Hunting for Illusive Moth Caterpillars with John Dill

(Photo of the infamous slug saddleback caterpillar, Archaria stimulea, courtesy of John Lill.)

I was in a bit of a panic. For weeks I had been promising the five-year-olds in my nature class that we’d study tent caterpillars, just as soon as we saw them emerge in late April or early May. Now the time had come. We were almost half way through the month of May, and the course was about to end for the semester. But the tent caterpillars had not been seen.

I had checked for them each day since late April. Tent caterpillars particularly like the native cherry leaves, and often you can find their huge webby tents in the park where those trees sprout up with abandon. But my morning walks this spring resulted in the discovery of only one bedraggled caterpillar, all alone… sans tent and tent mates.

Tent caterpillars, those little black insects with the sky-blue stripes down their backs are sometimes mistaken for the much more destructive gypsy moths. But they are also beloved by suburban children who like to gather them up on warm, sunny days and treat them like teddy bears. Perfect content for a nature class, I had assumed, because they are both benign and abundant.

What I didn’t anticipate was the variability of spring weather. The caterpillars don’t like to leave their tents when the weather stays cool and damp, and this spring that phrase pretty much described the entire month of May. Cool and damp. Those insects were only really out for a couple of weeks, and in many places their populations did not really reach their typical numbers. There weren’t the usual masses of them to be seen in many local parks.

A friend finally came to the rescue when her kids up in Germantown found lots of the caterpillars and brought them to my class where we watched them make cocoons and prepare to turn into moths, so I was saved.

John Lill, who studies caterpillars at George Washington University, has often had to face similar problems when he’s headed out to the woods with students. Caterpillars lives can be impacted by all kinds of variables and may sometimes prove very difficult to locate in any patch of woods or lawn.

This can be especially true for those which he refers to as the “ephemeral” species in our area. Until I heard him use that word for caterpillars I had only ever heard it used in reference to certain plants. But as Lill described it, there are some caterpillars which are like those spring wildflowers -- they only appear for a few short weeks each year before they quickly form cocoons and turn into moths. This category includes the beloved and friendly-looking tent caterpillar. It also includes the hickory-horned devil, a huge, green creature with red and black horns which Lill called the “holy grail of caterpillar scientists” because it is so difficult to find on local trees.

Lill discussed hunting for the hickory-horned devil and many other aspects of studying moth caterpillars in the eastern forests of the US at the May meeting of the Friends of Sligo Creek.

I really enjoyed his talk, but I couldn’t help feeling as if I were listening to Dr. Who describe aliens from outer space. There were descriptions of caterpillars who ambulate without obvious legs, and others which look like sea urchins and sting like jelly fish. There were some who give painful pricks with their spines to evade the mandibles of wasps. Others he’s been studying can secrete liquids from hidden holes in their skin, or are covered with white hairs that look and act like spun glass.

“I am completely obsessed with these lately,” Lill said with great warmth as he worked his way through photos of slug caterpillars. His enthusiasm was equal only to that of the five-year-olds in my nature class, so it was easy to assume he must be very popular when he visits classrooms each year throughout Montgomery County to teach students about his multi-legged study subjects.

When asked about collecting the insects with kids, Lill emphasized three things:

1) Only collect the caterpillars you find on foliage. Never take a caterpillar home from black top or sidewalks, because you will have a tough time figuring out what your study subject needs or likes to eat. Most caterpillars are very specific in their dietary needs and habits, and will restrict their munching to one or two kinds of plants or trees. Without the right kind of leaf they will quickly die.

2) Figuring out what they like to eat is important, because caterpillars can eat a lot in one day! In fact, some species can eat enough to gain more than 10,000 times their own body weight over the course of development. This would be, Lill says, “like a child becoming as large as an elephant just a few months after its birth.” You should gather a lot of fresh leaves on a frequent basis.

3) Poking air holes in the lid of a jar isn’t as important as most people think. In fact, the amount of air most caterpillars need is pretty small and the air can be refreshed each day just by opening and closing the container’s lid. But what caterpillars DO need desperately is moisture, which is often released in a jar with lots of air holes. Lill says you can even use a zipper style plastic bag to keep the caterpillar moist, happy and healthy. He also likes to use recycled plastic deli tubs for his study subjects in the lab and for his school visits.

Although the tent caterpillars I described earlier are only around for a short time each spring, there are many caterpillars which become more abundant as the summer wears on and fall approaches. Lill will lead a walk sometime in late summer for the Friends of Sligo Creek, in order to teach people about these dynamic creatures. Keep your eyes on the Friends of Sligo Creek website for more details.

This piece was published in the June 2011 Voice newspapers of Montgomery County, Maryland where Alison Gillespie is the author of the Sligo Naturalist column.