Tobacco Hornworms on Tomato Plants

One day when I was checking my tomato plants for ripe fruit I encountered my favorite tomato problem. I noticed some leaves towards the top of one of my six foot tall plants had been eaten down to the vein. Then I saw some green tomatoes with big bites taken out of them. I started looking around for the dragons of the insect world: the hornworm caterpillars. Within seconds I found not one, but two tobacco hornworms happily eating the photosynthesizing organs of my favorite summer vegetable. The hornworms are my favorite pest not because they do minimal damage, but because they are majestic creatures. Their odd beauty makes them hard for me to kill, so I was thrilled when my coworker volunteered to take them home to her insect loving neighbors who have plenty of tomato foliage.

Hornworms are named for the single spike that butts out from the posterior end of their bodies: tobacco hornworms (Manduca sexta) have red spikes, tomato hornworms (Manduca quinquemaculata) have black spikes. They can be up to four inches long and are bright green with white diagonal stripes (tobacco) or Vs (tomato) and almost eye-like circles down their sides. Their six jointed legs are up by their head but their main holding on power comes from the velcro-like prolegs in the back. They can rear up their front half to look menacing while holding on with those prolegs. The caterpillars become what are known as hawk or hummingbird moths.

Tobacco hornworm with parasitic wasp cocoons.

A couple days after I found the first two, I got a new thrill when I discovered the hornworm in the picture above! The white things sticking out of the caterpillar’s back are the cocoons of a parasitic wasp. The adult wasp inserts its eggs into the caterpillar. The eggs hatch and the larvae eat the inside of the caterpillar until they are ready to form cocoons, which they do on the hornworm’s back. When the adult wasps chew out of the cocoons, the caterpillar is left with tiny holes in its skin which, coupled with internal damage from the larvae, kill it. So when I see these parasitized caterpillars, I let them live so the wasp life cycle can continue and future hornworms can be kept from turning into moths, reproducing and making more caterpillars.

Ryan Mooney-Bullock

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