Tobacco Hornworms (Manduca sexta) and Tomato Hornworms (M. quinquemaculata) are beautiful native moths belonging to the Sphynx (Hawk) Moth family, Sphingidae. We don’t often see sphynx moths because most are night flyers. Their undulating markings and color patterns allow them to blend with tree bark, where they commonly rest during the day.

Tobacco Hornworm Moth

Tomato Hornworm Moth

Of course, their caterpillars may be less appealing. They are called “hornworms” because of their worm-like appearance and the horn-like protrusion on their posteriors. Hornworms consume plant foliage, and some are serious pests.

Tomato Hornworm with Breathing Spiracles Closed

Tomato Hornworm with Breathing Spiracles Open

Tobacco Hornworm

Although more tomatoes are grown in Ohio compared to tobacco, it’s the tobacco hornworm that is most commonly found on tomatoes. The hornworms are notorious for defying detection while chomping away on the leaves and fruits of tomatoes, as well as other members of the nightshade family, Solanaceae. It’s amazing how much damage can occur before the caterpillars are spotted and dispatched, perhaps using the highly effective (and satisfying) “caterpillar two-step.”

Tobacco Hornworm Feeding on Tomato

Hornworm Damage

Tobacco Hornworm on Eggplant

We’ve long recommended that gardeners keep an eye out for pellet-like excrement, called frass, collecting on upper leaf surfaces. Finding frass is a sure sign that hornworms are afoot. Of course, uttering “Oh, frass!” after finding the droppings is more socially acceptable compared to the other excrement expletive.

Tobacco Hornworm Damage and Frass on Eggplant

Tobacco Hornworm Frass

Carrie Jagger (OSU Extension, Morrow County) offered another more effective option for detecting these clandestine pillagers. In her BYGL Alert cleverly titled, “Glow Worms” (June 24, 2025), she illustrated that gardeners could spot hornworms at night using a black light flashlight.

It’s almost inevitable that at some point along the gardening learning curve, backyard vegetable gardeners will also observe hornworms festooned with small, white cocoons (not eggs!). And there lies a wasp’s tale worthy of a science fiction plot.

Tobacco Hornworm with Cocoons (Cotesia congregate)

The cocoons are the handiwork of the tiny parasitoid wasp, Cotesia congregata (Family Braconidae). The wasp is the nemesis of hornworms attacking tobacco and tomato hornworms as well as a few other sphingid caterpillars, including the catalpa hornworm (Ceratomia catalpae). In fact, the wasp is sometimes called the “hornworm wasp.”

Catalpa Hornworm with and without Wasp Cocoons (Cotesia congregate)

On a side note, the term “parasitoid” was coined to describe a lifestyle that’s somewhere between a parasite and a predator. Feeding by a predator is obvious because they consume their prey from the outside. Parasites are less impactful, commonly feeding on but not killing their hosts. Parasitoids kill their hosts, but are more subtle, particularly endoparasitoids, because they kill their hosts from the inside. Visualize the so-called Chestbuster that makes dramatic appearances throughout the “Alien” movie franchise.

The internal deconstruction of tobacco or tomato hornworms begins when a C. congregata female uses her ovipositor (ovi = egg) to insert eggs, teratocytes, venom, and a virus into a 2nd or 3rd instar hornworm victim. The resulting cascading of events is a fascinating story.

The Inside Story

The teratocytes are specialized cells associated with each egg and are released into the caterpillar when the eggs hatch. The teratocytes, in turn, release hormones that, along with the venom, suppress the caterpillar's development. Pupation would be disastrous for the wasp larvae by scrambling the internal structure of the hornworm.

Arrested Development

Another potential disaster is averted by the virus. It’s a specific type of virus called the Cotesia congregata Bracovirus (CCBV). Bracoviruses are always associated with braconid parasitoid wasps.

Here’s where the story becomes very interesting. The CCBV does not exist outside of the wasp. In fact, it doesn’t exist inside the wasp except in its ovaries. However, the wasp doesn’t come pre-equipped with the bracovirus; it’s not passed down from mother to daughter. Instead, the bracovirus is spun out of the wasp’s own DNA.

The CCBV suppresses the caterpillar’s immune response. Otherwise, the eggs and teratocytes would be rejected by the caterpillar's immune system.

I hope you are sitting down when you read this because here’s where the story becomes mind-blowing. The CCBV suppresses the caterpillar's immune response by integrating into the DNA in somatic immune cells. In other words, it turns the “attack switch” off from inside cells that would normally swarm the wasp larvae with bad intentions.

This relationship between the wasp and the bracovirus is mutually symbiotic, meaning that it’s mutually beneficial for both organisms. Without the wasp, the bracovirus can't exist. Without the bracovirus, the wasp larvae would be killed by the hornworm’s immune response to a foreign object.

