Several years ago I used to keep tomato plants on my deck (before my fenced in garden.) One summer I saw that my plants were wilting and not looking healthy, losing a lot of leaves and twigs. I figured I wasn’t watering enough. Despite better efforts, my plants continued to go downhill, looking worse every day, until they were missing most of their leaves and dying. I also noticed several partly eaten tomatoes still on the plants. I couldn’t understand what was wrong. While studying my plants, pondering what might have gone wrong, I suddenly discovered two large, fat, brightly green worms on one of the stems! I was both shocked and horrified at the sight! I had never seen anything like it! My husband did a quick search on-line and discovered that these two hungry caterpillars were tobacco hornworms, and that they are extremely common in North America. They cause a lot of damage to tomato plants, and will destroy them completely if left alone. These worms are often mistaken with tomato hornworms, which is closely related. It doesn’t matter much which type they really are, as they do the same damage, feed on the same plants, and are controlled in the same ways as well.
If you really want to know the difference between these two worms, there are a couple of things to look for: The tobacco hornworm has white stripes along its body and has a red horn, while the tomato hornworm has v-shaped markings and a black horn. I remember reading a little hint to help remember the difference: Think of the white stripes on tobacco hornworms as cigarettes, and the “V’s” on tomato hornworms as in “vine ripe tomatoes.” People also call them tomato worms, or hookworms. Tomato- and tobacco hornworms feed on tomato, tobacco, potato, pepper, and egg plants.
Since I did not known to be on the look-out for these nasty worms, they had been allowed to feed until quite large and plump, and finally nearly killing my tomato plants in the process. By the time they were removed, my plants were in such bad shape that there wasn’t much to try to salvage.
Look at the size of these worms!
As you can see, they also have markings along their bodies looking like rows of eyes; possibly to ward off predators? It can be hard to know which end is the head-end; it is the opposite end of where the horn is located. There are also some claw-like “arms” by the head. They use their bodies and even the hind area to hang onto the branch, and they can hold on quite well. Ask me how I know! OK, I’ll just tell you right away: When I try to pick them off, they hold on for dear life! I often will use gloves when I peel them off, since I’m a bit squeamish that way. They gross me out! Their bodies are soft and squishy, and they have green goo throughout their insides. Don’t ask me how I know that one, I don’ wanna talk about it.
But in all reality, I have to admit that I find them beautiful at the same time. Especially when I see a picture of one. When I see a large worm on my plant, however, quickly and actively destroying what I’ve been tenderly caring for, I don’t find them so pretty!
Ways to locate hornworms, clues to follow
On the next picture, you can see a small, black pebble on top of a leaf. This is a dropping, or poop, from a tomato / tobacco hornworm. These droppings, resembling small grenades, are a clear indication that a worm is nearby, and is one of the things I look for in order to find them. Finding the worms itself is tricky since they blend in so well, but if I see droppings like this, I typically look up a few inches and find a worm. Do you see the poop on the leaf and the worm toward the top in this picture?
Sometimes, especially when they are smaller, you may have to look under leaves in order to find the worms, but the poop is typically on top of the leaves as it obviously drops down from the worm and lands on top of leaves. Additionally, droppings can often can be found on the ground under the plants if several worms are present. You can also sometimes find little, green eggs underneath leaves, laid there by the adult moth; the moth of hornworms are sometimes called “hummingbird moths” as they can hover like hummingbirds when looking for nectar.
I also look for half eaten stems, twigs, or leaves on my plants, which is another indication of a hornworm infestation. If I find partly eaten green, or ripening fruit, that’s another clue to start looking around.
Here is a different plant with partly eaten twigs, and a worm directly to the left. This one is especially hard to see, but once you know where it is, you can see it.
See, right there!
Like I said, I’m a bit squeamish when it comes to huge caterpillars like that! Using my glove this time…
How to handle hornworms when you find them
Once you locate these worms, you can pick them off and discard of them by dropping them into a bowl of soapy water to drown, though some might find it more convenient to take care of things quickly, using a more direct approach. Otherwise you could put them on your driveway and let the birds take care of them. Chickens are also happy to receive such a juicy snack. Handpicking and destroying them is very helpful and makes a big difference, but if you are unable to check often, or simply have too many plants to control them that way, there are other things you can do to prevent them in the first place, or use for treatment once they show up.
Ways to prevent and treat for hornworms
One thing you might be doing already, is to till at the beginning and the end of the season; this will destroy a lot of the pupa in the soil; once worms reach a certain size or point of development, they will dig themselves down into the ground to over-winter, then hatch in the spring as mature moths (sometimes during the same season, even!), ready to mate and lay eggs.
You can also spray the plants with sprays that will control hornworms; there are a couple of different ones on the market that are considered safe for organic gardening. One of them is the beneficial bacteria bacillus thuringiensis (or Bt), which attacks and kills hornworms but is harmless to people, animals, plants, and the environment. Another spray for organic gardening that works well for a lot of different bugs, caterpillars, and beetles, is Spinosad.
When NOT to remove a hornworm
Another thing to be aware of, is that should you ever see a hornworm with protrusions on its outside looking like white grains of rice, you should leave these hornworms alone. Certain beneficial wasps will help you get rid of hornworms by laying eggs inside them, which will hatch and then feed on the worm, and finally make cocoons on the outside of it looking like white rice. If left alone to finish their life cycle, these parasitic wasps will help keep the hornworm population down as they seek out more worms to start a new cycle. The hornworms won’t do much damage at this stage as far as I understand, though I have never personally seen a hornworm infested by wasp larvae.
On the website Cahri’s Bugs Online, you can see pictures of hornworms in different stages of their life cycle. These pictures are helpful in learning what to look for in order to identify eggs and cocoons in addition to the worms themselves and their droppings.
Finding tomato or tobacco hornworms on your plants is not exactly fun, but it does add some excitement in the everyday gardening experience, as well as giving you another reason to get outdoors to enjoy the sunshine and warmth of summer. There might even be a story or two to be told in that regard, especially by your neighbors after they hear you shriek as you find your first one!