Vegetable gardening is not for the faint of heart. Even after months of nurturing tiny seedlings into big, vigorous food machines, you're still at Mother Nature's mercy.
Aside from weather, local wildlife often poses the greatest threat to homegrown crops. Pests are just part of horticulture, and shrewd gardeners can organically manage most moochers without losing their cool. Still, some invaders inflict so much damage so quickly that they rise to an almost mythical level of garden lore.
And for tomato lovers, few insect pests loom larger than a hornworm.
These huge caterpillars eviscerate tomato plants with alarming speed, leaving little time for gardeners to intervene. Yet while their notoriety is well-earned, hornworms are beatable if you know what to look for and how to react. To help with that, here's a quick primer on what hornworms are, what they do and how to stop them — including an ancient method every tomato gardener should know.
What is a hornworm?
Hornworms are larvae of hawk moths and sphinx moths, named after a horn-like spike on their butts that resembles a stinger but isn't. They're the largest caterpillars in much of North America, growing up to 4 inches long and ominously plump.
Two species are most infamous for raiding vegetable gardens in the U.S.: tomato hornworms (Manduca quinquemaculata) and tobacco hornworms (Manduca sexta). Despite their crop-specific names, both attack a range of plants in the nightshade family, including potatoes, eggplants, peppers, tobacco and tomatoes.
The tobacco hornworm is most common in the U.S. South, according to Florida's Institute of Food and Agricultural Sciences (IFAS), and the tomato hornworm is more concentrated in northern states. Yet their ranges overlap, and aside from subtle differences in horn color and markings, the two are "quite similar in appearance and biology," IFAS explains in a fact sheet. So if there's a hornworm in your garden, it probably doesn't matter which kind. Your tomatoes are in trouble regardless.
What do hornworms do?
Some gardeners are laissez-faire about caterpillars, many of which just nibble a few leaves without causing serious harm to the plant. And if you happen to notice a hornworm early in its development, it may seem innocent enough at first.
The process starts in spring, when adult moths emerge from overwintering sites and mate. Females lay tiny oval eggs on leaves, and those hatch within a week. After that happens, larvae go through five developmental stages known as "instars."
Young hornworms attack upper parts of a plant first, blending into the greenery as they eat foliage, flowers and even fruits. Their larval period is just three weeks, but they can grow 10 times larger in that span, from an average length of 7 millimeters (0.3 inch) to 81 millimeters (3 inches), moving around the plant as they mature.
Hornworms eat entire leaves, and at full size they can quickly defoliate a plant, with about 90 percent of damage occurring in the final instar. Once they're mature, they drop to the soil and burrow to form a pupal cell. Adult moths can emerge in two weeks, restarting the process up to three times per season, depending on climate.
How to handle hornworms on your own
Pull weeds near your garden, namely nightshades like horsenettle that can harbor hornworms. Tilling the soil kills some pupae, and light traps may lure adult moths, although IFAS notes this "has not proved practical" for pest control. Insecticides are rarely advised for home gardens, since they can kill beneficial insects like bees (or wasps), are less effective on big larvae and aren't typically needed for hornworms.
The University of Minnesota Extension Service (UMES) suggests checking tomato plants for hornworms at least twice a week in summer. If you find one, the best tactic is to remove it by hand, according to UMES, and drop it in soapy water to kill it.
First, however, always take a closer look. Tomato and tobacco hornworms are native to North America, and in healthy ecosystems, they're still kept in check by natural enemies. That includes not just predators like lady bugs and lacewings — which eat the eggs and young larvae — but also parasitoids: parasites that kill their hosts.
Hornworms, despite their size, are plagued by tiny parasitoid wasps. If you see their babies on your hornworm, Mother Nature has already solved your problem.
How to let wasps do your dirty work
"Wasp" may bring to mind big, predatory paper wasps, and those are known to prey on hornworms. But tiny parasitoid wasps also pose a grave threat to even the largest of these moth larvae, and their tomato-saving powers grow with every kill.
