The tomato hornworm (Manduca quinquemaculata) and the tobacco hornworm (Manduca sexta) are large, destructive caterpillars that cause significant damage to garden crops, leading many gardeners to assume they are invasive. While their appetite can be alarming, these species are categorically not invasive. Both hornworms are native to North America and have been integrated into the local ecology for centuries. Although native, they are considered agricultural pests due to their voracious feeding habits on cultivated plants.
Understanding the Hornworm
Hornworms are the larval stage of the Sphinx moth, also known as the Hawk moth or Hummingbird moth, which are large, stout-bodied flyers. The two primary species are often confused, but the tomato hornworm larva has eight V-shaped white markings and a dark blue or black horn, while the tobacco hornworm has seven diagonal white stripes and a red horn [cites: 6, 7, 9]. These caterpillars grow rapidly, undergoing four to five instars before reaching a mature length of up to four inches [cites: 4, 7, 23].
The life cycle begins when the adult moth lays small, spherical, greenish eggs, typically on the underside of host plant leaves [cites: 4, 5, 23]. After hatching, the larvae feed for about three to four weeks, then drop to the soil to pupate, a stage where they overwinter about four to six inches underground [cites: 5, 6, 23]. The large, reddish-brown pupa is characterized by a distinctive loop that encases the developing moth’s mouthparts [cites: 1, 11, 23]. In most regions, two generations of hornworms occur each year, with the adult moths emerging in mid-spring to begin the cycle again [cites: 2, 6].
Native Species Not Invasive
Both Manduca species are widely distributed across North America, from southern Canada down to Mexico [cites: 1, 3, 4]. They are long-established members of the continent’s biodiversity, having evolved alongside native predators and parasites.
Instead of being invasive, hornworms are classified as a “native pest,” meaning they are a native species whose population growth or feeding activity causes economic or agricultural losses. Their presence is a natural part of the local food web, where they serve as a food source for birds and other wildlife. This distinction changes the context of their management, relying on natural controls rather than eradication of a foreign species.
Damage to Plants and Gardens
Hornworms are specialists, feeding almost exclusively on plants belonging to the Solanaceae family, commonly known as the nightshade family [cites: 2, 3, 6]. Their preferred hosts include cultivated crops like tomatoes, peppers, eggplant, and potatoes [cites: 4, 7]. The damage they cause is significant because of their large size and voracious appetite [cites: 2, 23].
A single hornworm can defoliate a substantial portion of a tomato plant in a very short time, sometimes appearing to have done so overnight. Gardeners often first notice the rapid disappearance of leaves, leaving behind bare stems and veins. A secondary sign of infestation is the presence of large, dark green or black droppings, called frass, scattered on the leaves below where the caterpillar is feeding [cites: 7, 13].
Control and Management Strategies
The most immediate and non-chemical control method is manual removal, which involves carefully inspecting plants and hand-picking the caterpillars [cites: 6, 7, 17]. Because their green color provides excellent camouflage, searching for the tell-tale frass on the leaves is a good way to locate the hidden pests [cites: 7, 13]. Once found, the large caterpillars can be dropped into soapy water or relocated away from the garden.
For a more sustainable and biological approach, gardeners should be aware of the braconid wasp (Cotesia congregata), a natural enemy of the hornworm [cites: 6, 18, 21]. This tiny wasp lays its eggs just under the caterpillar’s skin [cites: 19, 21]. The wasp larvae develop inside the hornworm, eventually emerging to spin small, white, rice-like cocoons on the outside of the host’s body [cites: 7, 20, 21]. Finding a hornworm covered in these cocoons is a sign the pest is neutralized and should be left alone so the emerging wasps can parasitize other hornworms [cites: 6, 7, 22].
Chemical Control
If populations become overwhelming, an organic insecticide containing Bacillus thuringiensis (Bt) can be applied. Bt is a naturally occurring bacterium that specifically targets and kills caterpillars when ingested [cites: 18, 22].