Excited about Hornworms?

Manduca sexta is a moth whose larvae (caterpillar) is known as the Tomato Hornworm. This is one of the more destructive and dreaded pests for those who grow tomatoes, as the caterpillars can defoliate a plant seemingly overnight.

So why would I be excited to see this guy on our tomatoes this morning?

This pest has parasites!

Take a closer look at the back of the caterpillar.

This Tomato Hornworm has been parasitized!

Just think of the movie Aliens as I explain the life cycle of the Braconid Wasps, a Tomato Hornworm parasite.

Braconid Wasps are a family of parasitic wasps, and Cotesia congregata is one species in this family that prefers Tomato Hornworms. It has a sharp ovipositor… a long, tube-like, egg-laying appendage. The adult wasp uses its ovipositor to pierce the skin of a caterpillar and deposits eggs inside the caterpillar’s body. The eggs hatch and the wasp larvae slowly eat the caterpillar from the inside. Initially, the caterpillar will act as if nothing is wrong, but as the parasitic larvae grow, and continue to eat, the caterpillar becomes progressively less active. The wasp larvae save the vital organs for last, so the infected caterpillar will stay alive for as long as possible. Eventually the wasp larvae break through the caterpillar’s skin and spin a cocoon. The larvae pupate into adult wasps, and the cycle repeats itself.

I was disappointed to see the Tomato Hornworm on our tomatoes, but then I saw the cocoons adorning the caterpillar like some macabre decoration. My disappointment quickly shifted into excitement. I went from almost grabbing the Hornworm from the plant and feeding it to the chickens, to gladly leaving the Hornworm right where I found it on the tomato plant.

The parasitized Hornworm in my garden is the exact reason we do not spray insecticides on our farm. We may have to put up with a little loss for a short time, but then Nature turns the tables on the pests. This poor Tomato Hornworm is now a barely living but very viable nursery for some highly beneficial parasitic wasps. I left things alone, and in a few days I’ll have a couple dozen more allies patrolling my garden for pest caterpillars.

And I don’t mind letting others do the dirty work for me.