Rather than killing a hornworm outright, a female parasitoid wasp injects it with eggs and flies away, leaving her brood to hatch inside the live host. The eggs soon release little wasp larvae, which feed on the hornworm until they're ready to pupate.
The larvae form cocoons outside the host's body, and these white projections are easily visible to us. The hornworm is still alive at this point, and may continue walking around, but it has stopped eating. In fact, if you see a hornworm in this predicament, the best way to protect your garden is to just leave it alone.
After emerging from its cocoon, a parasitoid wasp pauses atop its hornworm host. (Photo: Cindy Funk/Flickr)
"If such projections are observed, the hornworms should be left in the garden to allow the adult wasps to emerge," UMES explains in a fact sheet on hornworms in home gardens. "These wasps kill the hornworms when they emerge from the cocoons and will seek out other hornworms to parasitize."
Parasitoid wasps are highly diverse, widely specializing in certain insects or life stages. They include broad families like the braconids, trichogrammatids and ichneumonids, the latter of which has an estimated 100,000 species — more than all vertebrate animals combined. Many use incredible tactics to find and control hosts, like Cotesia congregata, which injects a virus that limits caterpillars' growth and stops their immune systems from attacking her eggs. Microplitis croceipes finds its hosts by sniffing out a chemical in their feces, and can be trained to detect bombs. Some braconids in Brazil even take over their host's body and use it as a bodyguard.
These wasps may not be household names, but they perform irreplaceable jobs that illustrate why it's worth living and growing food in a balanced, biodiverse ecosystem. (Most are incapable of stinging humans, which is also nice.)
Braconid wasps are a nightmare for many pest insects like aphids, beetles, flies and caterpillars. (Photo: Katja Schulz/Flickr)
How to attract parasitoid wasps
As with any wildlife, parasitoid wasps are a lot more likely to inhabit a place if it has their preferred food and shelter. Some wasp species can be ordered online, including hornworm killers like Trichogramma pretiosum, but since wild wasps are free, it makes sense to try luring them first. And however they get to your garden, the wasps will only stay if you provide a suitable habitat. So here are a few tips:
1. Offer lots of tiny flowers.
While baby parasitoid wasps rely on host insects for food, adults feed on nectar. And since their small mouthparts can't reach into long, tubular blooms, they need flowers with relatively shallow nectaries. They also like a variety of tiny flowers, which are well-suited for them and often ignored by larger pollinators.
That includes plants in the carrot family (Apiaceae) like angelica, chervil, coriander, dill or fennel, as well as brassicas (Brassicaceae) like radishes or turnips. It also includes the mint (Lamiaceae) and aster (Asteraceae) families, which feature some late-season nectar sources such as goldenrod and yarrow. Here's a list with more options.
2. Provide shelter and water.
Removing certain weeds may limit hornworms, but it's wise to preserve a little wildness, too. On top of offering nectar, native plants can help shelter parasitoid wasps and other beneficial wildlife from extreme temperatures or weather.
Your wasps will also need water, albeit not much. If that's not already available, something like a bee bath should suffice. Just make sure it's shallow, with rocks or other objects to serve as perches, and check it regularly for mosquitoes.
3. Don't use insecticides.
When pests ruin your hard work, it can be tempting to use pesticides. But that often means a broad-spectrum insecticide, which is more hatchet than scalpel, killing helpful arthropods along with "bad" ones. Parasitoid wasps are no exception.
Growing food often feels like fighting with nature, forcing us to defend our crops against an onslaught of weather and wildlife. But while it's unrealistic to expect a problem-free growing season, it's also worth noting pests are only part of the picture. Entire species of predators and parasites have evolved to take out native troublemakers like hornworms, and in many healthy ecosystems, they still do.
Our gardens may be at Mother Nature's mercy, but if we're patient and give her space to work, she can be surprisingly generous with it.