Catalpa Worms

May 21, 2010 at 6:05 am (botany, plant-eating insects, Uncategorized)
Tags: Catalpa speciosa, catalpa sphinx, catalpa worm, Ceratomia catalpae, fishing, hardy catalpa

by Carl Strang

Growing up, I did not have a naturalist to mentor me. There were a couple adults who knew a little, and those contacts were important inspirations. For the most part, however, I had to learn through my own time exploring outdoors, and with the limited books available at the time in Culver, Indiana (2000 people then, 2000 now). On the other hand, in the narrower pursuits of hunting and fishing I learned a lot from Dad. That was an excellent extension of my studies, but it led to some odd misunderstandings. One of those had to do with catalpa worms.

Catalpa worms I knew as summertime bait for panfish. In the loose untutored taxonomy I was building in my head, catalpa worms were, in a sense, their own phylum. I did not think about what their relatives might be. They ate leaves of catalpa trees, usually small ones, and when you found the worms they were in large numbers. You caught them, put them in a cage, took them out on the lake for bluegill, and that was as far as my thinking went.

Not until college, in fact not until graduate school, did my thoughts go back to catalpa worms. I saw them on trees at the edge of the Purdue campus, and at long last realized that they were, as should be obvious from the photo, caterpillars. Furthermore, the single long spike on the end of the abdomen led me to the sphinx moth or hummingbird moth family, as this characteristic is found on larvae of nearly all species in that family, and not in others. The catalpa sphinx moth is strictly nocturnal, a generally nondescript brown color though with the long narrow wings and heavy body characteristic of the Sphingidae.

I knew from dendrology class that catalpa belongs to a plant family that is mainly tropical. In fact catalpa is native only as far north as Kentucky. Those in northern Indiana and Illinois have been introduced. The catalpa sphinx has followed the tree to northern Indiana, but I have not seen them in northeastern Illinois. The catalpa’s exotic defensive chemistry has protected it from nearly all consumers. The catalpa sphinx’s evolution of the ability to deal with those chemicals has given it nearly exclusive access to catalpa leaves, at the cost of narrow dietary specialization.

That chemical relationship the caterpillars have turned to their further advantage by using them for their own protection. Catalpa worms are conspicuous. Their striking black, white and yellow striping contrasts sharply against the tree’s yellow-green leaves. My reading revealed that this is one of the few members of family Sphingidae that lay their eggs in clusters rather than scattered singles. The caterpillars hatch, they become distasteful to birds by consuming catalpa leaves, and birds quickly learn to leave them alone. They are not completely safe, however.

Often you encounter caterpillars like this, covered with the cocoons of a parasitic wasp. Such caterpillars have had their exoskeletons so thoroughly breached by the emerging parasites that they are doomed to dessicate.

While researching the literature on this species I remembered that some fishermen turn the caterpillars inside out when using them for bait. This was exciting, because I thought I could tie this practice to the caterpillars’ chemistry. I know of no other bait treated this way. When I consulted with Dad, however, I learned that the practice is not universal and he, in fact, did not go to the trouble.

So, while I had an amusing incongruity in my childhood thoughts about catalpa worms, it was more than compensated by the enjoyment I found later in studying them from a mature ecologist’s perspective.