Tomato worms, the large green caterpillars that strip tomato plants seemingly overnight, transform into hawk moths, also called sphinx moths. The two species gardeners encounter most often are the tomato hornworm (Manduca quinquemaculata), which becomes the five-spotted hawk moth, and the tobacco hornworm (Manduca sexta), which becomes the Carolina sphinx moth. Both are hefty, fast-flying moths with wingspans that can reach about four to five inches, and both belong to the family Sphingidae. The metamorphosis from destructive garden pest to impressive pollinator is one of the more dramatic transformations in the insect world.
Which Caterpillar Is Which
Gardeners often use “tomato worm” as a catch-all for both species, and telling them apart takes a close look. The tomato hornworm has V-shaped white markings along its sides and a dark-colored horn on its rear end. The tobacco hornworm has diagonal white stripes and a reddish or reddish-orange horn. Both feed enthusiastically on tomato, pepper, eggplant, and other plants in the nightshade family. From a gardening standpoint, the distinction between the two rarely matters because their damage, life cycle, and management are virtually identical. But the adult moths they produce look different enough that knowing which caterpillar you had can help you recognize the moth later.
The five-spotted hawk moth (from the tomato hornworm) has brownish-gray wings and, as the name suggests, a row of five orange-yellow spots along each side of its abdomen. The Carolina sphinx moth (from the tobacco hornworm) has six orange spots on each side and slightly different wing patterning. Both are heavy-bodied, fast moths that are active at dusk and into the night, and both are frequently mistaken for hummingbirds when they hover at flowers.
Going Underground to Transform
Unlike many butterflies and moths that pupate above ground in a chrysalis or silk cocoon, hornworm caterpillars burrow into the soil to pupate. When a caterpillar reaches full size, usually after five growth stages called instars, it stops feeding, drops off its host plant, and digs several inches into the earth. There it constructs a smooth-walled pupal chamber where it sheds its caterpillar skin one final time to reveal the pupa: a hard, dark brown, oblong case with a distinctive jug-handle loop on one end that houses the developing proboscis.
Research on Manduca sexta has shown that these underground pupal chambers serve real protective functions. Laboratory and field experiments have examined the costs and benefits of the chambers the caterpillars construct, confirming that the structure helps regulate moisture and temperature around the vulnerable pupa while shielding it from soil-dwelling predators and pathogens.1PubMed. Costs and Benefits of Underground Pupal Chambers Constructed by Insects: A Test Using Manduca sexta If you have ever turned soil in autumn and found a brown, thumb-sized capsule with that telltale loop, you were holding a hornworm pupa.
How Long the Process Takes
Temperature is a major driver of timing throughout the hornworm’s life. During the caterpillar stages, cooler conditions slow everything down. At lower daytime temperatures, caterpillars spend a significantly greater proportion of each growth stage in the molting process itself. Larvae at around 20°C spent roughly nine percent more of each stage molting compared to larvae at 30°C.2PubMed. Growth versus molting time of caterpillars as a function of temperature, nutrient concentration and the phenolic rutin In warm summer conditions, the entire caterpillar phase from hatching to burrowing can take as little as three to four weeks. The pupal stage underground adds another two to four weeks in summer, meaning the total egg-to-moth timeline in midsummer is roughly six to eight weeks.
But that summer timeline only tells part of the story. The last generation of the season does not emerge as moths before winter. Instead, those pupae enter a dormant state called diapause and remain underground through the cold months, sometimes for six months or more, before emerging as moths the following spring or summer.
The Overwintering Pause
Diapause is triggered primarily by shortening day length and dropping temperatures, though the sensitivity to these cues varies by population. Classic research on Manduca sexta found that both photoperiod and temperature influence whether pupae enter diapause, and that strains from different regions respond differently. A Kentucky strain needed higher temperatures to suppress diapause under short day lengths, while a California strain required lower temperatures and shorter photoperiods to enter diapause at comparable rates. At a daytime temperature of around 31°C paired with a nighttime temperature of 21°C under a twelve-hour light cycle, the Kentucky strain entered diapause at essentially 100 percent, while the California strain entered diapause at about 81 percent.3Environmental Entomology. Induction of Diapause in Tobacco Hornworms from Kentucky and California by Temperature and Photoperiod
For gardeners, the practical meaning is straightforward. Caterpillars that finish feeding in late summer or early fall will produce pupae that sit in your garden soil all winter long. If you till or turn the soil in late fall, you can expose these pupae to freezing temperatures and predators, reducing next year’s moth population. If you leave the soil undisturbed, the moths will emerge the following season and start the cycle again.
