Teach Life Cycles with the Tobacco Hornworm
In the spring, many teachers and students look forward to studying life cycles because the accompanying hands-on activity is a highlight of the semester. This unit generally involves classroom observation of the metamorphosis of butterfly larvae or tadpoles. This is an experience most students won't forget. But teachers, are you ready to try something new?
Meet the tobacco hornworm (Manduca sexta), a member of the Lepidoptera order that emerges as a moth in adulthood. Most common throughout the southern and Gulf Coast states, this caterpillar is easily recognizable due to its distinct green hue, vertical lines trailing down each side, and its “horn” located on the posterior of its abdomen. They are easily confused with tomato hornworms, which look identical, except that they have V-shaped lines on their sides. In the larval stage, both are agricultural pests, and a single caterpillar can easily decimate an entire tobacco or tomato plant, respectively. Because of this, we discourage releasing these organisms, at any stage, into the wild.
The life cycle of tobacco hornworms is like that of butterflies. Both have complete metamorphosis, progressing from egg to larva to pupa to adult. At 27° C (81° F) it will take about 30 days for a hornworm to mature from an egg into an adult. Lower temperatures may extend the development time to 39 to 48 days. For observers, the difference between butterflies and tobacco hornworms comes from the visual appeal and substantial growth of the hornworm during the larval stage, and from its strange, fascinating pupal stage. Additionally, hornworms are novel organisms that students are usually unfamiliar with, leading to a heightened interest in the critters. Let's take a closer look at the life cycle of the intriguing tobacco hornworm.
Hornworm eggs are normally found on the underside of leaves but may also be found on the leaf surface. They are very small, translucent green spheres (1 to 2 mm in diameter). Hatching usually occurs 3 to 5 days after eggs are laid. In the wild, newly emerged larvae will immediately begin to feed on the leaf they were laid on. However, if you are hatching tobacco hornworm eggs in your classroom, Carolina recommends hatching them in an 8-oz cup containing artificial diet (item #143908 or item #143905). The reason for this is that once hornworms eat plants, they will rarely switch to artificial food sources. See the Hornworm Care Sheet for more detailed instructions on hatching tobacco hornworm eggs.
Once hatched, the larvae will eat and grow over the next 2 to 3 weeks, and as they do, they will progress through 5 instars, or stages. Between each instar the larvae molt, shedding their skin to allow for new growth. In the first instar, larvae are usually about 7 mm long. By the end of the fifth instar, they can measure up to 70 mm in length. During this time of growth, larvae need to remain in constant light at 27° C (81° F). If not, once the larvae pupate, they will enter diapause, extending the pupal stage to months instead of weeks.
Toward the end of the last instar (2 to 3 weeks), the dorsal aorta will become visible as a dark line running down the dorsal surface of the caterpillar. Once this line begins to pulsate, the larva will be ready to pupate. This is your cue to prepare pupation chambers for the larvae. Transfer 1 larva into each clean vial or bottle and add an inert material, such as sawdust, shredded paper, or potting soil, to loosely cover the caterpillar. Cap the container, then wrap it completely in newspaper or foil. In the wild, larvae burrow into the soil to pupate because they require total darkness.
Once the larvae have been secured in their chambers, pupation should be complete in about 7 days. After this time, it is safe to remove the pupae from the containers and examine their external anatomy. Handle the pupae with care, as they are easily damaged.
This stage marks the time for metamorphosis of the caterpillar to the adult moth. After pupation, the external features of the adult moth are apparent on the outside of the pupal case, as seen in figure 1. The strange appearance of this stage fascinates students. Many are surprised to learn that although the pupae seem rigid and inactive, the abdomens move. Be sure to warn your students about this, as many are startled when the pupae begin to wiggle in their palms.
After examination of the pupae, prepare them for eclosion (emergence) by placing them in the bottom of a butterfly habitat. At this point, they can be left at room temperature exposed to a normal day-night light cycle.
Emergence of the adult moths will occur 1 to 2 weeks after the pupae have been placed in the habitat. Once out, adults will climb onto a branch or other vertical surface to expand their wings. In your classroom, make sure the moths have something in the habitat to climb onto so they can properly unfold their wings. This is a great opportunity for students to compare the external structures of the moths with those of butterflies.
In nature these moths primarily feed on plant nectar, so provide them with a sugar water food source in the classroom habitat. The lifespan of the adult is usually 2 to 3 weeks. In order to begin the life cycle again, place a plant from the Solanaceae family (e.g., tomato plant, tobacco plant, jimsonweed) into the habitat. On the third night after emergence, females begin to deposit eggs on the underside of leaves. Again, because of the environmental impacts of agricultural pests, please do not release these organisms. If you need to dispose of the tobacco hornworms during any life stage, put them in a sealed plastic bag and place them in the freezer overnight. The bag and organisms can then be discarded in the trash.
As is the case with painted lady butterflies, hornworm eggs, larvae, pupae, and complete classroom kits are available from Carolina (see the list of additional resources below). So when it's time to teach life cycles, try something new and remember the tobacco hornworm. No matter what, the experience of observing living organisms grow and change will bring excitement and wonder into your classroom.