Worms with Horns

One of the most common questions we encounter in the Butterfly House is "What are those green things with horns on my tomato's?"  This tobacco hornworm above had just finished decimating a pepper plant which in other garden's would merit a death sentence.  Instead it ended up with a chance to pupate and become the attractive moth it aspired to be.

There are two common hornworms which thrive in a garden setting.  The tobacco hornworms,  Manduca sexta, are found predominately in the south along the Gulf Coast states.  Tomato hornworms,   Manduca quinquemaculata  are a more northern species.  In the midwest there is considerable overlap, sometimes even on the same plant.  They both feast on solanaceous plants, predominately tomato plants and their close relative, the tobacco plant.  They occasionally will lower their standards and eat eggplant, pepper, and potato plants.

For those of us obsessive enough to identify the species of every tick crawling up our pant leg, identification of the separate species is easy.  Both have a "horn" on the top hind tip of the abdomen pointing backward.  It is red on the tobacco hornworm, black on the tomato variety.  More distinctive, the tobacco hornworm has seven white diagonal lines pointing up and back, while the tomato hornworm has "Vs", formed by a horizontal line at the base of each diagonal line.  I think of this as pointing toward the mouth as if to indicate that "I don't use tobacco!".

Tobacco hornworm- James Castner

Tomato hornworm- James Castner

Larvae hatching from eggs- Peter J. Bryant

The growth of these caterpillars is amazing.  Think of a tiny egg, not much larger that the period of this sentence.  A caterpillar, the first instar, emerges, chewing at its egg and then growing out of its skin through 4-5 more molts over 20 days, each time with emerging with a larger skin to accomidate its future size.  When the time is right the final instar will burrow into the soil and form its pupa.  There are usually two generations in Missouri, the second entering diapause, a dormant period as a pupa to overwinter, avoiding the frost.

The moths look quite similar at first glance.  The tobacco hornworm moth, M. sexta, (sexta=six) emerges as the Carolina Sphinx moth, seen on the left.  Note that it has six spots on each side of its abdomen, the top one white.  The tomato hornworm M. quinquemaculatae (quint=5) ecloses as the five-spotted hawk moth, named for the five yellow spots on each side of its abdomen.

Tobacco- -Carolina Sphinx moth

Tomato- Five-spotted hawkmoth

If you grow tomatoes and peppers but like beautiful  hawkmoths like this one, here is a suggestion for when you find a horn worm.  Plant a tomato plant or two off to the side, possibly in a pot.  When you find a hornworm, transport them to your sacrifical plant and watch them grow.   For fun, feed them a tomato slice and watch their frass (poop to those under 12) turn red!

When they change color as above and reach full size, put them in a container with loose dirt, potting soil, spagnum moss or even some shreaded damp newspaper  or paper towels.  Throw in some tomato plant leaves every 1-2 days until they pupate.  Their time to eclose is variable.

Tomato hornworm moth  Peter J. Bryant

-  The University of Florida Entomology Department, John Capinera and Peter J. Bryant graciously permitted the use of their pictures to show side-by-side comparisons of these two species.  Their site has more detailed information.
-  There are good pictures of the larval instars at nathistoc.bio.uci.edu.