Tomato Hornworm

The tomato hornworm also called by some people a tomato worm isn't really a worm at all, but instead a caterpillar. Or to be more precise, the larvae of the five-spotted hawk moth also known as the sphinx hawk moth.

Before we discuss how destructive the tomato hornworm are to your vegetable garden, it would be helpful for beginner gardeners to get a "heads-up" regarding this unwelcome moth in your garden.

The hawk or sphinx moth species is easy to recognize because they have a wingspan between four and five inches easily making it the largest moth flying around in your backyard.

Photo courtesy Clemson University- USDA Cooperative Extension Slide Series,

Because of their size, they are also called the hummingbird moth by some vegetable gardeners.

Like most moths, they are attracted to a porch light or any other light you have on in your backyard. This is a great time to see if you have any of this moth species visiting your garden. If you don't great! On the other hand if you do, you can rest assured they have already mated and laid eggs in your garden soil. Soon a new generation of tomato hornworms (larvae from the eggs of this moth) will mature and begin to devour your vegetable leaves.

After the tomato hornworm has fully matured it buries itself in the soil and enters a pupae stage. When this stage is complete, a fully grown adult moth has developed whose sole purpose is to dig its way to the surface, mate and begin a new cycle of hornworms.

There's not much you can do to get rid of these moths but at least you are aware you probably have hornworms in your garden and if not now soon. Not to worry though, because a little later in this article we will discuss how to interrupt their life cycle and protect your vegetable garden from this insect pest.

The Tomato Hornworm Larvae

The tomato hornworm and its cousin the tobacco hornworm are the largest of the caterpillar species growing to a length of four inches. Both species are quite similar in appearance and biology and  get their name because of the prominent "horn" on its rear.

The tomato hornworm is usually found in northern states while the tobacco hornworm is common in southern states. However, it is not uncommon for vegetable gardeners to find both of these insect pests intermingling together in their vegetable garden regardless of which state they live in.

Because of the tomato hornworms large size they have a ferocious appetite and can quickly eat their way through a large portion of tomato, pepper, potato and eggplant leaves thereby affecting the plants ability to produce a decent yield of crops.

Identifying characteristics of the tomato hornworm have V shaped markings on its side and a blue-black tail.

Photo courtesy of Whitney Cranshaw, Colorado State University,

Identifying characteristics of the tobacco hornworm have diagonal shaped markings on its side and a red tail.

If you look closely at the tomato hornworm picture, you will see the tail is red.

 Plant Damage

Tomato hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season. Because of their size, (approximately four inches) at maturity and having a huge appetite, both the tomato hornworm and its cousin the tobacco hornworm can defoliate a plant in a very short time.

Hornworms are often difficult to see because of their protective coloring and can easily blend in with surrounding vegetation.

Photo Whitney Cranshaw Colorado   State University,

Because they are sensitive to heat and light they tend to feed on the interior of the plant during the day. It takes a sharp eye and careful inspection to spot a hornworm munching on your vegetable plants. Most experienced gardeners will tell you they are more easily spotted when they move to the outside of the plant at dawn or dusk.

If you don't have a sharp eye and the sleuthing technique of Sherlock Holmes, more often than not you will first become aware of these garden pests when you notice large areas of damage at the top of a plant before you see the culprit.

Another good point worth mentioning when inspecting your vegetable plants for hornworms, do be on the lookout for large black droppings (frass) either on plant leaves or on the top of soil. If you see any frass droppings keep looking for hornworms as they are on your vegetable plants and you may have over-looked them on first inspection.

So do be on guard for these insect pests around the middle of summer because they can chew their way through most of a plants leaves in just a couple of days.

Photo Courtesy Whitney Cranshaw, Colorado State University,

Preventative Measures

Even though the hornworm seem to be determined to destroy you vegetable crop and leave a wake of destruction in their path, there are other insects in your garden that are predators of hornworms and very effective in destroying them.

One of these "good guys" and friend to vegetable gardeners is called the parasitic wasp. There method of hornworm destruction appears to be gruesome to us folks, ugh! But never-the-less they are very effective.

The parasitic wasp lays eggs on the hornworm (appear as white projections on the hornworms body) and when the wasp larvae hatch they begin to feed on the inside of the hornworm and eventually kill the hornworm. Remember, I did say ugh!

Photo Courtesy Robert L. Anderson, USDA Forest Service,

Though not as gruesome as the wasp method but still quite effective is to hand pick hornworms. Yes, I know this takes a little work, patience and a sharp eye, but your vegetable plants will thank you by giving you a good yield of crops at harvest time.

Just in case you're wondering, hornworms are harmless to people, they will not bite or sting you.

Oher methods some gardeners use is to plant marigolds in with their crop as well as planting dill plants in the rows between your vegetables. Although not a guaranteed deterrent, some vegetable gardeners have reported success while other gardeners haven't.

If none of the above methods strike your fancy, another option is to use Bt spray as it will not hurt people or animals but is quite effective in destroying the tomato hornworm. Just be sure to read the manufactures instructions before use.

As with any insect pest, it takes dilligence and determination to win the battle. But any experienced vegetable gardener will tell you it is well worth the effort especially at harvest time when you see a beautiful healthy vegetable crop.

And finally, when you have harvested your crop, this is a good time to turn or roto-till your soil to destroy any hornworm larvae that are attempting to pupate and wait for next spring's crop.

For more information I've added an article from Colorado State University for your reading interest.

As always

Happy Gardening

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