Tomato hornworm: identification, parasitic wasps and hand-picking

At a glance

Tomato or tobacco hornworm?

Both species turn up on garden tomatoes and both feed mainly on the tomato family (Clemson). The sources give the same controls for both, so telling them apart is mostly a matter of curiosity, but the markings are easy to read on a full-grown caterpillar:

Young hornworms are yellow to white with no markings (UMN). Both species have five pairs of prolegs (USU). Utah State names two look-alikes: the white-lined sphinx caterpillar, which is mostly found on fruit and ornamental plants and rarely on vegetables, and the tomato fruitworm, which feeds on the same crops but is much smaller.

The moths fly in the evening and hover over flowers; Utah State says they can be mistaken for hummingbirds. They feed on nectar and do not harm tomato plants themselves (Wisconsin).

Life cycle

Minnesota describes two generations a season; Utah State says one or two, Wisconsin up to two in the upper Midwest, and Clemson at least two in South Carolina, with heavy egg-laying in August and early September. UC IPM says damage is usually worst in midsummer, with a smaller peak of larvae in late summer, and that infestations tend to be more severe in warm inland areas.

Damage and host plants

Tomato is the preferred host. Hornworms also feed on potato, eggplant and pepper, and on nightshade-family weeds such as horsenettle, jimsonweed and nightshade (UMN, Wisconsin). That puts the whole tomato family in a small garden at risk, including peppers and potatoes.

• Leaves first. Feeding starts on the upper leaves and works down (UMN, USU). Large infestations can defoliate plants quickly (UMN), and Wisconsin says a larva can eat up to four times its body weight a day.
• Then fruit. Larvae may chew large pieces from green fruit (UC IPM). Clemson says they graze the surface rather than boring in, leaving large open scars; Wisconsin says damaged fruit often rots on the vine or keeps a dry scar. Fruit damage is much less common than loss of leaves (Clemson).
• Sunscald. Heavy defoliation can reduce fruit production and raise the risk of sunscald (USU).

Scale matters: Wisconsin says hornworms rarely cause economic damage to tomatoes in that state, but because the caterpillars blend in, damage may be widespread before anyone notices them (UMN).

How to find them

• Look down first. Large black or green droppings on the ground under tomato plants usually signal hornworms (UC IPM). Utah State notes that droppings are abundant even when only a few larvae are present, so follow them up to the leaves above.
• Look up next. Check the upper leaves, where feeding starts (UMN), and stems stripped bare, which Maryland calls an early clue.
• Pick the time. Larvae hide during the day, so Utah State advises scouting in early morning or evening.
• Shake. Shake plants over paper or cardboard to dislodge larvae (USU).
• Keep at it. Check at least twice a week in summer (UMN). Utah State says to begin in early July, before populations peak; Wisconsin advises frequent checks from early July through August.

Parasitoid wasps: leave the hornworms with "rice"

A hornworm covered in small white cocoons is already doomed. The braconid wasp Cotesia congregata lays its eggs inside hornworms (UMN, USU). NC State Extension says many wasp larvae can develop in one caterpillar, feeding inside it for about one to two weeks; they then chew through the skin and spin silk cocoons on the outside while the hornworm is still alive. The cocoons are often mistaken for eggs. Adult wasps emerge about a week later, and the caterpillar dies a day or two after that.

Clemson says parasitized hornworms have stopped feeding, and Minnesota, Clemson and Utah State all say to leave them in the garden so the new wasps can attack other hornworms. NC State notes that Cotesia wasps are not sold commercially and that the adults feed on nectar from flowering plants.

UC IPM says natural enemies usually keep hornworm populations in check. Lady beetles, lacewings and other parasitic wasps also attack aphids, which is one more reason to avoid sprays that kill beneficial insects.

Hand-picking, step by step

1. Find them by following droppings and stripped stems, in early morning or evening (UC IPM, USU).
2. Check for cocoons. Leave any hornworm carrying white cocoons where it is (UMN, Clemson, USU).
3. Remove the rest. Pick them off or snip them with shears (UC IPM) and drop them into soapy water (UMN, USU). Clemson calls hand-picking actively feeding hornworms an effective control in home gardens.
4. Repeat at least twice a week through late summer, when the second generation is feeding (UMN).
5. Pull host weeds such as horsenettle, jimsonweed and nightshade to reduce egg-laying sites (UMN).
6. Turn the soil after harvest. Tilling destroys pupae and burrowing larvae (UC IPM, UMN); Wisconsin reports up to 90% mortality. Utah State says tilling in spring or fall disrupts overwintering pupae. The sources describe garden beds; none addresses pots or grow bags.

If a spray is needed

Minnesota says pesticides are generally not necessary for hornworms, and Utah State says they are usually unnecessary in small gardens. If other methods fail, Minnesota advises treating while the caterpillars are still small and before defoliation becomes severe. Wisconsin adds that late instars are hard to kill and that infestations are often localized, so spot treatments usually work.

• Bt (Bacillus thuringiensis var. kurstaki). A naturally occurring soil bacterium that caterpillars must eat; it affects only caterpillars and works best on very young larvae (UMN). Utah State's product table says it is effective only on caterpillars under ½ inch. UC IPM lists Bt and spinosad as effective against smaller larvae.
• Spinosad. Derived from a soil microorganism; works mainly when eaten and lasts about one to two weeks. Minnesota says it is toxic to bees when wet but much less so once dry.
• Insecticidal soap. Most effective on small caterpillars; it has to contact the insect and leaves no residue, so repeat applications may be needed (UMN). See the aphid guide for the soap precautions extension pages give.
• Broad-spectrum insecticides. Minnesota lists pyrethroids such as bifenthrin, cyfluthrin, lambda-cyhalothrin and permethrin, which last longer but can kill beneficial insects. Clemson advises using insecticides only when necessary to avoid harming beneficials, and notes that permethrin should not be used on tomato varieties with fruit less than one inch across. Utah State advises rotating insecticide classes.

This page gives no rates or intervals. Minnesota's advice: always follow the product label, make sure the plant you are treating is listed on it, and observe the required days between application and harvest. Clemson: "Use pesticides only according to the directions on the label."

Where sources differ

For spacing, staking and other tomato problems, see the tomato crop page and growing tomatoes in containers.

Hornworm checklist

• Tomatoes, peppers, eggplant and potatoes checked twice a week from early July.
• Droppings under plants followed up to the leaves above.
• Plants shaken over cardboard in early morning or evening.
• Hornworms picked or snipped into soapy water.
• Any hornworm with white cocoons left in place.
• Horsenettle, jimsonweed and nightshade pulled.
• Bed soil turned after harvest.
• If spraying: Bt or spinosad on small larvae only, crop listed on the label, days to harvest observed; spinosad treated as toxic to bees until it dries.