Organic Tomato Hornworm Control

4 June 2020, written by Barbara Pleasant

The most dreaded tomato pests in both of the Americas are tomato hornworms, which become active in early summer, just as tomato plants grow to full size. First you notice part of an outer leaf missing, then a small branch eaten to a nub, or maybe your eyes land on dark green pebbles of caterpillar poo (frass). These are the signs that tomato hornworms have arrived in your garden.

In northern states, you are probably seeing true tomato hornworms (Manduca quinquemaculata), while in warmer areas tobacco hornworms (Manduca sexta) are also likely to appear. Hornworms earn their names with their fleshy ‘horns’, which they raise to frighten enemies. The horns of tomato hornworms are blue-black, while those of tobacco hornworms are reddish. Both species are native to the Americas, being larvae of two sphynx or hawk moths that fly mostly at night.

The Tomato Hornworm Life Cycle

“Young — A young tomato hornworm dines on a green tomato

The hornworms on your tomatoes this year may have emerged from last year’s tomato patch, or the adult moths may have flown in from afar. Either way, female moths find a promising tomato plant and scatter single light green eggs on both sides of leaves. Eggs that are not consumed by lacewings, ladybeetles or other predators hatch in about a week, and the caterpillars start eating tomato leaves and rasping surface holes in green fruits. Three to four weeks later, caterpillars not nabbed by birds, wasps or assassin bugs grow to four inches (10 cm) long before dropping to the ground to pupate. In most climates, a second generation emerges after two to three weeks.

“Male — Male five-spotted hawkmoth, the adult form of tomato hornworm

You can use a Bacillus thuringiensis (BT) based organic insecticide to control young tomato hornworms, less than two inches (5cm) long, but more mature caterpillars may survive the treatment. Plus, more eggs are hatching all the time, which is why organic gardeners learn to be sharp tomato hornworm scouts. When you know what to look for, hand-picking tomato hornworms can be extremely effective.

Masters of camouflage, tomato hornworms give themselves away by leaving a trail of dark green frass

Hornworm Hand-Picking Techniques

If the idea of picking up caterpillars scares you, relax. Hornworms cannot bite or sting. Their mouthparts are built for chewing soft leaves, and their fleshy horn is meant to spook enemies. Their bodies usually stay intact when plucked, so bare hands are fine for this job.

Start by checking the tops and outer branches of your largest plants for missing leaves or branch tips. Then look under the consumed foliage for a trail of dark green frass. Hornworms often feed on outer branches early in the morning and at dusk, moving to the plants’ sheltered interiors during the heat of the day. If you see hornworms every year in your garden, plan to scout for hornworms once a day. The larger hornworms grow, the easier they are to spot and remove.

Some tomato hornworms are parasitized by braconid wasps, which feed inside the hornworms and then pupate in rice-size white cocoons. Most gardeners have a live-and-let-live policy for parasitized hornworms, because the braconids will go on to help control cabbageworms, armyworms and other plant-eating caterpillars. If you have an extra tomato plant, or a volunteer sprouting from your compost, you can transfer parasitized hornworms to these non-preferred tomato plants to live out their sad lives.

Most gardeners simply squash collected tomato hornworms underfoot, or you can drown them in water. Chickens don’t care for them (they may be toxic due to concentrated solanine), but should you know a reptile hobbyist, they will be delighted to take your collected hornworms. To a lizard, one big hornworm is better than a dozen crickets.

Gain Hornworm Control with Fall Cultivation

Unfortunately, even the most conscientious gardener will never manage to rid their gardens of this pest. A few hornworms may hide long enough to change from mature caterpillar to oblong, dark brown pupae burrowed four to six inches (10-15cm) below the soil’s surface, always right beneath the tomato plants. In bad tomato hornworm years, the best thing you can do to prevent problems the next season is to pull up the plants and cultivate the soil in the fall, which exposes the brown pupae, often leading to their death. If you do a good job of it, less than 10% of the pupae will survive, which will make organic control easier next year.

Photo of male five-spotted hawkmoth by Didier Descouens/CC BY-SA