Tomato Plants Being Eaten Overnight? Tomato Hornworms Could Be to Blame – Here's How to Stop Them
A second generation of tomato hornworms strikes in mid-to-late summer
Early August is often a time when tomato hornworms are at their worst. These destructive pests can decimate tomato plants as they munch through foliage, even defoliating them in extreme cases. Once you spot adult hornworms or their hungry larvae on your plants, act fast to save your tomato crop.
Tomato hornworms may be hard to spot. Their pale green bodies blend in with the leaves but are distinguishable thanks to their distinctive markings and horn. When growing tomatoes, if you see lots of holes or damage to leaves or fruits, the quickest tactic is to handpick the pests or use organic sprays, particularly Bacillus thuringiensis (available at Amazon).
Tomato hornworms are capable of decimating plants and wrecking crops. To help you preserve your harvest of tomatoes, I spoke to two experts about the signs to look for and the best ways to combat tomato hornworms. The pests don’t just target tomatoes either; they also affect other crops in the nightshade family, so heed this advice if you are also growing eggplants, peppers, potatoes, and tomatillos.
What Are Tomato Hornworms?
Tomato hornworms (Manduca quinquemaculata) are the larvae of the five-spotted hawk moth. They are large caterpillars found in vegetable gardens, measuring 4-5 inches long, and have pale green bodies.
Unfortunately, that color helps them blend in with the tomato foliage, but there are a couple of distinctive markings to look for. Tomato hornworms have white V-shaped chevrons along each side, black spots, and a distinctive black horn on the rear of their body.
The life cycle of a tomato hornworm is quick. This means gardeners can face two generations of these tomato pests each growing season.
There is one in early summer, after the adult moths emerge in spring and lay oval, light-green eggs on the underside of leaves. These hatch within a week and feed ferociously as they quickly develop into caterpillars.
After 3-4 weeks of feeding, mature caterpillars then burrow into the soil to enter the pupae stage, emerging as moths and starting the cycle again. This turns into a second generation, which strikes in late July and August and causes the most damage.
The Signs of Tomato Hornworm Damage
Tomato hornworms can cause severe damage in a short time. As Emily Jones, tomato expert and founder of Tomato Mentor, warns: ‘These large green caterpillars can devour tomato leaves and fruit, causing extensive damage.’
The pests strip leaves, chew stems, and nibble on tomatoes. Tomato plants are left stressed, and damaged leaves limit their ability to photosynthesize. This affects vigor and the ability to produce and ripen fruits.
While the caterpillars themselves may be difficult to spot, the signs that they are on your tomato plants are much easier to identify. Susan Mulvihill, author of The Vegetable Garden Pest Handbook, highlights three key things to keep your eyes peeled for:
- The leaves will show signs of chewing damage. Branches may be stripped to the point of being defoliated.
- You’ll notice fruits with large bites out of them.
- The hornworms blend in with their surroundings, so look below the damage for dark frass (the droppings produced by the larvae)
Regularly checking plants for signs of infestation means you can act quickly if anything is spotted. Tomato hornworms start feeding at the top of the plant, and the frass will be found underneath.
It may be tricky to spot these pests that destroy plants due to their camouflage, but Susan recommends: ‘If you go out to your garden at night with a UV flashlight, it’s easier to spot the caterpillars.’
Susan Mulvihill is the author of 'The Vegetable Garden Problem Solver Handbook', available at Amazon, and gardens on 5 acres in Spokane, Washington State. She has also been writing garden columns for the Sunday edition of The Spokesman-Review newspaper for nearly 20 years.
How to Get Rid of Tomato Hornworms
There are a few tools at your disposal to help fight against tomato hornworms. Emily Jones recommends: ‘For larger pests – such as hornworms, caterpillars and snails – a simple method to deal with them is handpicking them and removing them from the garden.’
Handpicking is an effective but time-consuming method to remove adult hornworms. It is most effective in small gardens and a small number of tomato plants. Scour plants to pick off the tomato hornworms and place them in a container filled with soapy water.
There are products you can spray that can help to control the pest. Susan Mulvihill highlights one particular organic pesticide that can be successful. ‘Apply Bacillus thuringiensis (Bt) to the foliage,’ she says. ‘Bt has beneficial bacteria that can target the caterpillar stage of insects.’
Bt is a quick and effective way to combat young caterpillars, as they need to eat sprayed leaves for it to be effective. Neem oil (get neem oil for plants at Walmart) and insecticidal soap (get ready-to-use insecticidal soap at Amazon) can also be effective against young caterpillars.
For a longer-term approach to preventing tomato hornworm issues, gardeners can try companion planting.
Companion planting tomatoes attracts predatory insects like lacewings and ladybugs that will feed on the eggs and young caterpillars. Plants such as marigolds, geraniums, nasturtiums (get double dwarf jewel mix nasturtium seeds at Burpee), and herbs like dill (you can get mammoth dill seeds at True Leaf Market) and fennel are great options for attracting ladybugs and lacewings.
Tomato hornworms are targeted by parasitic wasps that lay their eggs in the caterpillar and feed on it as they hatch. It is an excellent form of natural pest control. You can purchase parasitic wasps to release or attract them with companion planting of flowers and herbs.
As adult moths emerge in spring after spending winter in the soil, tilling the soil at the start of the year before planting tomatoes and at the end of the season can eradicate pupae overwintering in the ground and stop them from emerging as moths to start the life cycle of a tomato hornworm.
Emily has been growing tomatoes for more than 30 years, and she founded the blog Tomato Mentor as a way of helping others to improve their growing methods. She regularly writes about everything from watering tomato plants and common tomato pests to the best growing techniques people should use in their own space.
What to Shop
Companion plant around your crops with herbs to attract predatory insects to combat tomato hornworms and also repel other garden pests
Marigolds are great for companion planting. They have a strong smell to deter pests and attract beneficial predatory insects, including parasitic wasps.
A pair of 64-inch-tall tomato cages made from solid 6-gauge steel to support heavy harvests. The 8-leg design keeps the cage firmly grounded during the season.
FAQs
Can Tomato Hornworms Bite?
Tomato hornworms do not bite or sting with the little horn on their posterior, and the pests are harmless to humans.
As well as pests like tomato hornworms and others, there are also diseases, such as blight, powdery mildew, and blossom-end rot, that may strike.
Thankfully, ensuring good air circulation, correct watering, and properly fertilizing tomatoes can help avoid many of these troublesome tomato diseases.
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