Have you ever seen stripped leaves on your nightshade plants? Or maybe you’ve seen tiny white to light green discs sitting singly on a leaf. What you might be dealing with is the tomato hornworm.
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Tomato hornworms are destructive little beasts. While sometimes they don’t do a lot of damage, over a few years, their populations can increase significantly. This is why it’s important to identify them and the damage they do, so you can control them.
There are many options for keeping these alien-like caterpillars at bay. And combining multiple methods can cover more bases than choosing just one. So let’s discuss tomato hornworms and how to limit their damage, so you can have a healthy harvest.
- Monterey BT Caterpillar Killer (Bacillus thuringiensis)Neem Bliss 100% Cold Pressed Neem OilCaptain Jack’s Dead Bug Brew (Spinosad)Agribon Floating Row Cover
Tomato Hornworm Overview
What Are Tomato Hornworms?
Manduca quinquemaculata is commonly referred to as tomato hornworm. Once they are large enough, tomato hornworms are easy to identify. They are 3 to 4 inches when they’re fully grown, and have a striking appearance: bright green bodies with around seven diagonal V-shapes along their sides. A black or blue tail-like horn protrudes from the rear. They have breathing holes along their sides that are called spiracles. These look like little yellow-and-black spots near the white V’s or stripes.
The adult version of this caterpillar is the tomato hornworm moth or the five-spotted hawk moth. They have large wingspans of 4-5 inches. On the fuzzy abdomen of the adult moth, there are five yellow spots, hence the name. The hawk moth is greyish-brown in color. They lay eggs – which are green to white, and ovular – sparsely on their host plants, which are all members of the Solanaceae family. Most gardeners will see them on tomatoes, eggplant, pepper, tobacco, and moonflower.
A Word on Tomato Hornworm Look-Alikes
A close relative of tomato hornworms is Manduca sexta, also known as the Carolina sphinx moth, or tobacco hornworm. This caterpillar looks nearly identical to Manduca quinquemaculata, but has black margins on its white stripes. It also has a red horn, as opposed to a blue horn. The adult form, the tobacco hawk moth, is similar to its five-spotted cousin in size but has different markings as well. There are no spots on its abdomen and the overall appearance of the moth is much like that of oak bark. It too is grey, brown, and black. They prefer to lay eggs on tobacco plants. But they will use nightshades as host plants too. That’s why both the tomato hornworm and tobacco hornworms can be a threat to your spring yields. Therefore, making an effort to control hornworms of all kinds is a worthwhile endeavor.
One moth that isn’t damaging to your nightshades is the caterpillar from hummingbird moths. Although the nymphal stage of this moth looks remarkably like Manduca sexta and Manduca quinquemaculata, it has distinct differences. It lacks white diagonal lines and has a black underbelly rather than a green underbelly. The horn of the hummingbird caterpillar is yellow with a black margin, whereas the horn of Manduca quinquemaculata is one color: black or dark blue. The adult hummingbird moth has a completely different appearance overall and is a very important pollinator of native plants in your area. While all of these moths do pollinate plants, specifically hummingbird moths are daytime pollinators that you’ll love to see in your garden. They don’t damage your yields as they feed to become brightly colored red and yellow moths with a fanned tail end and clear wings.
Tomato Hornworm Life Cycle
The tomato worm life cycle begins as overwintering Carolina sphinx moth and tomato hornworm moth adults emerge in the late spring. These garden insects then mate and lay eggs on the underside of host plant leaves. The tomato hornworm egg is spherical and whitish to pale green. Within about 2 to 7 days, the eggs hatch, and young hornworms emerge. They often do not feed much farther than the site of intra plant oviposition, or where the eggs were laid in the first place. These larvae go through five or six stages of growth, shedding their skin and becoming larger over 3 to 4 weeks. This period after caterpillars hatch is when tomato hornworms and tobacco hornworms are most destructive, and when they can decimate the host plant they’re living on.
Once the larval stage of the tomato hornworm moth and the sphinx moth has concluded, the final form of the larva burrows into the ground and pupate. These two species form pupae around themselves for protection under the soil’s surface. The pupal stage can take 2 to 4 weeks to conclude, after which the adult moths, the five-spotted hawkmoth, and sphinx moth will dig their way to the surface and emerge to start another generation. This is how long hornworms live. In most areas, there are two full lifecycles per year. Regions with warmer climates may have a third lifecycle over the winter months, but generally, the hornworms will burrow into the soil to pupate and effectively hibernate in their pupa over the winter in a kind of suspended animation.
Identifying Tomato Hornworm Damage
Both caterpillar larvae of tomato hornworm and tobacco hornworm species feed on the plant foliage of various plants in the nightshade family. Their host plants include tomato plants, eggplant, peppers from bell peppers to jalapenos and beyond, tobacco, and sometimes potatoes. Often the first sign of damage in late spring comes in the form of chew holes that is visible there as the caterpillar larval stage backyard bugs feed.
