Northern Catalpa (Catalpa speciosa, family Bignoniaceae) trees are in full-bloom in southern Ohio. Last year, I posted a BYGL Alert! extolling the virtues of catalpa trees; both northern and its southern cousin (C. bignonioides). Of course, I recognized a few minor shortcomings, but no tree is perfect. I noted that 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!
My love affair with catalpa trees is rooted in a huge northern catalpa that grew in our farm yard in West Virginia; it shaded many family reunions. It also supported many tree-climbing expeditions and was a great teacher; it taught me the true meaning of gravity.
Lessons Learned From Catalpas
Catalpas attract many insects including a bevy of pollinators, particularly bumble bees. Indeed, catalpas support so many animals; some say catalpa trees have their own micro-ecosystems.
Caterpillars on the Prowl: Family Matters
One of the earliest lessons I learned was that catalpas occasionally play host to hungry hordes of their very own caterpillar; catalpa hornworms are the larval form of the catalpa sphinx moth (Ceratomia catalpae). The caterpillars only feed on catalpa trees. I also discovered that largemouth bass will feed on the hornworms; the caterpillars make wonderful, almost sure-fired bass bait! The catalpa tree fed the hornworms and the caterpillars 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 over-lapping 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.
The impact of the coevolution between native pests and native trees was demonstrated by observations in 2015 in the Boone County Arboretum (Union, KY). The Arboretum experienced an "outbreak" of hornworms, 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 may have evolved some defenses against their hornworms. Perhaps the defenses were lost when non-native genes were introduced to produce 'Purpurea'.
What Lies Beneath
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 an endoparasitoid wasp, Cotesia congregata (family Braconidae).
What is an endoparasitoid? Predators are defined as an organism that eats another organism (its prey). The big cats (lions, tigers, etc.) are good models of a predator. Parasites are generally described an organism that lives in or on another organism (its host) and benefits by deriving nutrients at the host's expense. However, the definition of a successful parasite often includes the caveat that the parasite does not kill its host. Mosquitoes are a good example. They may carry a disease causing pathogen that can kill their host, but the mosquitoes are just the carriers (vectors) of the pathogen, not the actual killers. What about a parasite that kills its host? That's a parasitoid. If it lives inside its host, it's an endoparasitoid; like a predator living inside the host. An ectoparasitoid does the same while attached to the outside.
C. congregata is a perfect model of an endoparasitoid. This is a "gregarious endoparasitoid" meaning that multiple wasps develop inside a single caterpillar. The female wasp uses her sharp ovipositor (ovi = egg; positor = lay) to insert her eggs into a hapless caterpillar. Once the eggs hatch, the wasp larvae feed on structures that will not kill the caterpillar; if they ate everything, everyone would die! However, once they near pupation, they consume all internal structures and emerge from their caterpillar host to spin cocoons. Zombie hornworm caterpillars festooned with the white, oblong, silken cocoons of this parasitoid wasp are a common site in Ohio.
C. congregata is a bit unusual in the parasitoid world. Most parasitoids have a relatively narrow host range; a product of the complicated "dance" that coevolved between the parasitoid and its host. However, C. congregata targets several species of sphinx moths. While it's a well-known nemesis of catalpa hornworms, you will also see the 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.
Digging Deeper (Optional Late-Night Reading)
The entire endoparasitic process practiced by C. congregata is a bit more complicated than presented thus far, and full of surprises. First, the wasp actually takes control of the caterpillar's development so the host does not complete its development before the wasp larvae are finished with their host. The wasp injects venom into the caterpillar when she inserts her eggs, then the wasp's eggs release special cells, called teratocytes, inside the caterpillar. The teratocytes release hormones that along with the venom suppress the caterpillar's development.
Second, the wasp relies on a mutually symbiotic relationship (a relationship that's mutually beneficial to two different species) with a virus to suppress the caterpillar's immune system so the caterpillar doesn't reject the wasp larvae and teratocytes. The virus suppresses the caterpillar's immune system, but what does it get in return?
The virus does not carry genes to replicate itself; the viral genes are carried in the wasp's genome! So, female wasps actually produce the virus which is injected into the caterpillar along with the wasp's eggs. Without the wasp, the virus can't exist. Without the virus, most of the wasp larvae would die (mutually symbiotic).
A virus hidden within the genome of a wasp?
It's like yesterday
I didn't even know your name
You're always on my mind
I never could have predicted that I'd feel this way
You are a beautiful surprise
- India Arie; Beautiful Surprise