (OSU Extension, Van Wert County) showed pictures of late-instar first-generation walnut caterpillars (Datana ministra) during our BYGL Zoom Inservice on Tuesday. Like yellownecked caterpillars (D. ministra) that were featured in a BYGL Alert posted earlier today, walnut caterpillars feed in "colonies" of 10-30 individuals throughout their development. Their defoliation of their namesake host is often focused on a single compound leaf with the caterpillars ganging up on individual leaflets before moving on to adjoining leaves on the same branch to repeat the process.
Walnut caterpillars practice the same interesting defense behavior that's seen with yellownecked caterpillars. When disturbed, the caterpillars will rear their front and tail ends, often in unison, presumably to confuse predators. Waving your hand near the colonies will solicit this entertaining behavior.
There are at least two generations per year in southern Ohio with some indications there may be only one generation in the northern part of the state. As with yellownecked cats, walnut caterpillars pass through distinct "color phases" during their development meaning they change their colors and markings as they mature. The early instars look nothing like final instar caterpillars which can present a challenge with identification.
There is some debate about whether or not it's 3 or 4 color phases. Regardless, the changes in colors, markings, and furriness may present a challenge with their identification.
Walnut caterpillars 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 or on tree trunks is a good way to identify the culprit behind the defoliation.
As their common name implies, walnut caterpillars are most often found on walnut trees. However, the literature notes that they will also feed on hickory and pecan trees and will occasionally infest apple, birch, honeylocust, oak, and willow. Large established trees can typically handle the defoliation with little long-term impact on overall tree health; even when infested by multiple colonies. However, multiple colonies on small trees can cause significant harm.
A Word from Management
Walnut caterpillars have a wide array of natural enemies from birds to insect predators to insect parasitoids and insect pathogens. These natural controls can keep populations in check; however, there may be occasionally caterpillar outbreaks as is common for native insects.
All instar stages can be effectively controlled on small landscape trees using a two-step control method. Step one involves knocking the caterpillar colonies onto the ground. Step two consists of performing the "caterpillar stomp." Thus far, no walnut caterpillars have become resistant to this control method.
As with many general defoliating caterpillars, early instar walnut caterpillars can be effectively controlled using the naturally occurring biological insecticide Bacillus thuringiensis var. kurstaki (Btk) (e.g. Dipel, Thuricide, etc.). Btk does not kill bio-allies that help provide natural control of the caterpillar populations.
Btk is a stomach poison which means it must be consumed to kill the caterpillars and it has relatively short residual activity. Thus, two applications may be required.
Btk is most effective on small caterpillars and becomes much less effective when caterpillars reach the middle instar stages. Larger caterpillars can be controlled using standard insecticides; however, this approach also risks killing bio-allies important for naturally controlling these caterpillars. Given that small, newly planted trees are most vulnerable to suffering negative impacts on overall tree health, and the caterpillar colonies are reachable by hand or broom to be knocked onto the ground (see above), it's hard to justify using standard insecticides.