When they first hatch, they’re very small caterpillars that can be hard to see. As they continue to feed, tomato hornworm and tobacco hornworm caterpillars can defoliate plants, leaving behind chewed branches and veins of tomato leaves and nightshade plants. Missing leaves are one of the next signs a hornworm has made your plant one of its host plants. Hornworm caterpillars also leave behind dark black balls of frass as they chew holes in tomato leaves and other nightshade plants. Late in the nightshade season, in early summer or early fall, depending on where you live, hornworms may feed on mature fruits. Therefore another sign of their presence is holes in the fruit of your nightshade plant. Eggplant flowers may also be consumed by these little greenies.
The most obvious sign they’re on your plant is seeing them there. Once they’ve fed enough, you’ll see their long hornworm body, with its protruding horn. Of course, tomato hornworm will likely be on tomato plants, whereas tobacco hornworm will concentrate on tobacco. But both are nightshade generalists.
Controlling Tomato Hornworms
Keeping hornworms at bay is very important, especially if you’re relying on an early summer or early fall harvest of nightshade fruits and veggies. Here are a few ways to control these destructive caterpillars and save your yields in the process. As with other pests, an integrated approach is best for covering all your bases.
Cultural Control of Hornworms
To keep them out of your nightshade foliage is to practice crop rotation. This confuses the five-spotted hawkmoth as it looks for a place to deposit eggs. It keeps the subsequent population of hornworms low. Handpicking is the most effective control. You can either feed the hornworms to your fowl or flick them into soapy water. To find them, spray your plant with water and watch them writhe around in response. Follow up with a black light at night to search for hornworm caviar on foliage and the soil surface.
Plant companion plants that repel these caterpillars. Basil improves the flavor of your tomato plants, and also keeps the little destructor away, along with mosquitos, flies, and other moths. Borage and marigolds also repel the caterpillars and attract bees and beneficial insects. They’re great for supporting pollination in your garden overall. Planting a trap crop of Colorado 4-o-clocks (Mirabilis multiflora) will attract hornworms and provide a source of nectar for hummingbirds and bees. Include them in your spring garden, and destroy them after they’ve seeded as the season cools. Collect their seeds from the soil before burning or destroying the hornworm-infested crop and plant the seeds during your next spring prep.
Biological Removal of Hornworms
Biological control stems from providing habitat for beneficial insects. Parasitic wasps are excellent natural enemies that lay their eggs inside tomato hornworms, and hornworms in general. If you ever see tomato hornworms, or their kin with clusters of white capsules along their backs, do not kill them. This is a sign that braconid wasps have successfully apprehended the larvae as a breeding ground.
Hatched pupae of these wasps will feed on the body of the worm preventing them from growing into moths. Trichogramma and Hyposoter exiguae are the parasitic wasps you’re looking for. By planting plants these wasps like such as yarrow, Queen Anne’s lace, zinnias, fennel, and dill, you’ll support their population and that of other insects. Speaking of other insects, green lacewings and ladybugs biologically control hornworms by consuming their eggs.
Insecticidal Removal of Hornworms
Tomato hornworms can be controlled by multiple applications of Bacillus thuringiensis var. kurstaki, also known as Btk. You can purchase a BT spray but you want to be sure you’re using the kurstaki variation of the bacterial insecticide because it is specific to hornworms. Neem oil also smothers eggs but isn’t great when it comes to the larvae themselves. Neem cakes and cold-pressed raw neem oil can disrupt the pupae of hornworms overwintering in the soil. Spinosad is also effective.
The chemical sprays Tebufenozide, Methoxyfenozide, and Novaluron are more effective. Reapply these according to the instructions on the bottles, but generally, reapplications can occur in intervals of approximately one week. These should be your last-ditch effort because they can be damaging to beneficial insects. Use all other methods before using these chemical controls. When it comes to tomato, and controlling hornworms on your tomatoes, and all other nightshades use cultural and biological methods first.
Preventing Hornworms
If you want to keep hornworms away from the foliage of your young peppers, tomatoes, and other Solanaceous plants, try a floating row cover for a short time after planting. This control is most useful for keeping newly hatched caterpillars off a young plant.
Diatomaceous earth will prevent adult moths and pupating larvae of tomato hornworms and tobacco hornworms from burrowing into the earth below your plants. It also prevents the emergence in the spring of adult forms of tomato hornworms and tobacco hornworms. Not only does it cut up the tender bodies of larvae, but it also cuts up moths, too.
While tilling can sometimes be stigmatized in gardening, light tilling at the end of the harvest season will unearth and sometimes destroy pupating larvae. Till again in spring to prevent any overwintering larvae.
Frequently Asked Questions
Q: Can a tomato hornworm hurt you?
A: No, tomato hornworms can’t hurt you. But they can ensure a lessened food quality on your nightshades.
Q: Do tomato hornworms turn into butterflies?
A: They don’t. They metamorphose into moths. But those moths are a part of the lifecycle of tomato hornworms too.
Q: Can you keep a tomato hornworm as a pet?
A: You sure can! While it may be best to keep it in an enclosure away from other Solanaceous plant matter, you can feed it branches of tomatoes, eggplants, peppers, and moonflowers.
Q: Will tomato plants recover from hornworms?
A: Determinate tomatoes have a harder time recovering. Indeterminate tomatoes can recover more easily. Overall, it’s possible. Monitoring and control of these pesky dudes help most.
