Plant Palette

Plant Palette

Tomato and Tobacco Hornworms

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture
jaschult@illinois.edu

I was so proud of my vegetable garden this year, especially the tomatoes. The plants looked spectacular. Turns out if you water regularly it makes a huge difference in how well the plants grow. I should have practiced what I preach years ago! This year I didn't let anything get in the way of getting the watering done. And I was rewarded with gorgeous plants—until recently.

The first warning signs actually came in my next door neighbor's garden, as I was talking to her one evening. I spotted the telltale stubs of what used to be leaves on one of her tomato plants and exclaimed, "You have a hornworm!" I think she thought I was crazy.

I quickly found the offender, a large green caterpillar about four inches long and approximately the diameter of my thumb. We found three additional accomplices. They all became fish food when we threw them in the large pond behind our houses.

My attention turned to the six plants in my own yard. That night I didn't find any hornworms, but the next morning I found eight! How do they escape detection? The answer is in their coloration and habits.

It turns out there are actually two different species of hornworms likely to be found on tomato plants. The tobacco hornworm, Manduca sexta, is a large green caterpillar with seven diagonal white lines on each side, and a curved red horn at the hind end. This is the culprit tormenting my poor tomato plants. The tomato hornworm, Manduca quinquemaculata, is also a large green caterpillar, but there are eight V-shaped marks on each side, and the horn is straight and bluish-black in color.

Both species are voracious feeders on tomato plants, and any plant related to tomato (solanaceous, or nightshade family), including potatoes, eggplants, peppers, or weeds such as nightshade or jimsonweed. Personally I had never seen a hornworm on a pepper plant until this year. I'm sure my neighbors thought I had really lost it when I was yelling "NO!" after finding hornworms on my pepper plants. They had stripped every last leaf off a plant in one night.

Both species of hornworm have evolved to escape detection quite well. Their color is a perfect match to a tomato plant. Their bodies resemble the underside of a tomato leaf. Plus they tend to hide deep in the foliage to escape the direct sun during the day, venturing onto outer stems in the early morning and evening. For most of their life, they easily go unnoticed.

After about three or four weeks after hatching from their egg, a hornworm is mature, usually measuring about four inches long. This mature hornworm can easily eat several tomato leaves and even fruit in a single day.

Hornworm damage is fairly easy to spot if you know what to look for. Leaves will be devoured, leaving only the petiole sticking out. You might see large green or black droppings at the base of the plant. Typically feeding begins on the upper part of the plant.

Many people are shocked to discover that hornworms actually do become something other than a tomato plant eating machine. When mature, the hornworms drop off the tomato plant, burrow into the ground and form a pupa. When they emerge, they are a large moth with a wingspan or four or five inches, commonly referred to as a hawk, sphinx, or hummingbird moth.

The first hornworms of the year eat, pupate and hatch out into moths during the summer, but the second generation overwinters as a pupa underground. In the spring or fall, you may find the pupas when working in the garden. They resemble brown cigars or jugs with a curved brown handle.

One great way to prevent a hornworm problem is to destroy these pupas, whether by tilling a large garden or individually picking by hand in a small garden. This is another reason to rotate where you plant different crops in the garden.

While it is possible to use insecticides such as carbaryl, permethrin, spinosad or Bt to control hornworms, most sources suggest hand-picking and destroying them is the most effective way to control them. It's also very satisfying to the psyche! I also found that paying my neighbor's daughter 25 cents per hornworm was effective.

Mother Nature has her own way of controlling hornworms. There are multiple species of parasitic wasps that will lay their eggs in a hornworm. The eggs hatch, feed on the hornworm from the inside, then emerge from the hornworm and form a cocoon. These will look like little white tic-tacs on the outside of a hornworm. If you find such a hornworm, leave this one alone. It will die when the new wasps emerge from the cocoons, and the wasps will move on to find a new hornworm to lay eggs in and the cycle continues.

It looks like I have won the battle of the hornworms, at least for this year. My plants are recovering, and soon I should have an overabundance of ripe tomatoes and peppers. Next year I think I will begin heavy patrols for hornworms just a bit sooner.

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