It’s happened to almost everyone who has ever tried to grow tomatoes: one day the plants are lush and full of promise; the next day they’ve been stripped of leaves and fruit. The culprit? The rapacious hornworm caterpillar.
This pest can be remarkably hard to spot for something that grows to be four inches long and is bright neon green. A single caterpillar can demolish an entire tomato plant with ninja-like speed and invisibility before the hapless gardener realizes what is happening.
There are actually two closely related species of hornworm that share the name. Manduca quinquemaculata, the “official” tomato hornworm is more common in the north. The hornworm found in the Santa Monica Mountains and throughout the Southern United States is usually M. sexta, the tobacco hornworm.
Quinquemaculata has a row of cream-colored “V” markings and a dark horn, M. sexta has diagonal lines instead ov v-shaped ones, and its horn is red. Both species have a ravenous appetite for tomato plants, although they feed on any member of the Solanaceae—nightshade—family, including flowering tobacco, potato, peppers, and datura, plants that contain some of the deadliest known alkaloid poisons, including solanine, nicotine, atropine, hyoscyamine, and scopolamine.
Once fully grown, the caterpillars drop from the plant and find a place under the soil to pupate. Hornworm pupae grow a smooth, dark-brown pupal case that enables the larva to transform into a moth in safety. Depending on the time of year, that transformation can take weeks or months.
The adult tobacco hornworm is known as a tobacco hawk moth, Carolina sphinx moth, or sometimes as a hummingbird moth, because of its size and flight pattern—the wingspan of this species can be as much as 4.75 inches.
Sexta is an important pollinator for the datura flower, the sacred plant of the Chumash people. The moth, like the caterpillar, is not affected by the powerful toxins in the plant. Tobacco hawk moths are also attracted to garden flowers, including hybrid daturas like angel’s trumpet and plants like evening primrose, moonflower, honeysuckle, and petunias.
Depending on host plant availability and weather conditions, this species generally has two generations a year, much to the dismay of vegetable gardeners.
Watching for the tell-tale sign of droppings can be the easiest way to detect hornworm caterpillars at work before they grow to monster size. Finding them when they are small makes removing them easier.
Emotions run high on the issue of hornworms. Some gardeners are repelled and appalled, others find the giant caterpillars captivating, so much so, they take pains to relocate them to another part of the garden, unwilling to dispatch the spectacular creatures.
Karen Klabin, who blogs on the Natural History Museum of Los Angeles County website, enjoys raising hornworms.
“My family has raised numerous hornworms to their glorious adult form,” she wrote in a 2016 post. “Gently disengaged from our tomato plants, the caterpillars are given a new home in a small, pink-lidded terrarium, with a supply of fresh tomato plant branches and a moist bed of soil.”
The captive caterpillars “are usually ready to pupate in the terrarium soil only a couple days after joining our household,” she wrote. “Most have typically emerged weeks after pupating, but once we had a moth emerge ten months later.”
Love them or loathe them, hornworms and hawk moths are a part of summer in the garden in Topanga and throughout the Santa Monica Mountains, as much as homegrown August tomatoes.