What the Adult Moths Actually Do
Once a hawk moth emerges from the soil, it has no interest in your tomato plants. Adult sphinx moths do not chew leaves. They drink nectar, and they are spectacularly good at it. Hawk moths hover in front of flowers much the way hummingbirds do, uncoiling a long, straw-like proboscis to reach deep into tubular blossoms. Their flight is powerful and sustained, and some species in the family can hover with remarkable precision.
Studies of hovering hawk moths have revealed complex wing-stroke mechanics. During each downstroke, the wings shed vortex structures from both the tips and the roots, creating ring-shaped vortices that direct a jet of air downward, holding the moth aloft. The upstroke generates weaker ring structures, making the downstroke the primary source of lift.4Journal of Experimental Biology. Hovering flight in hummingbird hawkmoths: kinematics, wake dynamics and aerodynamic power This hovering ability lets hawk moths feed from flowers without landing, which is unusual among moths and contributes to their reputation as hummingbird mimics.
The feeding apparatus itself is remarkably consistent across the hawk moth family. Morphological studies of the proboscis and associated pumping structures show that despite wide variation in proboscis length and diet among different sphingid species, the basic architecture of the tongue, the muscles that pump fluid through it, and the suction apparatus in the head are all highly conserved.5PubMed Central. Evolutionary functional morphology of the proboscis and feeding apparatus of hawk moths (Sphingidae: Lepidoptera) It is a design that works well: hawk moths are efficient nectar feeders, and their intake rate is affected by both the sugar concentration and the viscosity of the nectar they encounter.6PubMed. Nectar feeding by the hovering hawk moth Macroglossum stellatarum: intake rate as a function of viscosity and concentration of sucrose solutions
Hawk Moths as Pollinators
Because hawk moths visit many tubular flowers at dusk and at night, they serve as important pollinators for plants that other insects overlook. Pale, fragrant, night-blooming flowers are often specifically adapted to attract sphinx moths. Moonflowers, four-o’clocks, nicotiana, and certain orchids depend heavily on hawk moth pollination.
One dramatic example involves the endangered ghost orchid of southern Florida. Researchers documented at least five different hawk moth species visiting ghost orchid flowers and probing for nectar. Of those, two large species, the fig sphinx moth and the pawpaw sphinx moth, were photographed with the orchid’s pollinia (pollen packets) attached to their heads near the base of the proboscis, providing visual evidence that hawk moths are the likely pollinators of this rare flower.7BioOne Complete. Pollinia Removal and Suspected Pollination of the Endangered Ghost Orchid, Dendrophylax lindenii (Orchidaceae) by Various Hawk Moths (Lepidoptera: Sphingidae) The fact that your garden’s most annoying caterpillar grows up to pollinate endangered orchids is one of those ironies that makes the insect world so interesting.
How Hawk Moths Find Flowers in the Dark
Navigating at dusk and at night to find tiny floral openings while hovering requires a sophisticated sensory toolkit. Hawk moths combine vision, smell, touch, and even the ability to detect carbon dioxide and humidity gradients to locate and evaluate flowers. They use olfactory and visual cues to find rewarding blooms, mechanoreceptors on their proboscis to locate the nectary once they arrive, and gustatory receptors on the proboscis tip to evaluate the quality of the nectar before committing to a long drink.8PubMed Central. Fuelling on the wing: sensory ecology of hawkmoth foraging
Light levels matter a great deal. Under dim conditions equivalent to starlight or a crescent moon, Manduca sexta moths showed very low responsiveness to unscented artificial flowers but could be coaxed back into feeding either by increasing the ambient light or by adding floral scent. In dim light, the moths also showed a strong preference for white flowers over blue ones, a bias that disappeared as illumination increased.9Integrative and Comparative Biology. Influence of Ambient Illumination on the Use of Olfactory and Visual Signals by a Nocturnal Hawkmoth During Close-Range Foraging This explains why so many moth-pollinated flowers are white or pale: in low light, color is hard to see, but brightness still stands out. And the moths lean on scent to compensate when their eyes cannot do the full job.
Those White Things on the Caterpillar Are Not Eggs
If you have ever seen a tomato hornworm covered in small, white, rice-grain-like projections sticking up from its back, you might have wondered whether those are part of the moth’s transformation. They are not. Those are the cocoons of a parasitic braconid wasp, most commonly Cotesia congregata. The female wasp lays her eggs inside the living caterpillar, and the wasp larvae feed on the hornworm’s internal tissues as they develop. When they are ready, the wasp larvae chew their way out through the caterpillar’s skin and spin small white cocoons on its exterior.
