Catalpas (Catalpa spp., family Bignoniaceae) have long been one of my favorite trees. Both the Northern Catalpa (C. speciose) and its shorter southern cousin, S. bignonioides, sport huge, orchid-like flowers that attract a bevy of pollinators, particularly bumble bees.
Its leaves support a particular caterpillar; the Catalpa Hornworm (Ceratomia catalpae). The caterpillars support an intriguing parasitoid; Cotesia congregata (family Braconidae). Thus, begins our story.
The Tree and Its Caterpillar
Catalpa hornworms are the larval form of the Catalpa Sphinx Moth (Ceratomia catalpae). The hornworms only feed on catalpa trees and as I discovered in my youth, largemouth bass will readily feed on the hornworms. A catalpa tree in our family's farmyard fed hornworms and the hornworms fed my family many dinners of largemouth bass.
Sphinx moth (family Sphingidae) caterpillars 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 row of black spots 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 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.
I learned first-hand about the sometimes dramatic impact of the coevolution between native pests and native trees in the Boone County Arboretum (Union, KY). The Arboretum experienced an "outbreak" of hornworms in 2015, but the heaviest defoliation (100%) occurred on the hybrid Catalpa × erubescens 'Purpurea'. The hybrid mixes northern catalpa genes with those from the Chinese catalpa (C. ovata). It was planted closely in the Arboretum with our two North American species, northern and southern catalpas. However, our native catalpas had very few hornworms and minimal defoliation.
This was not a scientific study with replicated plots; however, one reasonable interpretation of the observation is that our native trees evolved some defenses against their hornworms. Perhaps the defenses were lost when non-native genes were introduced to produce 'Purpurea'.
The Caterpillar and Its Wasp
Although catalpa hornworm caterpillars can occasionally produce noticeable defoliation, they must run a gauntlet of beneficial insects including parasitoid and predatory wasps. The obvious white, oblong structures that occasionally sprout from the backs of the hornworms are NOT eggs! They are the cocoons of the endoparasitoid wasp, Cotesia congregata, which is also known as the "Hornworm Wasp."
The offspring of an endoparasitoid develops inside its 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's a bit unusual in the parasitoid world. Most parasitoids have a relatively narrow host range affected by the complicated coevolutionary dance between the parasitoid and its host. However, C. congregata targets several species of sphinx moths.
While C. congregata a well-known nemesis of catalpa hornworms, 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. Obviously, the parasitoid wasp takes full advantage of the effective tools provided through its coevolution with hornworms in general.
The Wasp and Its Virus
The female wasp uses her sharp ovipositor (ovi = egg; positor = lay) to insert eggs and venom into a hapless hornworm caterpillar. The eggs release special cells, called teratocytes, inside the caterpillar. The teratocytes release hormones that along with the venom suppress the caterpillar's development.
This is important because the wasp larvae would be doomed if the caterpillar pupated. It's also important that the wasp larvae don't kill their caterpillar food bag too soon. They are "programmed" not to eat any internal caterpillar structure that will kill the caterpillar.
Of course, once the immature wasps near pupation, all bets are off. They consume all internal structures and emerge from their caterpillar host to spin cocoons. Their emergence holes are often heralded by a droplet of straw-colored blood (hemolymph) oozing from the wound.
Zombie hornworm caterpillars festooned with the white, oblong, silken cocoons are usually much smaller than their non-parasitized siblings. This is the result of suppressed development caused by the combined effects of teratocyte hormones and wasp venom.
However, no wasp larva would survive if they succumbed to the caterpillar's immune response system. This is where a fascinating relationship between the wasp and a virus comes into play.
The wasp relies on a virus to suppress the caterpillar's immune system so its offspring and teratocytes are not rejected. But, where does the virus come from?
The virus does not carry genes to replicate itself. Those genes are carried in the wasp's genome. In fact, the virus does not exist until the proteins and genetic payload of the virus are coded from wasp DNA inside specialized cells in its ovaries, called calyx cells. The result is called a virion which is the complete, infectious form of a virus. The virus is created by the wasp then injected into the hornworm caterpillar.
A virus that is hidden within and created from the genome of a wasp?
"There's no mind-altering drug that equals nature's potency to blow your mind!" Anonymous