The Tomato Hornworm: Friend or Foe?
(Editor's Note: This article was originaly published on June 19, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
Years ago, I learned to keep an eye out for the fat, green hornworms that could strip a tomato plant overnight. My only concern was finding them and squashing them. Until I started lurking on the DG Hummingbird and Butterfly Gardening Forum, I hardly gave a thought to what these pests might turn into as adults. I figured it was probably something equally repugnant, if less voracious. Wrong!
If you've ever been out in the garden and glimpsed a flying creature darting among the blooms that reminded you of a miniature hummingbird, you've probably seen a sphinx moth, also known as a hawkmoth or a hummingbird moth. These remarkable creatures with their long proboscises have an important role in pollinating deep-throated flowers.
You're most likely to see sphinx moths at dusk. Their speed and agility in flight is just amazing. With their quick wingbeats and plump bodies, it's no wonder many mistake them for "baby" hummingbirds. They'll often pause long enough while feeding at a patch of flowers for you to get a really good look at them while they hover. The patterning on their wings is beautiful, and their fuzzy bodies look pettably soft. Here's what you may not have realized while admiring these buzzing little beauties: they were once hated hornworms, terrors of the tomato patch.
What many of us in the eastern US think of as the tomato hornworm, due to its appetite for our vegetable garden, is actually the tobacco hornworm, Mancuda sexta. The tomato hornworm, Manduca quinquemacula, has very similar habits and appearance but is unlikely to be found east of the Mississippi. You can tell the two apart because the tobacco hornworm has diagonal white stripes, and the tomato hornworm has v-shaped white markings on its sides. Also, the "horn" on the tomato hornworm is black or dark green, while that of the tobacco hornworm is red.
There's no question that tobacco or tomato hornworms can devastate your vegetable garden. A single big hornworm can strip a tomato plant of its foliage in a day or two. They generally start near the top and work their way down. Their camouflage coloring makes them very hard to spot, but their appetite gives them away. When you see bare stems stripped of their leaves, start looking for lots of dark droppings (frass), and you'll find the culprit. But wait! Don't squash it. Think of that beautiful sphinx moth it will become.
I'm not suggesting you sacrifice your tomato crop to hornworms. And if your garden is really overrun, you may need to break out the Bt or other method of control. But if you just have a few hornworms to deal with, you may be able to simply move them from your vegetable garden to alternative host plants.
|Nicotiana sylvestris before, during, and after a visit from the Tobacco Hornworm (photos by DGer wind)|
Since planting a ‘Wild Cherry' currant tomato by my lily bed, I have abundant volunteer plants there every year. That gives me a ready source of "extra" plants for hornworms to munch. From what I've read, it seems that any plant in the Solanaceae family may be a potential host for tomato or tobacco hornworms. This includes veggies such as tomatoes, peppers, potatoes, and eggplants and it also includes flowers such as datura, petunias, and flowering tobacco (both Nicotiana alata and N. sylvestris). So it should be easy to find them an alternative meal.
Braconid wasps can be a great ally in controlling hornworm populations. If you see a hornworm with what looks like grains of white rice stuck to its back, it has been parasitized by a wasp. When the eggs hatch, the larvae will make a meal of the hornworm. The wasps have been keeping the hornworms in check in my garden for several years now.
What's the cycle of the remarkable transformation between tomato destroyer and lovely pollinator? In spring and early summer, the moths lay their eggs, one at a time, on selected host plants. Tiny caterpillars emerge, and begin eating (and eating). They go through several larval stages, or instars, shedding their skin as they outgrow it. Hornworms are one of the biggest caterpillars I've seen, reaching several inches in length. When they're ready to pupate, they drop into loose soil or leaf litter and form shiny, jointed brown pupae. The characteristic "horn" is still evident as a loop at one end. In warm climates, a second generation may emerge in about three weeks. Otherwise, the pupae overwinter in the ground and emerge as moths the following spring.
An appreciation for sphinx moths convinced me to stop squashing hornworms on sight. Once I started paying more attention to them as something other than a menace to my tomato harvest, I became fascinated by the hornworms themselves. If you pick one up, you'll notice immediately the silky softness of its skin. I can't ever decide if their contrasting black and white markings are clown-like or elegant, but they're certainly distinctive.
Their reaction to being disturbed is often to go limp, like a fainting daisy. If you gently poke one, however, you may be treated to an entirely different defensive response. The hornworm will rear up, looking as large and threatening as possible, and making loud clicking noises. He's hoping to startle you into leaving him the heck alone. I've certainly dropped them more than once!
The next time you find a hornworm munching away on your tomato plants, I hope you'll think of the beauty of "hummingbird" moths. Before you snip him with your shears or stomp him into tomorrow, consider moving him out of your vegetable bed. Perhaps there's a plant in another part of your garden that could be offered up to his appetite. Next spring, he'll return as a moth to pollinate your brugmansia, moonflowers, and other deep-throated blooms.
The hornworm may be a vegetable garden foe, but the fascinating moth he'll become is definitely a garden friend.
For more information, see this article on Hornworms and "Hummingbird" Moths, posted by the Colorado State University Extension.
For an idea of the diversity of hornworms and their hosts, take a look at this page of photos of hornworms inhabiting forests in the Eastern US.
Searching for "sphinx moth video" will let you see one of these beauties in action. Video links tend to be short-lived, so I can't post a specific link here. If you've never seen one in your garden, it's worth looking to find one online.
And for those who are looking for the ultimate revenge on hornworms and don't give a hoot what they might become as adults, I must post a link to this recipe for Fried Green Tomato Hornworms.
Photographs for this article were a DG community effort! Many thanks to Badseed, BlountsCreek, IRIS, jmorth, sequee, vee8ch, and wind for sharing their photos for this article.