Parasitism by Cotesia congregata causes profound changes in the hornworm’s body. The total amount of protein in the caterpillar’s blood drops substantially, beginning almost immediately after parasitization. At the same time, the wasp’s presence induces the caterpillar to produce large amounts of novel proteins that appear in the blood as early as one to two hours after the host is parasitized.10PubMed. Effects of parasitism by the braconid wasp Cotesia congregata on host hemolymph proteins of the tobacco hornworm, Manduca sexta The caterpillar effectively becomes a living incubator. A hornworm bearing these white cocoons is doomed; it will not successfully pupate or produce a moth. Gardeners who spot a parasitized hornworm are generally advised to leave it alone, since the emerging wasps will go on to parasitize other hornworms in the area.
How the Caterpillar Survives on Toxic Plants
Tomato and tobacco plants produce alkaloids, including nicotine and related compounds, as a defense against herbivory. Most insects are deterred or killed by these chemicals. Hornworms, however, have evolved a biochemical workaround. Research on Manduca sexta has shown that nicotine genuinely does slow the caterpillars down: larvae reared on plants with normal nicotine levels gained weight more slowly and developed at a reduced rate compared to larvae reared on plants that had been genetically silenced for nicotine production. Given the choice, freshly hatched caterpillars preferred to start feeding on the low-nicotine plants.11PLOS Biology. Nicotine’s Defensive Function in Nature
So nicotine is not harmless to hornworms. It genuinely imposes a cost. But the caterpillars compensate through enzyme induction. When fed nicotine-containing diets, fifth-instar Manduca sexta larvae ramp up the activity of cytochrome P-450 enzymes in their midgut, enzymes that break down toxic compounds. Caterpillars that had been exposed to dietary nicotine showed increased tolerance to subsequent nicotine injection compared to caterpillars raised on nicotine-free food, supporting the idea that this enzyme induction is a specific adaptation to feeding on nightshade plants.12PubMed. Induction of cytochrome P-450 activities by nicotine in the tobacco hornworm, Manduca sexta The caterpillars are not immune to the plant’s defenses; they are just better equipped to handle them than most insects.
Managing Hornworms Without Killing Future Pollinators
For gardeners, the tomato worm dilemma is real. The caterpillars can defoliate a tomato plant in days, but the moths they become are beneficial pollinators active when few other insects are working. A few approaches balance both concerns.
Hand-picking is the simplest and most targeted method. Despite their size, hornworms are well camouflaged, so checking plants in the early morning or evening, when the caterpillars are more active and their frass (droppings) is easier to spot, helps. If you find a caterpillar covered with the white parasitic wasp cocoons described above, leaving it in place is the smarter move, since those wasps are free biological control.
Soil management also plays a role. Because pupation happens underground, tilling garden beds in fall or early spring can destroy overwintering pupae. Conversely, if you would rather tolerate some caterpillar damage in exchange for more hawk moths visiting your evening garden, leaving the soil undisturbed lets the cycle continue. Crop rotation and interplanting with strong-scented herbs can reduce how easily female moths locate your tomatoes for egg-laying, though determined moths will find them anyway.
Soil amendments have also shown promise. Research found that substituting vermicompost into the growing medium at rates of 20 and 40 percent significantly reduced hornworm damage to tomato foliage in greenhouse experiments.13Pedobiologia. Suppression of tomato hornworm Manduca quinquemaculata and cucumber beetles Acalymma vittatum and Diabotrica undecimpunctata populations and damage by vermicomposts The mechanism is not entirely settled, but changes in plant chemistry and microbial communities in the soil likely make the plants less appealing or less nutritious to the caterpillars. Vermicompost is not a silver bullet, but it adds one more tool to the toolbox without harming the moths or the broader garden ecosystem.
Hawk Moths as Research Animals
Manduca sexta in particular has become one of the most studied insects in biology. Its large body size, relatively simple nervous system, predictable life cycle, and willingness to grow in lab conditions have made it a go-to organism for research on insect physiology, neuroscience, flight mechanics, and chemical ecology. Much of what scientists understand about insect hormones, metamorphosis, olfaction, and flight control traces back to work done on this species. If you grow tomatoes, the pest in your garden is also one of the most scientifically productive animals on the planet.
That dual identity, garden villain and scientific workhorse, is part of what makes the hornworm life cycle worth understanding. The caterpillar eating your tomatoes is weeks away from becoming a large, fast, ecologically important moth that pollinates flowers at night, navigates by smell and vision simultaneously, and fuels research that has shaped our understanding of how insects work. Whether you choose to squish the caterpillar or relocate it to a patch of wild nightshade at the edge of your property, knowing what it will become gives you a more complete picture of what is happening in your garden.