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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Tough to be a hornworm: Tomato and tobacco hornworms, Manduca quinquemaculata and M. sexta


While cryptic coloration may help small hornworms escape searching eyes of hungry vertebrate predators, parasitic wasps are not so easily fooled and the guardians of my garden soon made short work of this little rascal.

White tailed deer are the scourge of my vegetable garden. In an attempt to grow tasty tomatoes, I planted vegetables in a container garden this year. Plants nestled in a flower bed close to the front door of my home with a hope that diabolical deer lacked the audacity to venture so close to the house. While the deer remained at bay, over the past month Mother Nature demonstrated her dominion by sending hordes of hungry hornworms to feast on my pitiful tomatoes.

Leaves of my tomato were the last meal for this hornworm and parasitic wasps had the last laugh.

Hornworms are one of the most prodigious eating machines on the planet. During the course of development the caterpillar will increase more than one thousand times in weight from the time the tiny larva hatches from the egg until the four inch long caterpillar transforms into a pupa. This remarkable metamorphosis is equivalent to an eight pound baby growing into an eight thousand pound man. No wonder my tomato plants have been disappearing before my very eyes!

The gorgeous hawk moth pollinates plants at night and lays eggs on tomatoes that hatch into naughty hornworms.

How did the hornworm arrive on the tomato in the first place? It was deposited as a tiny egg on the undersurface of a tomato leaf by a beautiful nocturnal hawk moth, kin to the day-flying hummingbird moths we met in a previous episode. Adult hawk moths are important pollinators servicing several native and introduced plants, especially ones with tubular flowers such as honeysuckle, trumpet vines, phlox, petunia, and tobacco. From these plants, hawk moths obtain energy rich nectar.

Once a female moth discovers suitable food for her young, in this case my wretched tomatoes, she lays eggs on the leaves. The eggs hatch in few days. The hatchling has five or six larval stages and its development takes about three weeks. The pale green color of the caterpillar enables it to blend with the foliage of the plant and probably helps prevent detection by birds and other hungry predators. Often the larval droppings, a.k.a. frass, or leaf damage are the first clues of a hornworm attack. After completing larval development, the hornworm moves to the soil to pupate. Early in the season moths emerge from pupae in the soil and the life cycle will repeat. But as autumn grows near, pupae suspend development and prepare to spend the winter hidden in soil or leaf litter.

In addition to leaves, tomato fruit are on the hornworm’s menu as well.

When the warmth of spring arrives and the presence of food plants is assured, pupae complete development and moths emerge to feed, mate, and lay eggs. In addition to tomatoes, hornworms feed on other members of the nightshade family including tobacco and occasionally eggplant, pepper, and potato.

After popping the lids of their pupal cases, these wasps are raring to hunt and parasitize caterpillars.

Ah, but in the Yin and Yang of the natural world, good things didn’t last forever for the voracious hornworms in my garden. Two weeks ago, cocoons of parasitic Cotesia wasps like the ones we met last week attacking saddleback caterpillars appeared on the back of a moribund hornworm. After being deposited as eggs into the flesh of the hornworm by the mother wasp, mature wasp larvae emerged and formed dozens of snowy white cocoons on the hornworm’s back. I captured one of the hapless caterpillars and placed it in a Petri plate on the kitchen counter. Within a few days, scores of new wasps emerged from the cocoons.

After a brief repast of organic honey in the kitchen, wasps gained their freedom in my garden to seek mates and to hunt other nocent caterpillars. These tiny assassins have done a splendid job. While several hornworms appeared on my tomatoes in the past two weeks, nary a one survived the wrath of the parasites. Large and small, all hornworms are now festooned with Cotesia cocoons. Having bested deer and hornworms, perhaps I will enjoy abundant tomatoes before the killing frost.


Whoa, that’s a lot of wasps to emerge from a single hornworm! Note the exit holes made by emerging parasite larvae near the posterior horn of the hornworm.


Bug of the week thanks Dr. Shrewsbury for discovering parasitized hornworms on the tomatoes. To learn more about hornworms and their parasites, please visit the following web site: