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Nature's Superheroes

Have you ever seen a hornworm?

By Missy Weiss, Environmental Educator

One Tuesday in July, during Group for the East End’s Summer Field Ecology Program, I felt a slight tug on my shirt.  A young girl was eager to show me the critter that she had caught and quickly threw a blue and yellow bug box in my face.  “See,” she said.  “My Dad found it in his garden last night! What is it?”  My eyes focused and a lime green caterpillar came into view.  My mind quickly flipped through the rolodex of animals in my brain.  I had seen this animal before, but when, where, why, and what were the circumstances?  Her eagerness for answers left me a bit nervous, but I blurted out “Tomato Hornworm!”  A sense of relief came over me but the girl, a skeptic even at age seven, stared at me with question in her eyes.

I led her to the reference book and a lesson on how to use a field guide began.  We discussed body type, color, and distinguishing features, and finally we both agreed that the larva living in this bug box was a hornworm, but not a Tomato Hornworm.  It turned out that the caterpillar was strangely similar to the Tobacco Hornworm and one of the only differences between the two was the color of the rear horn.  Tobacco Hornworms have a red horn while Tomato Hornworms have a green and black horn.

A couple of weeks later, I saw this same caterpillar resting on my father’s tomato plant but this time, it looked a bit different.  It appeared as if white Tic-Tacs were glued to its back and sides.  This was where my fascination with the hornworm began.  Tobacco Hornworms are the larval form of the Carolina Sphinx Moth and are common throughout the United States.  Caterpillars can reach up to 6 inches long and often feast on tomatoes, potatoes, and other related vegetation.  Their voracious appetites can often cause gardeners and farmers great agitation.

All parties, however, should rest assured because Mother Nature’s design has eliminated the need for insecticides and pesticides.  She has a natural biological control for the pesky hornworm.  Cotesia congregatus is one of more than 1,000 wasps in the Braconid family and is black with yellowish legs and clear wings.  Although this parasitic wasp is harmless to humans, it is a silent assassin of hornworms.  Female wasps stealthily oviposit or insert their eggs just under the skin of the helpless caterpillars.  As the eggs hatch, the larvae begin to feed on the innards of the hornworm and literally emerge from the inside out.  Much like a scene from a horror movie, the plot only seems to thicken from here.  Once the larvae emerge, their attack continues and they begin to weave white, oval cocoons along the back of the victim.  A short time later, the adult wasps hatch in search of a new tomato plant to protect.  The dying hornworm is nothing more than a hollow shell and will soon perish.

Like every good superhero film, the hunky hero defeats the villain and this dramatic story is not any different.  The only real difference is that the wasp, C. congregates, is not really all that hunky, measuring less than 1/8 inch!  I guess good things really do come in small packages!


Information derived from Peterson First Guides: Caterpillars
Galveston County Master Gardener Class

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