At some point along the gardening learning curve, most backyard vegetable gardeners will confront a head-scratching tomato conundrum. Leaves are eaten and green fruit is nibbled high up on the plants. Tiny, black droppings litter the leaves and ground. A thorough search of the plants yields no culprits. Tiny flying rabbits?
Of course, the culprits are Tomato Hornworms (Manduca quinquemaculata) or Tobacco Hornworms (M. sexta). They are called “hornworms” because of a prominent horn-like projection rising from the top of their rear ends.
The host range of these sneaky caterpillars is a reminder that tomatoes and tobacco both belong to the nightshade family, Solanaceae. Indeed, the hornworms will also feed on other family members including eggplants, peppers, potatoes, and certain weeds.
Tomato and tobacco hornworms are the caterpillars of Sphinx Moths (a.k.a. Hawk Moths, family Sphingidae). The sneak attacks by the tomato-blitzing hornworms are initiated by the adult moths.
Sphinx moths are exceptional fliers. However, tomato and tobacco hornworm moths rest during the day and fly at night. So, unless we use a flashlight to watch tomato plants at night, we aren’t likely to see them laying their eggs. I don’t have my own pictures of the moths, but thanks to OSU’s agreement with Adobe, you can see what they look like.
Sphinx moths have variable coloration, but most employ some form of camouflage, so their daytime rest is not disturbed. As you can see with my picture of the Waved Sphinx (Ceratomia undulosa), the coloration allows them to blend with their background which may be a tree trunk, fence rail, etc. I walked past this moth several times while mowing my lawn without spotting it.
Of course, it’s the same with the caterpillars. I'm always amazed at how well large, “finger-sized” tobacco and tomato hornworm caterpillars can remain hidden from our probing eyes! Both types of hornworms can grow to a truly impressive size of 4" in length and 1/2 - 5/8" in diameter.
However, despite their size, these cleverly camouflaged caterpillars may go undiscovered for weeks owing to their coloration and white markings. Both hornworms have white diagonal lines along their sides.
The tobacco hornworms have a series of white diagonal lines and their “horns” are red to orangish-red. I most often find tobacco hornworms on tomatoes in southwest Ohio. I seldom find tomato hornworms.
The lines on tomato hornworms appear as a series of white sideways "V's" and their “horns” are dark colored. Also, they have more prominent spiracles compared to tobacco hornworms. These are breathing holes that are connected to branching tubes called trachea which function like lungs. Insects don’t breathe through their mouths.
Backyard vegetable gardeners need to be alert for the symptoms of feeding activity by these luminous green to bluish-green caterpillars which include missing leaves and stems, and hunks bitten out of developing fruit. They should also be on the lookout for small, black to blackish-brown, barrel-shaped caterpillar excrement (frass) deposited onto leaves and the ground beneath infested plants.
The caterpillars can be controlled through hand-picking and doing the "caterpillar stomp dance." Hornworm caterpillars are also subject to the depredations of several predators including paper wasps, yellow jackets, and baldfaced hornets. The wasps grab the hornworms, chew them up, and take the remains to their nests to feed their larvae. Tomato and tobacco hornworms are also targeted by a particular parasitoid wasp that can have a substantial impact on reducing caterpillar populations.
A Tiny Wasp, a Virus, and a Complex Story
The tiny parasitoid wasp, Cotesia congregata (Family Braconidae) is the nemesis of 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.”
Feeding by a predator is obvious because they consume their prey from the outside. Praying mantises, lady beetles, lions, and tigers are good examples. Parasitoids, specifically endoparasitoids, are more subtle because they kill from inside their prey.
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 hornworm victim. The resulting cascading of events is a fascinating 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. If the hornworm caterpillars pupated, the scrambling of the internal structure would be disastrous for the wasp larvae.
Another potential disaster is averted by the virus. It’s a specific type of virus called a bracovirus because these viruses are always associated with braconid parasitoid wasps. The bracovirus suppresses the caterpillar’s immune response. Otherwise, the eggs and teratocytes would be rejected by the caterpillar's immune system.
Here’s where the story becomes very interesting, maybe even mind-blowing. The bracovirus 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. I hope you were sitting down when you read that.
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. Throughout most of their development, the larvae only eat tissue that will not kill the caterpillar; if they ate everything, everyone would die!
However, all bets are off once the wasp larvae are near pupation. They eat everything. The wasp larvae then break out of their hapless hornworm victim and spin cocoons in which they will pupate.
The end of the wasp’s behind-the-scenes insecticidal work is heralded by white cocoons that look like cotton swabs sprouting from tobacco and tomato hornworms. It’s important to note that multiple wasps will not lay eggs in the same hornworm. This means all of the cocoons blossoming from an individual hornworm are the work of a single female wasp.
Even more significant, 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. They don’t exist.
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.
The bottom line is that gardeners should leave hornworms alone that are festooned with cocoons. Each cocoon foretells the same story replayed inside another hornworm. It’s a tragic story for the hornworms, but good for our tomatoes.
Selected References
Bézier, A., Louis, F., Jancek, S., Periquet, G., Thézé, J., Gyapay, G., Musset, K., Lesobre, J., Lenoble, P., Dupuy, C. and Gundersen-Rindal, D., 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), p.20130047.
Chevignon, G., Thézé, J., Cambier, S., Poulain, J., Da Silva, C., Bézier, A., Musset, K., Moreau, S.J., Drezen, J.M. and Huguet, E., 2014. Functional annotation of Cotesia congregata bracovirus: identification of viral genes expressed in parasitized host immune tissues. Journal of virology, 88(16), pp.8795-8812.