Tobacco Hornworm Manduca sexta. Photo used with permission
Note the Asterix. Similar to other "worms" in this forum, hornworms are actually caterpillars. And like all caterpillars, it will eventually turn into a pollinator. So while the caterpillar stage of this critter is a gardening foe for reasons we will discuss below, the adult version is a great nocturnal pollinator.
"Hornworm" is actually a name for several different types of caterpillars found in North America of the Sphingidae family. The name comes from the horn-like projection that is actually a tail. Host plants of hornworms vary. Two hornworms with common mistaken identity both feed exclusively on plants of the nightshade family. These two hornworms are almost identical and their common names are misleading. Here's why.
Tomato Hornworm Manduca quinquemaculata Photograph by Paul Choate, University of Florida.
The Tomato Hornworm, or Mand
uca quinquemaculata, is named for its tail and because it is commonly found efficiently destroying tomato plants. It pupates into the Five-spotted Hawk Moth. The Tobacco Hornworm, Manduca sexta, is almost identical in appearance and is commonly found on tobacco plants. It pupates into the Carolina Sphinx Moth. Sounds simple enough. BUT, most backyard gardeners are not growing tobacco. Instead they are commonly trying to grow edible plants such as tomato plants. When they see hornworms on their tomatoes, they assume it has to be tomato hornworms. It's in the name right? In Florida, what gardeners are more likely to see on their tomato plants is actually the tobacco hornworm.
Tobacco Hornworm with yellow tail instead of red. Photo by Patricia Croft used with permission
While tobacco hornworms can be found throughout most of the United States, tomato hornworms are generally only found in the northern United States. Besides the native distribution, telling apart the tomato hornworm from the tobacco hornworm is relatively easy once you know what to look for. The tobacco hornworm's tail is commonly red while the tomato hornworm has a black tail. The tobacco hornworm has seven diagonal stripes down its sides with black margins, while the tomato hornworm has "V" shaped markings along the back that point towards the head. While the tobacco hornworm's tail is commonly red, it can also be combinations of yellow, green, orange, and white.
Tobacco hornworms, or Manduca sexta, are caterpillars of the Carolina sphinx moth. These moths are a grey/brown in color and nocturnal. Moths benefit the garden by pollinating flowers at night. This is especially beneficial to night blooming flowers, or early morning blooms. Like most butterflies, the moths are attracted to nectar producing flowers that are typically uniquely shaped to allow their long proboscises to reach the nectar but deny other insects the same meal.
Tobacco Hornworm Manduca sexta. Photo used with permission.
Eggs of the female moth are laid on host plants, or plants that are specific to housing and growth of the moth's larvae (aka "worm" aka caterpillar). The host plants for these two species of hornworms are multiple plants of the Solanaceae family. I want to emphasize the word plants. You can find tobacco and tomato hornworms on multiple plants commonly grown in the garden. Solanaceae refers to plants of the nightshade family. This includes tomatoes, potatoes, eggplant, peppers, and tobacco. There are also some ornamentals, herbs, and weeds that can host these hornworms as well. These include jimsonweed, devil's claw, and lochroma. According to the University of Florida's Entomology & Nematology article on hornworms, they prefer tomatoes and tobacco over other nightshades. It is rare to find them on other host plants when tomato and tobacco plants are available.
It is interesting as to why tobacco and tomato hornworms prefer plants within the nightshade family. The reason is similar to why monarchs prefer milkweed. The caterpillars are using the toxins of the plant in order to make them less suitable to potential predators. Plants of the nightshade family contain toxic alkaloids. When foliage of these plants is ingested by the hornworms, the toxins build up inside their bodies similar to the toxins from milkweed building up inside the monarch caterpillars. The build up of toxins can lead to a foul taste to would be predators who attempt to eat the hornworm. This tells the predator that hornworm would be toxic to them, and that they should find different prey.
Caterpillar of the Rustic Sphinx Moth Manduca rustica. Photograph by Lyle J. Buss, University of Florida
Caterpillars of the Rustic Sphinx Moth, Manduca rustica, are also similar in color, shape, and size. They are green with with diagonal yellow stripes and "horn" that is the same color as the body. Often times the diagonal yellow stripes are bordered by either a darker green stripe, purple stripe, or even black stripe. The horn of this species is the only one that has granulations along it. This helps differentiate it from the other species who have smooth horns. It also lacks the large "false eyes" of the tobacco and tomato hornworms. There are small circles on each side of the body that are barely noticeable when compared to false eyes of other hornworms. Another interesting characteristic is the rustica hornworm changes colors to a reddish brown right before it pupates in the soil. Host plants for this species are not in the nightshade family further helping potential correct ID of this caterpillar. Host plants for this hornworm are lantanas, gardenias, bigonias, and other ornamental plants. Their distribution includes FL, and the southeastern United States.
