The massive, bright green caterpillar that can strip a tomato plant bare overnight is one of the most destructive garden pests, known widely as the tomato hornworm. This creature, with its intimidating yet harmless posterior “horn,” represents the larval stage of one of the most fascinating insects in North America. This voracious caterpillar eventually completes a biological transformation, emerging from the soil as a large, fast-flying insect commonly called a “hummingbird moth.” Both the garden pest and the elusive pollinator are two distinct phases of the same animal, belonging to the Sphinx moth family, Sphingidae.
Confirming the Transformation
The adult moth that develops from the tomato hornworm is a species of sphinx or hawk moth. Specifically, the tomato hornworm is the larva of the Five-spotted Hawk Moth (Manduca quinquemaculata). Its close relative, the tobacco hornworm, transforms into the Carolina Sphinx Moth (Manduca sexta). Though they possess slightly different markings, both species cause similar damage and are grouped under the common “hummingbird moth” name.
These large adult moths, with wingspans reaching up to five inches, are generally mottled gray-brown with distinctive yellow or orange spots along their abdomen. The adult phase is harmless to plants and serves as an important pollinator. This contrast between the destructive, leaf-eating caterpillar and the beneficial, nectar-sipping moth highlights the significant life cycle shift in this insect.
Stages of the Hornworm Life Cycle
The journey from garden pest to pollinator is one of complete metamorphosis, involving four distinct stages: egg, larva, pupa, and adult. The cycle begins when the female moth lays tiny, pearly-green eggs, often deposited singly on the underside of host plant leaves. These eggs typically hatch within about a week, initiating the destructive larval phase.
The larval stage, the hornworm, lasts approximately three to four weeks, during which the caterpillar progresses through five growth stages, known as instars. During this period, the larva consumes foliage from plants in the nightshade family, such as tomatoes, peppers, and eggplant, quickly growing to several inches in length. Once fully mature, the larva drops from the plant and burrows into the soil to prepare for pupation.
The pupal stage is the resting period where the larval body is reorganized into the adult moth form. The pupa is a reddish-brown, hard casing, often featuring a distinctive loop-like structure that encases the developing proboscis. First-generation pupae may emerge as adult moths within a few weeks. However, later generations remain in the soil to overwinter, emerging the following spring to restart the cycle.
Why the Adult Moth Mimics a Hummingbird
The common name “hummingbird moth” is earned because the adult insect displays behaviors and physical adaptations that closely resemble the tiny bird. This similarity is known as convergent evolution, where two unrelated organisms evolve similar traits. The moths are diurnal or crepuscular, meaning they are active during the day or at twilight, unlike many other large moths.
Their flight pattern is characterized by rapid wing beats, which generate an audible humming sound and allow the moth to hover in front of a flower. This ability to remain stationary is coupled with a long, coiled feeding tube called a proboscis, which the moth extends deep into tubular flowers to access nectar. The combination of hovering flight and rapid feeding leads to their popular, avian-inspired name.
Controlling Hornworms in the Garden
Because the larval stage can defoliate entire plants in a short period, management is a priority for many gardeners.
Manual Removal
The most direct method of control is manual removal, which involves inspecting plants daily for the large, camouflaged caterpillars and simply picking them off. Once removed, the hornworms should be dropped into a container of soapy water.
Biological Control
Gardeners can utilize natural biological controls, such as encouraging the presence of Braconid wasps. These parasitic wasps lay their eggs beneath the hornworm’s skin. The resulting wasp larvae feed internally before emerging to spin small, white, rice-like cocoons on the caterpillar’s back. If a hornworm is spotted with these cocoons, it should be left alone, as the emerging wasps will help control future generations.
Cultural Control
Tilling the soil in the fall or early spring can destroy up to 90% of the overwintering pupae, significantly reducing the moth population for the following growing season.