Parasitic Wasp Cocoons on Tomato Hornworm

Look closely - the first wasp cocoon is visible in the middle of the hornworm larva

A couple of weeks ago I was visiting a friend’s garden when I noticed that her tomato plant in the container on the front porch looked strange.  It seemed to be missing most of the leaves.  When I looked closer, I came face to face with my first tomato hornworm!  I guess I’m a bit of a garden geek because I thought it was very cool; my friend was not nearly so enthusiastic.

I ran for the camera and took a photo. That’s when I noticed a small whitish protrusion on the side of the hornworm.  It was a parasitic wasp cocoon emerging from the hornworm!  Even cooler   As I watched over the next hour, more and more cocoons emerged, until the entire larva was covered with them.

More cocoons appear on the hornworm

It turns out that tomato hornworm larvae are parasitized by a number of insects, but one of the most common is a small braconid wasp called Cotesia congregatus. Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate. The cocoons appear as white projections sticking out from the hornworm’s body (see photos). Eventually, the wasps emerge from the hormworm and kill it. Then they’ll head off to seek out other hornworms to parasitize.

Completely covered - this hornworm is doomed

If you see wasp cocoons on a hornworm, leave it alone. Hornworms can defoliate a tomato plant practically overnight so anything that keeps their population in check is a welcome addition to the garden!