Though a picture may be worth a thousand words, does anyone really want to see a hundred hairy caterpillars? Many would cringe at the sight, then run screaming from a copious corpus of downy caterpillars. 

But not Felix Neck volunteer Sam Moore, who did what any good nature photographer would: he got his camera and took some close-ups. Sam has a naturalist’s curiosity and a fascination for the furry. The creatures he caught with his camera were walnut caterpillars, also called walnut datanas, as their scientific name is Datana integerrima.

Walnut datanas are nuts for nut trees. Their preferred hosts during their eggs-and-caterpillar stages are walnut, pecan, butternut and hickory trees, which provide a leafy location and food source that serve the needs of both stages.

These insects are relatively common in areas with nut trees, but can be easily missed during much of their life cycle. In their moth stage, they appear as a nondescript reddish-tan night flyer. During their egg and caterpillar stages, they exhibit interesting appearances and behaviors.

Adult female moths lay a few hundred eggs on the leaves of their favored trees. Their cylindrical white eggs with a central black dot are vulnerable. Parasitoid wasps can lay their eggs inside the caterpillar eggs and when the wasps hatch, they feed, eventually emerging from the egg after eating out the egg’s contents.

The tiny walnut caterpillars that don’t get eaten and appear in the late summer are ravenous. They will skeletonize all leaves within their crawling range. During their molt, they have peculiar habit. There are five stages of growth for the black walnut caterpillar. Each is called an instar and will end with the shedding of the caterpillar’s too-small skin after it becomes too tight, thus enabling the caterpillar’s growth. During each instar, the caterpillar looks different, with early ones being red, then brown, and finally, the last ones appearing black with fine white hairs.

What is unique is that when they are ready to molt, individual walnut datanas will crawl down from their leafy buffets and gather into a big group ball or cylinder on the tree’s branches or trunk to do the deed. Unless it is the last instar, they will return to the leaves individually to eat. After the last instar they travel downward to the ground where they will overwinter in the soil. With this behavior, each molt becomes a group growth spurt.

If bothered during any instar, walnut datanas will raise their bodies in a U-shaped defensive position to try to deter predators. This behavior is only marginally successful since at least half of the caterpillars will be eaten by predators, such as other insects (wasps, spiders, beetles, etc.), birds and mice, which can consume 15 per cent of walnut caterpillar larva.

Another deterrent is their white hairs. While those hairs are the not true uticating (stinging) hairs that some caterpillars possess, they do provide a warning sign to predators that don’t want to take the risk of being stung.

For people, these caterpillars may cause irritation to those with sensitive skin, but generally won’t cause any harm as they do not have the toxins that other hairy caterpillars possess.

In times of significant walnut datana outbreaks, trees can be weakened and even nut harvests affected, though in a typical, non-eruptive year, both plant and animal can cohabitate. If you have a problem on your trees, either the eggs and/or the caterpillars can be manually removed or the ground around the tree can be dug up to remove their overwintering brethren.

Sam’s photo may not have inspired a thousand words in this article (which at its lesser length might be called the Datana 500), but his enthusiasm for these crawling critters certainly deserves extensive comment and praise from this naturalist. Grabbing the camera or field guide is an understandable response to such an unusual display of insect activity for nature lovers.  But big bunches of bugs isn’t for everyone, so the insect-phobic shouldn’t be criticized for heading in the other direction, as quickly as possible, when they find these terrifying teammates occupying a local tree.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown, and author of Martha’s Vineyard: A Field Guide to Island Nature.