“Follow, follow, follow, follow, follow the yellow brick road… We’re off to see the wizard, the wonderful Wizard of Oz… because, because, because, because, because, because of the wonderful things he does…”

~The Munchkins of Oz

In this particular case, the road is not so much yellow and brick as white and silky… but these little critters certainly do some wonderful things…

Eastern tent caterpillars on a cherry tree branch

Hey, Roads End Naturalist readers, it’s Melissa here. With a little extra time on my hands I’ve been thinking a lot about ways to help teachers who are trying to teach virtually and parents who are now homeschooling to use the natural world as a teaching tool in this time of social distancing. So yesterday, I went out to document one of the most ubiquitous and friendly critters at this time of year – eastern tent caterpillars – as an idea for a subject to explore in your backyards or local parks (while staying 6′ from other people, please!).

Eastern tent caterpillars are the furry little guys who come out just as tree buds break in early spring and construct a silken web in the crotches of wild cherry tree branches. They’ve spent the winter as tiny larvae housed inside of an egg case that looks like a shiny, swollen growth on the tree branch. Our eastern tent caterpillars have been out and about for a little more than a week now, if memory serves me right. (Let’s see, I started to notice them in the branches before I stopped going to work, and that was a week ago Friday.) Most of them now are about 3/4″ long and starting to show some of the patterning that they’ll have later in life. And their tent homes range from about 4″ to 6″ across at this point.

Eastern tent caterpillar nest in between branches of a cherry tree with one caterpillar out on the surface. The dark spots inside the nest are frass (caterpillar poop).

Yesterday afternoon I got to observe a few of their really interesting behaviors when I went out for a look with the camera. I was very excited to note the silvery trails of silk running along most of the branches on the tree. As the young caterpillars go off to feed, and as they return, they leave a silken trail that is laced with chemical scents that guide the other caterpillars between the nest and leaves that they are feeding on. Over time, the silken “roads” become quite substantial. Apparently, a lot of research has been done on this behavior, and studies have shown that they are able to change the scent of the trail depending on the quality of food resources at its end – when a caterpillar comes back having fed successfully on a tasty branch, the next round of feeders will follow its trail back to the same spot.

I was even more excited when I saw a caterpillar follow a silk trail down one branch and back up another. Eventually, the caterpillar turned around and marched back to its nest – perhaps because at the end of the trial in the other direction was a different nest? It moved surprisingly quickly, so I had to anticipate its position along the branch to get a shot.

Tent caterpillar following a silken trail down a cherry tree branch.

A little later, after borrowing Mike’s fancy new camera with its twin light flash unit to try to better capture this phenomenon, I went out to a different, lower-down nest to try to get some closer images. The scene was totally different from earlier in the day! If you read this blog often, you probably know a little about caterpillars, as it’s a favorite topic in our household. But as a refresher, caterpillars shed their skin, usually five times (six for eastern tent caterpillars), to accommodate their dramatically increasing size as they grow. An analogy Mike once shared with me is that if a human baby grew as much as a caterpillar, it would end up the size of a blue whale! No one wants to live in a crowded house (especially when you’re not allowed to leave it… ah, pandemic humor…), so eastern tent caterpillars add additional layers of silk over time to increase the size of their nest.

The caterpillars have multiple “stories” to their house and can move between the layers of silk. You can see caterpillars within different levels of the nest in this image, as well a few out on the surface.

And since they don’t have HVAC systems, they also use their nests for thermoregulation. One study I read said temperatures were typically 4 degrees C higher in the nest than outside of it (that 7.2 degrees F if you’re like me and need the conversion)! Of course, on a warm day like Friday, that might not be a good thing, and the caterpillars were congregated on the shady side of the nest when I went back out to see them. I’m not sure if this was because it was cooler, or if this was their typical nest-expansion aggregation behavior.

I’m not sure if these caterpillars were expanding the nest or thermoregulating by hanging out in the shade, or maybe both. But it sure was cool to see. They moved around a good bit while on the outside of the nest.

These caterpillars really are fascinating. Numerous references (like this one and this one as well as a favorite classic field guide, Observing Insect Lives by Donald Stokes) note that they typically have 3-4 periods of feeding during the day. Apparently, the pattern is that they aggregate outside the nest and add layers of silk to expand it, then go off and feed. Sure enough, a little later (once I’d solicited Mike’s help to bend the tree down so I could get a better look at the nest), lines of caterpillars were marching along their silken roads back from the outer tree branches, I assume after having fed on the tasty, tender cherry leaves. This was the coolest moment yet – seeing the little caterpillars nose to tail marching down the branch and back home!

These caterpillars were marching down the branch toward their nest. You can see the silken “road” if you look closely under their bodies. Take another look at the first image in this post for another example.

They all marched right into the nest to hang out until their next bout of feeding.

One corner of the nest is at the right side of the image. The caterpillars entered through a hole where it meets the branch.

Here’s a few other interesting tidbits that I gleaned in researching this subject…

• If there’s more than one nest in a tree, eventually the trails from caterpillars in each nest will run into each other. Apparently, the caterpillars will share nests. Friendly little guys!
• Caterpillars in the same nest may be at different stages of development. It seems that development rate is linked to temperature, with warm temperatures being correlated with faster development.
• Eating eastern tent caterpillars can cause abortion in horses! In 2001, more than 3000 mares aborted fetuses as a result of eastern tent caterpillar ingestion. My initial guess was that the concentration of cyanide, which is naturally found in cherry trees, by caterpillar feeding was perhaps the cause of the issue. However, it turns out that the hairs on the caterpillar are able to penetrate tissues inside the horse, and those hairs carry bacteria which, when they reach the uterus, can infect the horse and/or fetus and cause abortion. Fascinating!

So, here’s my challenge to you. Get out in nature (with proper social distancing), and see if you can observe some of these fascinating behaviors of eastern tent caterpillars!

If you’re looking for ideas for teaching kids at home, maybe challenge them to observe and document behavior over time. Here’s some questions and ideas to consider:

• Can you document which times of day the caterpillars like to go out and feed? How does weather affect their behavior? What happens when they’ve eaten all the leaves on a branch?
• Use a thermometer to measure the temperature on different sides of the nest – where is is warmer or cooler?
• Use a compass to note the direction nests are facing – do they tend to be built at a certain orientation? Are the silk trails typically on a side of the tree facing in a particular direction, or is it more random than that?
• How many caterpillars can you count in a single nest?
• Using your observations, write a story or poem describing what it’s like to be a tent caterpillar, hanging out in a tiny silk nest with hundreds of your brothers and sisters… maybe not too dissimilar from your life right now? If anyone does this activity, feel free to post it as a comment on the blog – Mike and I would love to read some of your writing!
• And if you really want to get into it, this book I found online has a whole bunch of experiment ideas using tent caterpillars – it’s chapter 17. I can’t find a way to link to the specific page, or even a page number to give you, but I found it by searching “eastern tent caterpillars” within the text.

I’d love to hear if any of the ideas are useful, or if you observe any interesting behaviors in your tent caterpillars. Please share in the comments on the blog or tag Mike and I on social media!