Tomato hornworm
Manduca quinquemaculata
also called Tobacco hornworm, Sphinx moth caterpillar
A large green horned caterpillar that can strip a tomato plant in days. Hand-pick (look for dark droppings first); leave any carrying white parasitic wasp cocoons.
How to know it
A caterpillar the size of a finger — up to 10 cm — soft green with pale V-shaped chevrons along its sides and a curved horn on the tail. Astonishingly hard to see against tomato foliage, which is why you find the droppings first, then look directly above them.
What you'll see
- Whole leaves and stem tips gone overnight, leaving bare stalks at the top of the plant.
- Dark green-black droppings the size of grape seeds on the lower leaves and the ground.
- Chewed patches and scars on green fruit.
- A plant defoliated from the top down while its neighbors are untouched.
- Large gray-brown hawk moths hovering at flowers at dusk earlier in the season.
Where it comes from
Hawk moths lay single green eggs on leaf undersides in early summer; the caterpillars feed for three to four weeks, then drop and pupate in the soil in a hard brown case. One to two generations a year, with the pupae overwintering a few inches down in the bed the plants stood in.
Warm summers in North America especially, on unsprayed tomatoes and their relatives. Numbers are usually small — one or two caterpillars can defoliate a plant on their own.
Dealing with it
- Follow the droppings up the plant and pick the caterpillar off by hand; drop it in soapy water or move it well away.
- Search at dusk or with a UV torch after dark, under which hornworms glow and stop being invisible.
- Leave any caterpillar studded with white rice-grain cocoons — braconid wasps have already killed it, and the emerging wasps will hunt the rest.
- Use Bacillus thuringiensis on small caterpillars if a crop is under real pressure.
- Check the plants every day or two in high summer; a hornworm doubles in size alarmingly fast.
Keeping it out
- Cultivate the bed in autumn and spring to expose the overwintering pupae to birds and frost.
- Rotate tomatoes, peppers and potatoes away from last year’s ground.
- Grow dill, parsley and alyssum nearby to keep braconid and trichogramma wasps resident.
- Interplant basil and marigolds, which appear to reduce egg-laying somewhat.
- Inspect the tops of the plants weekly — that is where the eggs are laid and the feeding starts.
What it goes for
Tomatoes above all, plus peppers, aubergines, potatoes, tomatillos and other nightshades.
Who hunts it for you
- BatsChiroptera A single bat takes hundreds of moths and mosquitoes in a night, and the moths are the parents of your cutworms, corn earworms and hornworms. Put up a bat box on a warm south wall, leave a night-scented corner unlit, and never spray after dark. Grow to recruit it: Evening Primrose,Honeysuckle,Nicotiana,Summer Jasmine
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com