Groups tend to be more extreme than individuals.

~Daniel Kahneman

In spite of the heat and humidity (or maybe because of it), this is one of my favorite times of year – caterpillar time. The period between mid-August and mid-September tends to be the best time to find the greatest variety of interesting caterpillars in this part of the world. Most species are nearing the end of their larval life and are getting larger, the amazing parasitoids and predators are reaching a peak, and many of the caterpillars are getting ready to pupate, which usually means they will be on the go looking for a suitable spot making them more likely to be encountered out in the open.

Caterpillars clustered on an oak leaf (click photos to enlarge)

This past week I had a few nice encounters with a bizarre behavior – caterpillar aggregations (or clusters). The first was a group of Datana sp. (they can be tough to tell apart) larvae clustered on the back of an oak leaf in front of one of our buildings at work. Garden staff had alerted me to them so I walked over and grabbed a couple of quick pics in between what has been constant rain these past two weeks.

Aggregation of walnut caterpillars on tree trunk

The next day, I checked on the oak leaf, and the cluster was still there, huddled together. I wasn’t sure whether it was the weather, or perhaps an impending molt, causing this behavior. When I got back to my office, another coworker texted me a photo of what looked like a blob of caterpillars on a tree trunk next to her car. This one was a bit more dramatic (as she said, it looked like a squirrel had been squashed on the side of the tree). This group looked like walnut caterpillars, Datana integerrima. They are hairier than the other species from earlier in the day, and this species feeds only on trees in the Juglandaceae family (black walnut, hickories, pecans, and butternut).

The pile of larvae was huge and several caterpillars thick

This species is known to form large aggregations, often 2 or 3 caterpillars deep, on tree trunks (this larval pile was about 12 inches long and wrapped about 5 inches around the hickory trunk). It turns out this is a phenomenon called synchronous molting, and is fairly common in several species of Datana larvae. I looked online for information on how they manage to coordinate the timing of their molt, but was unable to find a definitive answer. I assume they are able to communicate with pheromones which may impact hormone levels that trigger the molt.

A close-up view of the hairy mass

This process may take a couple of days before they finally shed and head back up into the tree to continue feeding. But why do these caterpillars cluster? Looking through several resources it seems there are a few theories: thermoregulation (clustering together increases body temperatures of the individuals); feeding advantages (feeding together in groups may help overwhelm plant defenses); and the most likely, anti-predator/parasitoid defense. Datana larvae typically display a defensive U-shape, raising the tip of the abdomen and arching the head and thorax back. They exude what is presumably distasteful liquid if the threat continues. They also exhibit synchronized head-flicking in response to an approaching flying predator or parasitoid. The time just before and after a molt is a very vulnerable one for caterpillars, so it makes some sense to aggregate and combine defenses, although it may also make them more vulnerable to being discovered by certain caterpillar-seekers like hungry birds or curious humans.

Shed skins of walnut caterpillar cluster (compare to second photo above)

After work that day, I decided to go back out to the walnut caterpillar cluster for a few more photos. As I approached, I could see something was different. The cluster was not as noticeable as I approached their tree. They had all shed! The cluster now looked like a week-old squirrel carcass stuck to the tree, and almost all of the caterpillars were gone, presumably up in the canopy feeding. What a difference a few hours made. As I looked at the pile of shed skins, a slight movement caught my eye a few inches away from the heap of husks…a line of small caterpillars clinging to a green briar (Smilax sp.) vine,

Early instar turbulent phosphilas on Smilax sp.

These little guys are turbulent phosphilas, a favorite of mine, both for their odd name and their strange color patterns. I am more familiar with their latter instar where they are adorned in bold black and white stripes and more prominent fake eye spots that make it hard to discern which end is which. Learn more about them in an earlier blog post. I am glad to find these small larvae. Maybe a few will make it until BugFest (coming up September 15!).

I walked over to the other caterpillar cluster on the oak. They had also molted after at least two days of hanging out on that one leaf.

Newly molted Datana sp. larvae on oak leaf (compare to first photo above)

I am still not quite sure which species of Datana these are…most likely either Contracted Datanas, D. contracta, or Yellow-necked caterpillars, D. ministra.

Arched defensive posture of Datana larvae

As I maneuvered the branch for a better photo, the cluster began to twitch and assumed the defensive U-posture common to this genus. Teamwork against a perceived threat.

Cozy caterpillars

Though they looked content all gathered together, I assume they eventually dispersed back up into the leafy regions of the tree. I am thankful to work in a place with such biodiversity and that these larval congregations were all in places so accessible that they allowed me to share a few moments of wonder with them.