The Fascinating Tale of the Tomato Hornworm Life Cycle

The plump, green menace known as the tomato hornworm is a significant pest for anyone who grows tomatoes and other members of the nightshade family. (These include peppers, potatoes and eggplants.)

These pests have been uninvited guests in my tomato garden on more than one occasion. But as the old adage goes, “Know your enemy.” Understanding the tomato hornworm lifecycle helps you keep the population under control and away from your tomatoes.

I use lots of natural methods, from plucking them off by hand to using companion planting to deter them.

What Is a Tomato Hornworm?

The tomato hornworm is an intriguing insect, both a garden pest and a curious study of nature.

About the size of a thumb, these large caterpillars feature striking bright green bodies and white, V-shaped markings. At the end of their body, they sport a harmless yet distinctive horn-like tail, hence the name.

The tomato hornworm is the larval stage of the five-spotted hawkmoth. They’re primarily found across the northern United States, with a particular affinity for your beloved tomato plants. In the North, you’re more likely to find the tomato hornworm, and in the South, the tobacco hornworm. (They’re both usually labeled the tomato hornworm.)

They live as far south as Mexico and north into southern Canada. Across the world, you’ll find these caterpillars or their close relations in any temperate climate where nightshade family plants grow.

The Tomato Hornworm Life Cycle

Understanding the tomato hornworm’s fascinating life cycle lets us spot these creatures at different stages and prevent them from harming our gardens. Plus, if you’re not growing tomatoes or other nightshades, watching nature in this way is amazing.

The egg stage

In late spring, female moths lay small, spherical, greenish-yellow eggs on the underside of foliage. Although this stage doesn’t pose a direct threat to your plants, early detection can prevent a future infestation.

Look underneath leaves, often close to the point where the leaf joins the stem. Gently rub off the eggs with your fingers or a gentle spray of water from the hose.

The larval stage

Hatching after about a week, the larvae begin to feed on your plants. They go through five instars, or stages, where they shed their old skin to allow for new growth.

Their green color provides perfect camouflage amongst the foliage, since they’re almost the same shade of green as tomato leaves. Nature’s sneakiness at its finest. The most damage occurs at this stage, as the caterpillars eat voraciously to fuel their growth. You’ll likely notice the caterpillars, or at least the damage they’re doing to your tomato crop.

The pupal stage

After gorging themselves for two to four weeks on your poor tomato plants, the larvae burrow into the ground to pupate. They can reach up to six inches deep in sandy soils. They overwinter in their protective cocoons before emerging as moths to start the cycle anew.

The cocoon, or chrysalis, of the five-spotted hawkmoth is a dark red-brown color with a distinctive hook structure, called a maxillary loop, at one end. That’s actually the developing mouth parts of the adult moth.

During this stage, they pose no threat to your garden. Tilling the top few inches of soil exposes the cocoons to harsh weather and to birds and other predators who’ll happily gobble them up. They way, they never metamorphose into adults and lay their eggs on your young plants.

Pest control is one of the key reasons I till my vegetable beds in late fall and again in early spring.

The adult stage

Emerging from the pupae, the adult five-spotted hawkmoths are impressive creatures with a wingspan up to five inches. As the name suggests, they have five pairs of yellow spots running down their abdomens. The wings are a blurry gray-brown that offers good camouflage.

Feeding on nectar, they’re known for their rapid, hummingbird-like flight. While they don’t damage plants, their egg-laying sets the stage for the next generation of garden pests.

How to Get Rid of Tomato Hornworms

While tomato hornworms can be a significant challenge if you don’t take action, in my experience a combination of vigilance and organic methods can effectively manage them.

Manual removal: Because hornworms are large, you can easily pick them off your plants. Doing this regularly, especially in the early morning or late evening when they are most active, can greatly reduce their numbers. I drown them in a bucket of soapy water. I know it’s not nice, but it’s a safe, easy way to get rid of them.

Attract natural predators: Braconid wasps are natural predators of the tomato hornworm. Attract these allies to your garden by planting flowers that draw them, like dill, parsley and mustard. Attract insect-eating birds like sparrows, bluebirds, orioles and woodpeckers by creating a bird-friendly landscape, with cover, water and nesting sites. Bats eat hawkmoths, too.

Use Bacillus thuringiensis (Bt): Bacillus thuringiensis is a naturally occurring bacterium that’s harmless to pets, wildlife and humans but lethal to these caterpillars. Spray it on your plants at the first sign of an infestation, and it’ll kill the hornworms after they ingest it.

Plant basil: The strong scent of basil masks the smell of the tomato plants, disorienting and repelling hornworms. Plus, basil improves the flavor, health and vigor of tomato plants.