Although I wrote about this phenomenon a few years ago, I’m revisiting the subject now because they have returned. The ‘they’ consists of masses of catalpa hornworms currently consuming the leaves of the catalpa tree that grows behind our garage.
I first noticed their appearance a few days ago by a substantial coating of a gritty gravel-like substance grating beneath my feet just outside our garage. I knew instantly what this meant: they were back.
I looked up into the leaves and, sure enough, there they were: catalpa hornworms. The blanket of grit beneath my feet was produced by their unbridled defecations, sometimes falling from the branches above me like a gentle rain shower.
Their return signaled the rekindling of an annual life and death drama, produced and directed by Mother Nature, that movie buffs might describe as a combination of the circle of life (Lion King) and Invasion of the Body Snatchers. The three principal players in this production are the catalpa hornworm, the parasitic braconid wasp, and the catalpa tree.
The dramatic interaction between these three adversaries plays out every summer in my own Northbrook backyard where a mature catalpa tree throws considerable shade on our driveway and garage. The catalpa tree (also called catawba tree) is distinguished by its large elephant ear-like leaves and long seed pods that resemble green cigars. If you were to Google ‘cigar tree’ the catalpa/catawba will appear at the top of the list.
But these trees have another unique characteristic — their leaves represent the sole source of food for the catalpa caterpillar, also referred to as the catalpa worm. It’s a relationship suggestive of the narrow diet of the koala bear that eats only the leaves of specific kinds of eucalyptus trees in Australia.
These caterpillars are the larval stage of Ceratomia catalpae, aka the catalpa sphinx moth. In late spring and throughout much of the summer, the female sphinx moth lays clusters of eggs on the undersides of catalpa leaves.
After they hatch, the black and yellow caterpillars boast a horny structure at the end of their abdomens that’s apparently intended to discourage predators. These kinds of caterpillars are often classified as ‘hornworms,’ most of which, like tomato hornworms, are considered serious garden pests.
These hornworms are ravenous eating machines, mowing through my catalpa tree’s leaves like little buzz saws and quickly growing to three inches or more in size. In past years significant hornworm infestations have served to completely defoliate my catalpa tree. A sure sign that my tree has been infested (other than the leaves being stripped away) is the layer of grit that appears on my driveway courtesy of the caterpillars’ copious fecal droppings.
But even gnawed down to bare branches, the resilient catalpa tree always bounces back, immediately sprouting a fresh batch of leaves. Unlike gypsy moth caterpillars which can defoliate and eventually kill oak trees, the connection between catalpa worms and catalpa trees is far less lethal as the tree appears to suffer little harm.
While defoliation of the tree might be unwelcome and unsightly, there is a major upside here, particularly if, like me, you happen to be a fishermen. For the upside, just Google ‘fish bait tree’ and see what happens.
You’ll find that the ‘cigar tree’ doubles as the ‘fish bait tree’ because, as it so happens, catalpa worms make for excellent fish bait, especially for catfish, sunfish, and crappie. In fact, some folks plant catalpa trees specifically to attract these caterpillars for use as bait.
They then harvest their worm crop and either hook them up fresh off the tree or freeze them for later use. You can even go online and purchase preserved catalpa worms that may sell for as much as $7 a dozen or more. I can testify firsthand that the bluegills in our pond can’t resist them.
So now we’ll cue the parasitic braconid wasp, Cotesia congregata, the actor that might be regarded either as the hero of our story or the villain, depending upon your point of view. If you’re a catalpa tree seeking relief from catalpa worms or a gardener plagued by other hornworm pests, the wasp represents the protagonist.
If you’re a fisherman collecting fresh catalpa worms for bait, the wasp is definitely antagonistic to your quest. Here’s why.
While these tiny, half-inch long wasps don’t sting and pose any threat to humans, they prove deadly to the catalpa caterpillar in the most sinister of ways. Braconid wasps are parasitoids, meaning they eventually kill their hosts in a most unpleasant manner.
The female braconid wasp actually pierces the hornworm’s skin and deposits her eggs inside its body where they hatch and become larvae, feeding on the caterpillar’s flesh from the inside out. When the time comes to pupate, the larvae chew their way out (reminiscent of the movie ‘Alien’) and then create tiny silk cocoons on the surface of the caterpillar’s skin. These pupae may number well into the hundreds attached to a single caterpillar. Soon after, tiny adult wasps emerge from these cocoons.
The hornworm host, although doomed, might survive through the stage where the wasp larvae are feeding and developing inside its body, but in the end it’s a walking dead zombie worm that will die long before it can spin its own cocoon and emerge as a sphinx moth.
Since these wasps cut short the hornworm’s life cycle and prevent it from reproducing, they are considered beneficial parasites (although some fishermen might disagree). Ultimately, in this movie, there are really no good guys or bad guys, just compelling raw evidence that Mother Nature can indeed be a cruel mistress. Nowhere is this more apparent than in the unique mini-ecosystem encompassing the catalpa hornworm, the braconid wasp, and the catalpa tree.
Tom Tatum is the outdoors columnist for the MediaNews Group. You can reach him at tatumt2@yahoo.com.
