There is nothing I appreciate more than hearing from folks with inquiries, observations and article ideas. Susan Gomez and I ran into each other in the grocery store, where she told me that she had an idea for an article, but couldn’t remember what it was. She followed up with an email, not only remembering her topic idea, but also with a flurry of great questions about hornworms.
Susan got more than she bargained for. I know I did when I researched answers to her questions and got deep into vivid photos and descriptions of the sex organs of pupating insects. It was like the entomological equivalent of Penthouse magazine.
Hornworms are those large green caterpillars that are eating your tomato (and eggplant, pepper and other nightshade-family) plants. You can’t miss these massive monsters. At four inches long and with the circumference of a pretzel rod, they are among the largest caterpillars I know of, and are especially identifiable due to their curved thorn-like horn. Two species of hornworms, tobacco and tomato, voraciously consume the goodies in your garden—even if you are not specifically growing tobacco or tomatoes.
To distinguish the two common species, count the lateral white stripes on their large bodies. There are eight for the tomato hornworm and seven for the tobacco hornworm.
Lifecycle questions were on Susan’s mind, first and foremost. It seems like they appear out of nowhere, so where do they come from? Eggs on the underside of leaves is the answer, laid by the caterpillar’s adult forms; moths known as hawk moths for tomato hornworms and sphinx moths for tobacco hornworms. Eggs will hatch into small caterpillars. In early stages, these caterpillars are small and feed on the underside of leaves, so it is possible to miss them.
They become more noticeable as they grow. At their largest, these caterpillar species can weigh up to 10 grams. Compare that to a hummingbird, which weighs an average of three to four grams!
These caterpillars eat through five instars or stages, during the last of which the caterpillar eats more than all the earlier stages combined! The scientific name for their genus, Manduca, appropriately means glutton or chewer.
The large caterpillar crawls down from its food source and wanders for up to five days to ready itself for its pupa stage. Then it will dig itself down 9 to 15 inches into the leaf litter or soil. A new generation can emerge in a few weeks’ time or overwinter until the next spring.
It is during the pupa stage when things get interesting. One question Susan posed was how to tell the gender of these insects. As an adult moth, a male has broader antennae than a female does. Gender can’t be easily observed in the caterpillar stage. However, when it is a pupa gender becomes clear. During the pupa stage, the insect resembles a wrinkly brown bullet. On the posterior or tail end, you can see the anal opening. Below that orifice will either be a slit indicating a female or two roundish knobs representative of the male.
I hope I wasn’t too graphic, but Susan, you did ask. And yes, although I do feel a bit like a voyeur after this one, keep those questions and topics coming. Inquiring minds want to know!
Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.