Once the table is set by the venom, teratocytes, and bracovirus, the C. congregata larvae begin to feed. They chow down on the caterpillar’s blood (hemolymph) and fat (fat body). The caterpillars continue to feed, providing nutrients to the wasp larvae. If the wasp larvae ate everything, it would be like pulling the plug on a ship halfway across an ocean à laTitanic; it would be bad for everyone.

Of course, the caterpillars are eventually sunk by the collective damage caused by the wasp larvae. However, before that happens, the wasp larvae board lifeboats by breaking out of their hapless hornworm victim and spinning cocoons in which they pupate. You can see this in the following images with C. congregata larvae emerging from catalpa hornworms.

Wasp Larvae Eiting Catalpa Hornworm (Cotesia congregate)

Wasp Larvae Spinning Cocoons on Catalpa Hornworm (Cotesia congregate)

Substances secreted by the wasp larvae numb the wounds. This isn’t out of consideration for the hornworm. It’s to suppress the normal caterpillar behavior of striking at irritants, which could damage the immature wasps.

The end of the wasp’s behind-the-scenes insecticidal work is heralded by a mass of white cocoons looking like cotton swabs sprouting from the hornworms. Multiple wasps do not lay eggs in the same hornworm; thus, all of the cocoons blossoming from an individual hornworm are the work of a single female wasp.

Tobacco Hornworm with Cocoons (Cotesia congregate)

Also, each cocoon will give rise to a new female wasp. How do we know they’re all going to be females? Because there are no males. C. congregata is parthenogenic, meaning the females can produce fertile eggs without input from males. It saves time. The females don’t need to waste time going on dates, feigning interest in braggadocious tiny-wasp stories, etc., etc. The females emerge from their cocoons ready to wreak havoc on other hornworm caterpillars.

Opened Cocoons (Cotesia Congregata)

The bottom line is that gardeners should leave hornworms festooned with cocoons alone. Each cocoon foretells the same story involving the same characters that will be replayed inside another hornworm. It’s a tragic story for the hornworms, but good for our tomatoes.

Thankfully, we don’t have such a horrific relationship with a parasitoid. Unless FX’s Alien Earth isn’t science fiction ...

Selected References

Adamo, S. A., Kovalko, I., Turnbull, K. F., Easy, R. H., & Miles, C. I. (2016). The parasitic wasp Cotesia congregata uses multiple mechanisms to control host (Manduca sexta) behaviour. Journal of Experimental Biology, 219(23), 3750-3758.

https://doi.org/10.1242/jeb.145300

Adamo, S. A. (2019). Turning your victim into a collaborator: exploitation of insect behavioral control systems by parasitic manipulators. Current opinion in insect science, Science Direct, 33, 25-29.

https://doi.org/10.1016/j.cois.2019.01.004

Beckage, N. E., Tan, F. F., Schleifer, K. W., Lane, R. D., & Cherubin, L. L. (1994). Characterization and biological effects of Cotesia congregata polydnavirus on host larvae of the tobacco hornworm, Manduca sexta. Archives of Insect Biochemistry and Physiology, 26(2‐3), 165-195.

https://doi.org/10.1002/arch.940260209

Bézier, A., Louis, F., Jancek, S., Periquet, G., Thézé, J., Gyapay, G., ... & Drezen, J. M. (2013). Functional endogenous viral elements in the genome of the parasitoid wasp Cotesia congregata: insights into the evolutionary dynamics of bracoviruses. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1626), 20130047.

https://doi.org/10.1098/rstb.2013.0047

Chevignon, G., Thézé, J., Cambier, S., Poulain, J., Da Silva, C., Bézier, A., ... & Huguet, E. (2014). Functional annotation of Cotesia congregata bracovirus: identification of viral genes expressed in parasitized host immune tissues. Journal of virology, 88(16), 8795-8812.

https://doi.org/10.1128/jvi.00209-14

Chevignon, G., Periquet, G., Gyapay, G., Vega-Czarny, N., Musset, K., Drezen, J. M., & Huguet, E. (2018). Cotesia congregata Bracovirus circles encoding PTP and Ankyrin genes integrate into the DNA of parasitized Manduca sexta hemocytes. Journal of virology, 92(15), 10-1128.

https://doi.org/10.1128/jvi.00438-18

Gastanaga, D. B. (2025). Fatal attraction: Parasitized Manduca sexta caterpillars halt feeding despite a functional motor system. Ottawa-Carleton Journal of Neuroethology, 1(4).

https://ojs.scholarsportal.info/ottawa-school/index.php/OCJN/article/view/517

Lem, M., Rh, H., Dg, B., Barkhouse, A., Miller, D. W., Raun, N., & Sa, A. (2024). The caterpillar Manduca sexta brain shows changes in gene expression and protein abundance correlating with parasitic manipulation of behaviour. Scientific Reports, 14(1), 31773.

https://doi.org/10.1038/s41598-024-82506-4

Ron Wilson