What's Eating the Tomatoes? A Tobacco Hornworm Food Web

A large tobacco hornworm (Manduca sexta) munching on your tomato plant might look a little

menacing. But look closer, and the hornworm reveals something far more interesting: it's an integral part of a living food web, in which even the plants we grow for food provide habitat and resources for a wide range of species.

The tobacco hornworm has two particularly important ecological paths. If it escapes parasitism and completes its development, it becomes a Carolina sphinx moth—a large, nocturnal moth that feeds on flower nectar and contributes to pollination after dark. If it is parasitized, it instead becomes host to Cotesia congregata, a native braconid wasp that provides natural biological control of hornworm populations.

From Tomato Leaf to Sphinx Moth

Unparasitized hornworms eventually leave the garden to pupate, emerging later as Carolina sphinx moths—impressive nighttime pollinators especially drawn to flowers that release scent and nectar after sunset.

Our Children’s Garden and broader landscape on the Cuvilly campus offers excellent habitat for

them. These moths favor deep-throated, night-blooming flowers. Evening primrose (Oenothera

biennis), which opens at dusk, is especially well suited to their needs. Other long-tubed

flowers—including buttonbush (Cephalanthus occidentalis), beardtongue (Penstemon digitalis),

and bee balm (Monarda fistulosa)—also provide valuable nectar for sphinx moths.

Why Are We Seeing More Tobacco Hornworms?

The tobacco hornworm—caterpillar of the Carolina sphinx moth—is closely related to the tomato hornworm (Manduca quinquemaculata), caterpillar of the five-spotted sphinx moth. Both feed on nightshade plants such as tomatoes. While tobacco hornworms have traditionally been more common in warmer parts of the country, they're becoming increasingly noticeable farther north, including in northern New England.

If you feel like you're seeing more of them in recent years, you may be right. Changing temperatures and seasonal patterns can influence when these moths emerge, how long they stay active, and how many generations they produce each year.

Why Are They Moving North?

• Warmer winters: Milder winters may allow more pupae to survive underground in areas that were once too cold.

• Longer growing seasons: Warmer temperatures can speed up the moth's life cycle, potentially allowing additional generations farther north.

• Shifting habitat: As northern climates warm, conditions may become more suitable for insects once limited to warmer regions.

These changes are part of a broader ecological pattern sometimes called "latitudinal range shift" in which warm-adapted species expand into temperate areas as temperatures rise.

Our gardens offer a small but fascinating glimpse of this larger shift. As temperatures and seasons change, the insects living in—and visiting—our ecoregions change with them.

When the Hornworm Becomes a Nursery

Some of the hornworms we've observed carry the unmistakable white cocoons of Cotesia

congregata. These tiny native wasps use hornworms as hosts: their larvae develop inside the

caterpillar before emerging to spin clusters of white cocoons on its body.

Once parasitized, the hornworm's feeding declines sharply, reducing the damage it does to tomato plants. Meanwhile, the developing wasps complete their life cycle and emerge to continue the next generation of this natural biological control.

Adult Cotesia wasps feed on nectar, and the Children's Garden already offers several good

sources, including dill, yarrow, and parsley. Sweet alyssum (Lobularia maritima) could be a

valuable addition. Though not yet established on the property, it produces abundant, shallow

flowers over a long season and could extend forage for small parasitoid wasps and other beneficial insects from spring through frost.

Challenges and Choices

These relationships are a reminder that a vegetable garden is more than a place where we grow food—it's also habitat.

A tomato plant feeds a hornworm. That hornworm may become a sphinx moth and later visit

flowers as a nocturnal pollinator, or it may become a nursery for a parasitoid wasp that helps keep future hornworm populations in check. The gardener is part of this system too, making decisions about which organisms to encourage, relocate, or leave alone.

For that reason, not all hornworms should be treated the same way. Hornworms carrying visible

white Cotesia cocoons should generally be left undisturbed so the developing wasps can complete their life cycle—by the time they emerge, the caterpillar will already have greatly reduced its feeding.

In our gardens, we've taken this a step further: extra tomato plants are grown away from the main garden beds so hornworms can be relocated there and allowed to complete their development into moths. This lets us protect our primary crops while still supporting the larger food web.

What looks, at first, like a battle between a gardener and a caterpillar turns out to be something far more nuanced and interesting: a network of plants, insects, pollinators, predators, parasitoids, and people, all participating in the life of our gardens and habitats.