Papaya Hornworm Erinnyis alope. Photo used with permission
Another species of hornworm found in Florida gardens is the Papaya Hornworm of the Alope Sphinx Moth, Erinnyis alope. The distinguishing characteristic among this hornworm is the obvious "false eye" spot on the back of the head. As you may guess, it gets its name by commonly feasting on papaya leaves. But, it will eat other plant species as well. Gardeners may see these hornworms on cassava and jatropha.
As if there weren't enough hornworms in Florida gardens, just wait.... There's more. Next is the caterpillar of the Tersa Sphinx Moth Xylophanes tersa. These hornworms have the most variation in color. They can be pale green, brown, or even a shade of red. The eye spots down each side of the body can be very vivid or dull and pale. The vivid eye spots and dark color is thought to be a protective measure by imitating a snake. Each variation has a black horn in the year like other hornworms. Their host plants include pentas as well as broadleaf buttonweed, candy corn vine, catalpa, firebush, and smooth buttonplant. They have also been reported from joe-pie weed.
Tersa Sphinx Moth Xylophanes tersa
Hornworm infestations in the garden can be catastrophic to the host plant. They are aggressive and quick eaters often chewing foliage down to the midrib. On tomato plants, new unripe fruit is also a target. Since tobacco hornworms blend in so well with their host plants, gardeners are often confused as to why their plants are experiencing such severe damage. After careful inspection of the plant, hornworms are often found under leaves near stems. Another identifying marker is frass left behind on leaves or stems. Poop! These caterpillars are one of of the few garden pests that leave behind large noticeable frass that appears as rounded balls of brown/black feces.
Management of hornworms consists of several different approaches. The most effective ongoing management of hornworms is allowing your garden to establish its own ecosystem. This allows balance in the garden between garden pests and garden predators. Allowing beneficial predators the opportunity to prey on the harmful pests ensures an ongoing balance between the two. These beneficial predators do not get the opportunity to be present in the garden if their prey (the harmful pests) are removed too quickly by either mechanical, biological, or chemical methods. Once predators are aware that there is a food source in your garden, they will likely patrol those areas frequently. This leads to ongoing management with minimal input of the gardener.
Predators of the hornworm include birds, lizards, frogs, and other mammals. Lacewing and ladybugs/lady beetles eat eggs laid by the moth. They may also eat young small hornworms that have recently hatched. One of the most efficient predators are wasps. Wasps of different species use two main methods to prey on hornworms. One method is a paralyzing sting
Red Marked Mason Wasp Pachodynerus erynnis. Photo by FOL
The other method of predation is performed by parasitoid wasps. Just like it sounds, it involves using the hornworm as a host for their young like a parasite would. The wasps will deposit eggs inside the hornworm that will eventually hatch and feed on the hornworm as they grow. Next, the wasp larvae will pupate on the outside of the caterpillar. If seen by the gardener, it will appear as if there are small white crystals on the back of the hornworm. Leave them alone and allow the hornworm to continue to host these wasps. It will eventually die and the wasps will now be new predators seeking out harmful pests in your garden.
Hornworm hosting several wasp cocoons. Photo by Rachel Elaine used with permission
If damage caused by hornworms is causing significant loss to garden plants, I recommend hand removal and relocation if possible. This management method requires a lot more effort by the gardener. I suggest relocation vs. killing the hornworms because they eventually become nocturnal pollinators. Incorporating "sacrificial plants" to relocate hornworms to is best practice with mechanical removal. That way sacrificial plants will receive the damage and prized tomato plants will continue to provide food.
Hornworm hosting wasp cocoons. Photo by Shiloh Pendergrass and used with permission.
If all seems lost, another method of control is by a biological agent called B.t. Bacillus thuringiensis is an organic and naturally occurring bacteria fatal to caterpillars. It is specific to caterpillars and will not harm other insects, including butterflies. B.T. works best when applied early, when hornworms are small. If large hornworms are already present, B.t. and mechanical removal may need to be done simultaneously. B.t. is available in most nurseries, box stores, and online. Keep in mind that the use of B.t. may limit the introduction of beneficial predators into the garden since it will kill young caterpillars before the predators can find them.
Do you have any thoughts about these bugs in the garden? If so, comment below to keep the conversation going!