Participants in the Greater Cincinnati BYGLive! Diagnostic Walk-About held this past Monday in the Boone County Arboretum (Union, KY) viewed second-generation Catalpa Hornworms (Ceratomia catalpae) enjoying a last hurrah before pupating this season. Their discovery led to a discussion on host preference, parasitoids, and a virus spun out of a wasp's genome
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 bumblebees.
Catalpa hornworms are the larval form of the Catalpa Sphinx Moth. The hornworms only feed on catalpa trees. An intriguing native parasitoid wasp, Cotesia congregata (family Braconidae), feeds on the caterpillars. Thus begins a fascinating story.
The Catalpa 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 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 - tree host relationship has been studied for many years to learn how coevolution affects relationships between native trees and their native pests.
The sometimes dramatic impact of the lack of a coevolutionary connection between a native pest and a non-native plant host was clearly displayed in the Arboretum. Walk-About participants found caterpillars along with some minor feeding damage on a Golden Southern Catalpa (S. bignonioides, 'Aurea').
However, the amount of damage paled in comparison to the almost 100% defoliation suffered by a nearby hybrid Catalpa ×erubescens 'Purpurea'. The hybrid mixes northern catalpa genes with those from the Chinese catalpa (C. ovata). The stark contrast in defoliation between 'Aurea' and 'Purpurea' was also observed in 2015. One reasonable interpretation 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'.
Hornworm Caterpillars and the Wasp
Although catalpa hornworm caterpillars can occasionally produce noticeable defoliation, they must run a gauntlet of beneficial insects including parasitoid and predatory wasps. Plundering by the endoparasitoid wasp, C. 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 Mind-Blowing Relationship between 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 did the virus come from? This puzzled researches for years because the virus 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.
"There's no mind-altering drug that equals nature's potency to blow your mind!" Anonymous