Tomato hornworm: Friend or foe?

I don’t often write about highly controversial subjects. For one thing, it’s hard to alter someone’s mindset and secondly, I don’t appreciate hate mail. However, having just written about pollinator gardens, I felt the need do a bit more than just touch on a particular species I mentioned.

I’m guessing that most of you have observed those adorable little clearwing moths moving from flower to flower, resembling miniature hummingbirds. Everyone I know seems to “ooooh” and “ahhhhh” over the cute little insects as they move from blossom to blossom, their long tongues reaching down into the structures’ recesses to sip up the sweet nectar, all the while performing the important act of pollination.

Not the least bit shy, hummingbird clearwing moths fly by day and are members of the family Sphingidae, a group known as sphinx moths or hawkmoths due to their straight, narrow wings and streamlined abdomens.

Unlike the giant silkmoths that arrive into adulthood with no mouthparts and only live around seven days, long enough to mate and lay eggs, the sphinx moths have a longer lifespan. With a super long tongue, or proboscis, the moths are able to sip nectar from long tubular blossoms which are unreachable by other insects.

As they poke their heads into each flower, they collect and deposit pollen, acting as important pollinators for many plant species, some surviving up to 30 days. Sphinx moth caterpillars are often referred to generally as “hornworms” due the characteristic stiff horn on the last segment of their bodies.

There are nearly 1,500 species of sphinx moths worldwide and nearly all of them play an important role in pollination.

Tomato and tobacco

Nevertheless, there are a couple of species that are loathed, rather than loved, simply because the host plants to which they were assigned by Mother Nature just happen to be plants that we don’t want to share with them.

The Carolina sphinx, Manduca sexta (tobacco hornworm) and five-spotted sphinx, Manduca quinquemaculata (tomato hornworm) are big, beautiful moths that happen to lay their eggs on members of the Solanaceae, or nightshade family. Unfortunately, that often includes tomato and tobacco plants.

As the moths feed and pollinate flowers during the crepuscular hours, they stop to deposit a single egg, here and there upon their host plant’s leaves as they encounter them. The tiny larvae hatch in about a week and remain deep within the recesses of the plant, for safety, doing little if any damage to the leaves as they mature and grow through five instars of shedding their skins.

As they mature into the final and largest instar of their growth, signs of their huge appetites tattle on their presence. On tomato plants, stems devoid of leaves and large droppings known as frass (which act as fertilizer for the soil) can be seen quite easily. It’s actually quite startling to suddenly realize that this caterpillar has been on your plant for weeks, slowly eating and growing until it can no longer hide.

When the caterpillar can simply eat no more, it begins its metamorphosis. Dropping off the plant, it thuds onto the ground, where it begins to crawl. The insect searches until it finds soil that is just the right consistency, then burrows in.

Within this subterranean environment, it forms a cozy little chamber around itself and begins its transformation. Within a few days, its sheds its skin one more time, revealing an astonishing, reddish-brown pupa.

Unlike other moth pupae, the sphinx moth pupa has a loop at one end, resembling a handle. This is the future moth’s tongue. After 2-3 weeks beneath the ground, the pupa undergoes major changes until finally, it cracks open, and the adult moth works its way up through the soil.

Resting on the first substantial stem it encounters, it sits quietly pumping fluid into its powerful wings. All day it rests here, drying, and as evening approaches, it takes to the air to begin its pollination duties. Here in the northeast, it will mate and a second generation of sphinx moths will be produced. When those caterpillars dig down into the soil to pupate, they will remain there throughout the winter, to emerge the following spring.

Being a lover of Lepitdoptera, you can imagine how I cringe when I hear people proudly telling stories of finding hornworms on their tomato plants. Ripping them off and stepping on them, feeding them to their chickens, lighting them on fire, etc… It is incredibly disturbing to me.

These caterpillars are so close to becoming adult moths at this stage. They have worked so hard to stay alive. And now, during their final hour, they are viciously eliminated.

A closer look at the hornworm and sphinx moth

1 of 7

Compromise

Known as the “moth lady” in my area, I often have people saving these caterpillars for me when they find them, so that I can finish out their metamorphosis safely. Yet the best way to co-exist with an insect that will always persist, is simply to move the caterpillar off your plant and onto a new food source. Nightshade that often grows on the outskirts of gardens is also a favorite food host, as is nicotiana, a flowering tobacco.

Yet, the easiest thing to do is to simply plant a couple extra tomato plants. These can serve as a site on which to relocate the caterpillars when you discover them, or simply a source for added produce that may be lost to the caterpillar’s appetite.

Tomato hornworms have plenty of their own natural predators. Songbirds love them and can be seen grappling with the larvae as they gulp them down. Box turtles seek them out when they are on the ground searching for a place to burrow. Braconid wasps lay eggs in the young caterpillars.

These eggs hatch and the wasp larvae grow inside, breaking out through the caterpillar’s skin and forming rows of white cocoons from which the adult wasps will hatch, dooming it to a slow and painful death.

I don’t expect you to fall in love with the tomato hornworm if you have an aversion to it, I only ask that you try to understand it. Perhaps someday you might find it in your heart to plant an extra tomato plant or two to accommodate its innocent, yet ferocious appetite. Also, I don’t expect any hate mail!