I came across a colony of Walnut Caterpillars (Datana integerrima, family Notodontidae) yesterday as I was slogging out of a local park in Butler County around the time the “feels like” temperature hit self-ignition. Of course, photography took precedence over survival instinct. Natural selection has lost leverage; I have grandchildren.
Walnut caterpillars feed in groups, or "colonies," throughout their development. The colony that I photographed yesterday was feeding on its namesake host. However, the native caterpillars may also be found on hickory and pecan (Carya spp.), and occasionally on apple (Malus spp.), birch (Betula spp.), honeylocust (Gleditsia triacanthos), oak (Quercus spp.), and willow (Salix spp.).
The caterpillars undergo a dramatic change in appearance as they develop through 5 instars. They gradually develop striping, become hairier, and change colors, with early instars looking nothing like final instars. The changes may present an identification challenge.
The series of photographs below illustrates the different phases in their appearance. I’ve labeled them “Phase 1” through “Phase 4”, but I’m only confident that Phase 4 is the final instar.
All instars practice an unusual molting behavior. 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 a good way to identify the culprit behind the defoliation.
The caterpillars also practice an interesting behavior throughout their development that may be a physical defense strategy. When disturbed, the caterpillars across the entire colony will raise their front and rear ends in unison, presumably to confuse predators. They have great entertainment value!
Chemical Warfare
Unfortunately, I discovered yesterday that walnut caterpillars also possess a messy chemical defense. As I was holding what was left of a walnut compound leaf covered in Phase 4 caterpillars, I noticed that my hand and arm were gradually being covered by a dark, sticky, smelly liquid. My “Eww YUCK” response, fired by my overheated pre-delirium, worsened the experience.
At first, I assumed the caterpillars were regurgitating a foul substance as in the "defensive regurgitation" used by grasshoppers and other insects. However, I could not see globules of gut contents being exuded from their mouths.
A light literature review revealed that late instar walnut caterpillars possess a specialized pouch-like gland inside their first thoracic segment (prothorax), located just behind their head, connected to a small slit on the underside of the prothorax. Various authors call it a “ventral prothoracic exocrine gland” or “prothoracic defense gland.” When disturbed, the caterpillar’s thoracic muscles squeeze the gland to spew the noxious (it seemed that way) brew through the slit.
Research has shown that the liquid exuded by walnut caterpillars contains terpenoids, which are a class of chemicals commonly used by plants to defend against pests. The caterpillars had extracted the defense chemical from the walnut tree and repurposed it to defend against predators and misguided entomologists. I could not find information on whether the caterpillars are able to acquire the same mix of chemicals from other hosts, but they had hit pay dirt with black walnut.
Two-Step Management
Walnut caterpillars have a wide geographical range, as shown in the iNaturalist map below. The scientific literature notes that the native moths have two generations in their southern range and one generation in their northern range. Based on past observations, I believe we have one generation in Ohio, but that may change with climate change.
Although walnut caterpillars can be a serious pest in southern pecan orchards and occasionally in walnut orchards, they are seldom considered serious pests in Ohio’s woodlands. The native caterpillars have many native enemies, so localized populations tend to rise and fall dramatically from year to year.
Also, the caterpillars don’t disperse throughout tree canopies to produce general canopy defoliation. The literature notes that final instars separate from their colonies; however, they are primarily focused on dropping to the ground, where they pupate in the duff.
Throughout the rest of their development, the caterpillars feed in colonies, focusing on a single branch or a group of adjoining branches. They pose little threat to established walnut trees.
On the other hand, multiple walnut caterpillar colonies can quickly defoliate small trees. However, the caterpillars can be easily managed using a two-step control method. Step one involves positioning the colonies on the ground (handle with care!). Step two consists of performing the "caterpillar stomp."
This digital control method is approved for Integrated Pest Management (IPM) programs. So far, no walnut caterpillars have become resistant to this control method.
Selected References
Farris, M.E., Appleby, J.E., and Weber, B.C. (1982). Walnut caterpillar. US Department of Agriculture, Forest Service, Forest Insect & Disease Leaflet 41.
https://www.fs.usda.gov/foresthealth/docs/fidls/FIDL-41-WalnutCaterpillar.pdf
Cutler, B. L., & Harris, M. K. (1979). Foliage consumption and damage by the walnut caterpillar on pecan in Texas. Journal of Economic Entomology, 72(3), 315-318.
https://doi.org/10.1093/jee/72.3.315
Cutler, B. L., & Harris, M. K. (1976). Head capsule widths of the walnut caterpillar. Journal of the New York Entomological Society, 156-158.
https://www.jstor.org/stable/25009002
Weatherston, I., MacDonald, J. A., Miller, D., Riere, G., Percy-Cunningham, J. E., & Benn, M. H. (1986). Ultrastructure of exocrine prothoracic gland of Datana ministra (Drury)(Lepidoptera: Notodontidae) and the nature of its secretion. Journal of chemical ecology, 12(11), 2039-2050.
https://link.springer.com/article/10.1007/BF01041995
