Words and Photos by Marisa Thompson
If you know tomato hornworms, you probably hate them. I know I did.
Who can blame us? These chubby caterpillars sure are pest-like: sneaky, creepy, and very, very hungry. I understand. I’ve killed plenty. And I shrieked every time because they’re fleshy and wiggle as you pull them off the plant.
If you haven’t seen one yet, imagine a huge, green worm with six tapered, segmented true legs up near the head; another ten squishy, stumpy prolegs with pads that hold on to leaves (and fingers) with an uncannily firm Velcro grip; and a cute little horn on the far end.
Yes, hornworms decimate some of our garden plants. And yes, some hornworm species target tomato plants. But we don’t have to grow tomatoes with the idea that we’re fighting against hornworms. Instead, let’s grow with the hornworms. Stick with me on this—it’s going to be fun.
The caterpillar is just one stage of the hornworm’s life cycle. After that they’re wrapped up snug in the pupal form, buried in loose soil for a few weeks or overwintering for several months before emerging as amazing winged pollinators. Common names for moths in this Sphingidae family include hawk moths, sphinx moths, and hummingbird moths. Call them what you will, but I’ll select hummingbird moth because it might help us honor them instead of demonizing them, like saying butter beans instead of lima beans.
Though they are caterpillars for only a few weeks, this is the stage that worries gardeners. Most hummingbird moths never harm vegetable plants, even in the caterpillar stage. The only two species that affect plants in the Solanaceae family (including tomatoes, eggplants, potatoes, tomatillos, and chiles) are the five-spotted hawk moth or tomato hornworm (Manduca quinquemaculata) with a black horn, and the Carolina sphinx moth or tobacco hornworm (Manduca sexta) with a red horn. As adults, both are important native pollinators, particularly in the desert Southwest. As caterpillars, the tobacco species seems to pop up on tomato plants in our region more readily. Common names can be silly like that. You say “tobacco,” I say “tomato.”
Left: Hornworm with squished-up face and six underdeveloped segmented true legs. Right: Sphinx moth pupae.
Unsure if you have hornworms or not? Telltale signs include almost completely defoliated leaves with only a few green midribs sticking out like very sore thumbs, and blackish pellets of caterpillar frass (a.k.a. insect poop) on lower leaves or on the ground (the frass is always greener when it’s fresh). Hornworm droppings tend to be fairly large—large enough to distinguish segmented grooves. Imagine a Barbie doll–sized hand grenade.
Even after confirmation with frass and denuded leaves, finding the worms can be surprisingly difficult, especially considering how portly and obvious they are when you do see them. Time and patience usually do the trick, like waiting to see the hidden picture in a stereogram.
Beat the heat and find hornworms in a fraction of the time by searching at night using an inexpensive black light flashlight. While some insects fluoresce under black light, hornworms do not. At least not technically. Luckily, they do reflect light differently than tomato leaves, so hornworms are much easier to spot under black light, and their body stripes offer extra-helpful contrast.
Okay, you found one. Now what? Squeamish? Gloves help. So does squeezing your eyes closed as you pluck it off the plant. And yelling loudly.
Keep in mind that hornworms are not our biggest garden hurdle by any means. Water stress is the top tomato issue in our hot New Mexico gardens, followed closely by the curly top virus. For more on protecting young tomato plants by covering them with shade cloth, visit nmsudesertblooms.blogspot.com and enter “tomato” in the search prompt. There you’ll also find a video tutorial on finding hornworms.
Now you have to decide what to do with your hornworm collection. One option is to simply move them over to other host plants in the area, like that tomatillo that doesn’t seem to be growing very well, or perhaps sacred datura or even silverleaf nightshade. Next year, plant some extra datura and eggplants as designated hosts for hornworm relocation.
Placing the wormies near bird feeders is another great option—at least for the birds. Hornworms also make nutritious feed for chickens, as well as lizards, chameleons, and other reptiles.
Another option is to rear them in a lidded terrarium, feeding them daily with fresh foliage from any of the Solanaceae plants, watching them pupate, and releasing them as pupae or after they emerge as moths. We did this last year at the New Mexico State University Agricultural Science Center at Los Lunas with tomatoes grown for a research study. After noticing the leaf damage and frass on July 16, I went out after dark with a friend and two blacklight flashlights. We started finding hornworms within minutes, collecting a dozen within the hour. Over the coming weeks, research staff, volunteers, and more friends removed over a hundred hornworms by hand from as many tomato plants. I lost count of how many made it to the moth stage, and it became a little bit of a hassle to arrange for their care when I went out of town, but it still felt worth it. One night in late August, my headlights shined on the sacred datura plants lining my driveway, and I was greeted by multiple hummingbird moths deep-diving into each huge flower before flying busily to the next. I was so excited that most of my photos came out blurry.
Don’t worry, even with a heavy population of hornworms in 2021, we still harvested over 5,400 pounds of tomatoes from our tomato study plants. The key is to start scouting in early July. You may not have tomatoes yet, but you definitely won’t have any if these voracious caterpillars are left alone for long.
And scout often. Before long, the chore of scouting becomes interesting and fun itself. Any time you slow down and look closer, you’ll see more.
White-lined sphinx moth (Hyles lineata) sipping golden currant (Ribes aureum) nectar at the NMSU Agricultural Science Center at Los Lunas.
ABQ Backyard Refuge Program (friendsofvalledeoro.org/abq-backyard-refuge)
Backyard Beneficial Insects in New Mexico, NMSU Extension Guide H-172
Integrated Pest Management (IPM) Strategies for Common Garden Insect Pests of New Mexico, NMSU Extension Guide H-176 (pubs.nmsu.edu/_h/H176)
Integrated Pest Management (IPM) for Home Gardeners, NMSU Extension Circular 655
Prominent red horn on tail end of tobacco hornworm at the NMSU Agricultural Science Center at Los Lunas.
Marisa Thompson is New Mexico State University's Extension Urban Horticulture Specialist, responsible for active extension and research programs supporting sustainable horticulture in New Mexico. In addition to studying landscape mulches and tomatoes, her research interests include abiotic plant stressors like wind, cold, heat, drought, and soil compaction. She writes a weekly gardening column, Southwest Yard & Garden, which is published in newspapers and magazines across the state and on her blog. Find her on social media @NMdesertblooms.