Hornworms on the Tomatoes

A few years ago, I had planted a tomato in my small, raised bed garden at my home. By early July, the tomato hornworms started showing up. The first batch was about five worms I found over the course of a weekend. I could see where the plant was eaten, looked for fresh poop, then looked for the worm above. Then came round two with about 10 worms; then came round three with 27 worms over a period of four days. Fast forward to the end of the season, and I estimate my one Juliet tomato plant had been munched on by over 60 worms that year.

I’ve been growing tomatoes for a long time and have seen plenty of hornworms, but it has always been just a few in the garden area, maybe at the most 10 worms a year in the entire garden. I was beginning to wonder if they just had an affinity for this variety of tomato.

With all these worms and the looking for worms, I became somewhat of a hornworm hawk. I found it easier to spot them by looking at the plant from further away and just doing a general scan waiting for the unique color patterns and posture to jump out at me. It has always amazed me how such a large caterpillar can be so hard to spot.

  • Tobacco hornworm (Manduca sexta) egg.
    Tobacco hornworm (Manduca sexta) egg. Paul Thompson, ©2022, Clemson Extension

There is a condition that tomato plants get during the hottest part of the summer called physiological leaf curl. The leaflets roll upward at the margins for the entire length. The rolled-up leaflets are tube-shaped. The veins on the underside run diagonally across the tube. The leaf margins are toothed and undulate back and forth along one edge of the tube. The overall effect of this rolled-up leaflet with the colors and patterns mimics almost exactly the appearance of the hornworm, or to be more precise, the color and patterns of the worm mimic the rolled-up leaflet of the tomato. The diagonal markings on the sides of the worm mimic the veins of the leaf; the leaf margins look like the fat legs of the worm. The resemblance is striking once you pick up on it.

One evening I was sitting on the steps of my deck, and suddenly I heard a humming noise and perceived the movement of something large flittering around some four-o’clock flowers nearby. As I strained my eyes to see in the dim light, I picked out the presence of a sphinx moth about the size of a hummingbird visiting each flower and using its long proboscis to drink the nectar. The moth would sip a little nectar for a few minutes and then proceed to the tomato plant about five feet away and briefly alight on a tomato leaf to lay a single egg each time. Back to the snacking, then back to the egg laying; this went on and on until I could see no more.

I had inadvertently created a tomato hornworm factory by planting night-blooming flowers that attract the adults who are only active at night. And right beside the adult food is the food for the larval stage of this insect. The moral of the story: if you want to grow tomatoes, don’t plant any four-o’clocks.

For more information on this and other pests, see HGIC 2218, Tomato Insect Pests.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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