Tomato hornworms love tomatoes and can strip tomato plants of their leaves in very little time. The destruction starts when the large, attractive five-spotted hawk moth lays its eggs on the underside of tomato leaves. They hatch for about a week and the white larvae start to devour the leaves. The life cycle continues and the hornworms grow to about 4 or 5 inches in length within 3-4 weeks.
Tomato hornworms are a common garden pest that is found in many parts of North America. Luckily, once you know what you are looking at, they are relatively easy to get rid of. Tomato hornworm control includes picking them off by hand, encouraging its natural enemies to visit your garden, and using low-risk pesticides.
What are hornworms?
Hornworms, Manduca quinquemaculata, are very large green caterpillars that have a tail that looks like a horn. As they grow, they develop 8 white, V-shaped markings on either side of their long bodies.
The horn is a small, dark, blue-black projection on the last abdominal segment. They continue to feed until late summer or early fall before they burrow into the soil and pupate.
The pupae of the tomato hornworm remain in the soil through the winter months. Then, in spring, the mottled gray, white, and gray moths with a wingspread of up to 5 inches emerge.
Each Moth has five spots down each side of its abdomen. If you look out for them, you might see them at dusk hovering over garden flowers in search of nectar.
The female moths lay their small, light green eggs on the underside of tomato and other solanaceous plant leaves. The eggs hatch in about 7 days and start to feed, and it only takes about 3-4 weeks for them to be fully grown greedy green caterpillars.
Are hornworms harmful to your tomatoes?
The short answer to this question is yes! If you don’t do anything to control the infestation, you can kiss your tomato patch goodbye.
In a short publication titled Tomato Hornworms in Home Gardens, Jeffrey Hahn and Suzanne Wold-Burkness of the University of Minnesota Extension discuss the ins and outs of these destructive tomato worms. While they definitely prefer tomatoes, they have also been spotted feeding on potato, pepper, and eggplant leaves.
They are also known to feed on solanaceous weeds including nightshade, jimsonweed, and horsenettle.
The bigger the caterpillars get, the more damage they do. In addition to defoliating tomatoes, they also eat and destroy the fruit.
Hahn and Wolf-Burkness say that in the last caterpillar stage, the tomato hornworms consume almost as much as all the other life stages combined.
They advise checking your tomato plants for caterpillars at least twice a week in summer. As a precaution, they also advise that you keep your garden weed free, especially if you have solanaceous weeds the caterpillars may lay their eggs on.
Another suggestion is to till the soil after you have harvested your tomatoes. This will help to destroy any burrowing caterpillars and the pupae that form.
How to get rid of tomato hornworms
There are various steps you can take to get rid of tomato hornworms. These are the accepted methods.
The simplest way to get rid of tomato hornworms is to pick them off the plants by hand. It’s a bit of a tedious process, but not difficult because they are so big.
Add liquid dish soap to warm water and keep a bucket of soapy water next to you. As you remove the caterpillars, throw them into the bucket. That will kill tomato hornworms.
Hornworms, from the egg stage to full-grown caterpillars, have many natural enemies These range from birds and wasps to lady beetles and green lacewings.
Lady beetles and green lacewings will feed on the eggs and young caterpillars while wasps and birds like to eat mature adult caterpillars. You can encourage these good bugs and other creatures to visit your garden by practicing natural, organic gardening methods.
It’s also useful to know that tomato hornworms can be parasitized by some insects. As Jeffrey Hahn points out on the University of Minnesota Extension’s website, a small braconid wasp, Cotesia congregatus, lays eggs on hornworms.
When the larvae hatch from the wasp eggs, they feed on the inside of the hornworm until they pupate. The result is bizarre!
The cocoons that emerge from the caterpillar’s body look like fat grains of white rice. If you see this, leave the hornworms alone because the wasps that emerge will kill tomato hornworms even more effectively than you can.
Better still, it means that the wasps are busy in your garden environment and will help to keep the hornworms from killing your next tomato crop.
Pesticides and insecticides
If you feel you need to go this route, choose a product that has low risk in terms of toxicity. One effective pesticide Hahn mentions is Bacillius thuringiensis var. kurstaki (Bt), a bacterium that occurs naturally in the soil.
It only affects caterpillars, not bees and so on, but the worms must eat the bacterium if it’s going to kill them.
Another option is Spinosad, which is sold in a spray form. This is derived from microorganisms that occur naturally in the soil.
It kills caterpillars when they ingest it, affecting the nervous system. Unfortunately, though, it is toxic to bees when it is wet.
Another option that doesn’t affect beneficial insects is insecticidal soap. On the downside, it isn’t as effective as the other two options because the residual is minimal, and treatments may need to be repeated.
Also, it is best used to control small caterpillars, and they must come in direct contact with the soap if it’s going to be effective.
Some people recommend making a spray with cayenne pepper. It’s environmentally friendly and you can make your own by mixing one teaspoon of dish soap with a teaspoon of cayenne pepper. Mix with 2 cups of water and decant into a spray bottle.
Like insecticidal soap, you will need to use it often.
Many people prefer using Neem oil, which is a good solution for many garden pests. Spray it on tomato plants during the growing season and it’s likely to repel most pests, including tomato hornworms.
How to prevent tomato hornworms from coming back
Destroying the larvae of tomato hornworms or, if they survive, handpicking them from your tomato plants, is a good way to prevent them from coming back. Tilling the soil at the start and end of the growing season is an excellent way to destroy the larvae.
Weeding your tomato beds will also help to prevent them from coming back. In fact, anything you can do to stop the lifecycle is going to work.
Our garden tips provide you with several proven ways to get rid of tomato hornworms. But first, you need to know what you are dealing with.
Once you can identify these nasty tomato worms, you will find that it isn’t very difficult to get rid of them. But do take steps to ensure that they don’t keep coming back year after year.
Then try the methods we suggest. See what works best for you.