Few garden pests are as startling as a tomato hornworm: a caterpillar the size of your thumb, sitting on a stem that was fully leafed yesterday. The good news is that hornworms are one of the easiest tomato pests to control in a home garden — you can see them, you can pick them off, and in a small planting that is usually all you need.
Identification
Tomato hornworms are the caterpillars of large sphinx (hawk) moths. There are two very similar species: the tomato hornworm (Manduca quinquemaculata) and the tobacco hornworm (M. sexta). Both feed on tomatoes, and for garden purposes they are managed the same way.
- Size: about 3–4 inches (7–10 cm) when full-grown, with a horn-like spike at the rear end
- Colour: bright green with white diagonal or V-shaped marks along the sides, which camouflage them against tomato foliage
- First sign: large dark green or black droppings (frass) on the ground under the plant, and stripped leaves near the top of the plant
- Damage: leaves eaten down to the stem, sometimes overnight, and feeding scars on green fruit
Hornworms are hard to spot at first because their green colouring blends into the plant. Look for the droppings first, then look directly above them. They are easiest to find at dusk or dawn, when they feed on the outside of the plant, and a UV flashlight at night makes them glow.
Step 1: hand-pick — the best control in a small garden
The University of Minnesota Extension states that physical removal is usually the only management tomato hornworms need, and UC IPM lists hand-picking first among its recommendations. Check plants at least twice a week in summer.
- Grip the caterpillar firmly near the middle and pull — they hold on with surprising strength.
- Drop them into a bucket of soapy water, or snip them with shears.
- Wear gloves if you prefer; hornworms are not dangerous to handle.
In a bed of a few tomato plants, a twice-weekly check through the summer is usually enough to keep damage negligible.
Step 2: leave the ones with white cocoons
If you find a hornworm covered in what looks like grains of white rice, leave that one alone. Those are the cocoons of a parasitic braconid wasp, which has already killed the caterpillar from the inside. The wasps that hatch will go on to parasitise more hornworms — free biological control, and the main reason hornworm populations are normally kept in check. UC Master Gardeners give the same advice: the wasp-covered hornworm is doing you a favour.
Other natural enemies include lady beetles, lacewings, paper wasps, and Trichogramma egg parasites.
Step 3: Bt for small larvae, if spraying is needed
Bacillus thuringiensis (Bt, specifically the kurstaki strain) is a naturally occurring soil bacterium that caterpillars must eat to be affected. UC IPM lists Bt and spinosad as effective against smaller larvae. The critical detail, repeated by every extension source, is size: Bt works best on very young caterpillars — Utah State notes it is effective only on caterpillars under half an inch — and much less well on large ones. By the time a hornworm is 3 inches long, hand-picking is the answer, not spraying.
If you use Bt:
- Apply in the late afternoon or evening, because sunlight breaks Bt down, and coat the leaf undersides where caterpillars feed.
- Spray when you see small hornworms or fresh hatching, not after the damage is done.
- Follow the product label exactly — including the crop, the dose, and the days to wait before harvest.
- Spray only the affected plants. Bt kills any caterpillar that eats treated leaves, including butterfly and moth larvae you may want to keep. Keep it away from milkweed and other host plants grown on purpose.
Spinosad is also listed by UC IPM for smaller larvae, but it can harm bees and natural enemies for one to four days after spraying, so Bt is the gentler choice when bees and beneficial wasps are active. University extension sources agree that pesticides are generally not necessary for hornworms in home gardens at all.
Step 4: prevention for next year
Hornworms overwinter as pupae in the soil. Tilling the bed in autumn can destroy overwintering pupae, and rotating tomatoes to a different bed each year helps. Watch for the adult moths — large, grey-brown sphinx moths, sometimes called hummingbird moths — at dusk in early summer; their eggs hatch into the caterpillars you will be picking a few weeks later.
What not to do
- Do not reach for a broad-spectrum insecticide first. Products such as pyrethroids or carbaryl also kill the parasitic wasps and other beneficial insects that keep hornworms in check, and they carry harvest waiting periods you must observe.
- Do not spray Bt on large hornworms and expect results. It is a young-larvae tool; the big ones need picking.
- Do not kill the wasp-covered ones. They are the reason you will have fewer hornworms next year.
Sources
- UC Master Gardeners (UC ANR) — Ask a Master Gardener: Hornworms: https://ucanr.edu/blog/savvy-sage/article/ask-master-gardener-1
- The More One Sows — Tomato Hornworm: ID, Parasitic Wasps, Hand-Picking, Bt: https://THEMOREONESOWS.COM/pests/tomato-hornworm/
- Pest Control Basics — Tomato Hornworm: Spotting It, the Tobacco Hornworm Look-Alike and the Rice-Like Cocoons: https://pestcontrolbasics.com/pest-profile-tomato-hornworm.html
- Ambitious Harvest — Tomato Hornworms in California: ID, Control, and the Wasp Signal: https://www.ambitiousharvest.com/learn/tomato-hornworms-california