I came across a collection of first-generation catalpa hornworms (Ceratomia catalpae) feeding on a southern catalpa (Catalpa speciose, family Bignoniaceae)) planted in a county park in southwest Ohio. The caterpillars were 2nd instars when I found them on Monday; however, these native moth caterpillars develop rapidly.
The caterpillars consume the entire leaf with the exception of the main leaf veins. Thus far, the damage is barely noticeable, but that will soon change as the ravenous caterpillars increase their size and appetite.
Catalpas have long been one of my favorite trees. I recognized that there are a few minor shortcomings, but no tree is perfect. Whether viewed as a beautiful, resilient native tree that will compliment any urban landscaping, or a coarse, messy, tree best confined to forested bottomlands, no one can ignore the beautiful bell-shaped blooms. Both the Northern Catalpa (C. bignonioides) and its shorter southern cousin sport huge, orchid-like flowers that attract a bevy of pollinators, particularly bumble bees.
My love affair with catalpa trees is rooted in a huge northern catalpa that grew in our farmyard in West Virginia; it shaded many family reunions. It also supported my tree-climbing expeditions and was a great teacher. It taught me the meaning of gravity coupled with the rigidity of the Earth's surface.
The tree also taught me that catalpa hornworms make wonderful fish bait! I can say from experience that both largemouth and smallmouth bass, as well as channel catfish, seem incapable of resisting the offer of a big, wiggly, catalpa hornworm. Our tree fed the hornworms and the hornworms fed my family many fish dinners.
The Tree and Its Caterpillar
Sphinx moth caterpillars (family Sphingidae) are called "hornworms" owing to a distinctive "horn" on their posterior end. Catalpa hornworms sport an obvious black horn that's very apparent on all caterpillar instar stages.
The caterpillars have two "color forms;" a dark form and pale form. Dark form caterpillars have a broad, "black-velvet" stripe running down their backs, and their sides are yellow to yellowish-white with black spots. Pale form caterpillars are light green or greenish‑yellow and may have a rows of black markings down their back rather than a black stripe, or they may appear almost albino-like by lacking any noticeable black markings.
There are two overlapping generations per year in Ohio with large late instar first generation caterpillars commonly found feeding alongside early instar second generation caterpillars. Winter is spent as pupae buried 2 - 3" inches beneath the soil surface.
Although the caterpillars of this native moth are capable of producing substantial defoliation of their native host, the hornworms seldom cause significant long-term injury to the overall health of catalpa trees. Indeed, this pest - host relationship has been studied for many years to learn how coevolution affects relationships between native trees and their native pests.
Hornworm Caterpillars and the Wasp
Catalpa hornworm caterpillars must run a gauntlet of beneficial insects including parasitoid and predatory wasps. Plundering by the endoparasitoid wasp, Cotesia congregata, is revealed when white, oblong cocoons sprout from the hornworms. Endoparasitoids develop inside their host; it's like having a predator living inside (see the movie Alien (1979)). An ectoparasitoid does the same while attached to the outside.
C. congregata is a gregarious endoparasitoid meaning that multiple wasps develop inside a single caterpillar. It also targets several species of sphinx moths. This is unusual in the parasitoid world. Most parasitoids have a relatively narrow host range owing to the complicated coevolutionary dance between the parasitoid and its host.
C. congregata is a well-known nemesis of catalpa hornworms; however, you will also see its white cocoons sprouting from the backs of tomato hornworms (Manduca quinquemaculata), tobacco hornworms (M. sexta), and laurel sphinx moth (Sphinx kalmiae) caterpillars. This is why C. congregata is sometimes called the "Hornworm Wasp."
The developing wasp larvae are "programmed" not to eat any internal caterpillar structure that will kill the caterpillar. Once the immature wasps near pupation, all bets are off and wasp larvae consume all internal structures. Doomed hornworm caterpillars festooned with the white, oblong, silken cocoons signal that the wasps have completed their development.
The Wasp and a Virus
The female wasp uses her sharp ovipositor (ovi = egg; positor = lay) to insert eggs, venom, and a virus into a hapless hornworm victim. All three are required for wasp larvae to successfully develop inside the hornworm caterpillar.
Obviously, the eggs give rise to the wasp larvae. The eggs also release special cells, called teratocytes. The teratocytes release hormones that along with the venom suppress the caterpillar's development. This is why parasitized caterpillars are often much smaller than their non-parasitized siblings. Keeping their caterpillar food a caterpillar is critical to the survival of the wasp larvae; they would be doomed if the caterpillar pupated.
Insects have immune systems, just like us, to defend against foreign intruders. The caterpillar's immune system could potentially reject the wasp eggs, teratocytes, and wasp larvae. If this occurred, you wouldn't be reading about C. congregata.
This is where the virus comes into play: it suppresses the caterpillar's immune system. But, where does the virus come from? This puzzled researches for years.
As we know from our crash course with the coronavirus, most viruses take over certain cells in their host and then use the cell's machinery to replicate more virus particles. This process is governed by genes in the virus's genome. However, the virus found in the hornworms does not carry genes to replicate itself.
That's because the virus is hidden within and created from the wasp's own genome. The actual virus does not exist until the proteins and genetic payload of the virus are coded from wasp DNA inside specialized cells in the wasp's ovaries, called calyx cells. The result is called a virion which is the complete, infectious form of a virus.
"A virus is not an individual organism in the ordinary sense of the term, but something which could almost be called a stream of biological patterns."
Sir Macfarlane Burnet, 1960 Nobel Prize winner for predicting acquired immune tolerance