Walnut Caterpillars

Last August I was invited to the Polly Hill Arboretum on Martha’s Vineyard to give a presentation about my insect research on Nantucket. After the slideshow I led a walk around the arboretum in search of insect tracks and signs. It’s always a little challenging to show the tiny things I focus on to a large group of people, but right outside the barn where I gave the presentation, there was a striking bug-related phenomenon I hadn’t encountered before: a big ball of hairy caterpillars hanging from a branch of a black walnut tree.


The caterpillars had devoured all the leaves on some parts of the tree.


Some mature caterpillars had dropped to the ground and were wandering off in search of a place to pupate. On close inspection, they didn’t look like anything I recognized.


Then I noticed some immature ones that were still feeding. Their stripes and posture (abdomen bent upward) reminded me of the genus Datana (Notodontidae).


So I grabbed the arboretum’s copy of Dave Wagner’s Caterpillars of Eastern North America and opened to the Datana section. Sure enough, the first species I saw was D. integerrima, which has been given the common name of “walnut caterpillar.” I gather from Wagner’s remarks section that the caterpillars had collected into that dense cluster in order to molt. Apparently I didn’t bother to take a photo of it, but I seem to remember looking the next day and seeing that all that remained of the caterpillar ball were some shed skins hanging in the loose webbing.

I was going to make this post a miscellaneous collection of things I found on Martha’s Vineyard, but upon rereading Wagner’s notes on Datana integerrima just now I realized that the mystery moth eggs I found nearby were in fact D. integerrima eggs: “The white eggs, small for a prominent, are deposited by the hundreds in one or more adjacent rafts—each egg bears a small black central spot.” (Members of the family Notodontidae are generally referred to as prominents.)


What had caught my attention about these eggs, apart from being something distinctive that I didn’t recognize, were the tiny parasitoid wasps that were emerging from them. Fortunately they were still emerging the next day when I got around to bringing out my macro lens.


Having reared identical wasps from eggs of a tawny emperor butterfly (Nymphalidae: Asterocampa clyton), I recognized these as male eulophids in the subfamily Tetrastichinae, though I have no idea what genus they might be. I mostly encounter (very different-looking) tetrastichines as parasitoids of gall insects, but some of them parasitize leafminers.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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3 Responses to Walnut Caterpillars

  1. Hi Charlie, I recently moved back to the east coast and would love to go to your talks when I can. Is there any way to find out if you’ll be speaking at all within an hour’s radius of Bourne, MA? Maybe Falmouth, Woods Hole, Rhode Island…. Do you post your speaking engagements anywhere? Best wishes, Iris

    • Hi Iris– I don’t give talks very often, but all of my events are posted in the “Schedule” link at the top of this page. I can also add you to the Northern Naturalists email list if you want–at this point I just use that a few times a year to let people know about upcoming programs.

  2. Great post! Another link in the walnut caterpillar community. In my prior garden, I noted adult wheel bugs stationing themselves between clusters of young walnut caterpillars and the uneaten portions of the pecan branches, then systematically eating the caterpillars as they tried to move to fresh feeding grounds. I love learning about the varied and complex interrelationships between plants and various insects.

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