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The Fly (actually, a Wasp)

October 11, 2009


An unusual sight met my eyes as I searched for green fruit in my fall tomato plants. This hornworm was clearly parasitized by something as it perched on an uneaten leaf. It didn’t appear to be eating at all, just clinging to life. I headed to the house for my camera and Howard Garrett’s Texas Bug Book to learn the name–I remembered seeing his photos of these things on a caterpillar in the book.

The Braconid wasp (I have removed reference to the incorrectly identified Tachnid fly) hovers and drops down on the caterpillar (according to Howard, much to the caterpillar’s twitching consternation) and lays eggs on the host. They bore into the caterpillar and after feeding for a while emerge in a “barrel-shaped pupal case” (159). That appears to be the stage of this stunted hornworm.

After watching robust hornworms reach the size of my index finger, it was easy to see that this insect’s size was considerably stunted. These things on it’s back weren’t just hitching a ride. There are lots of insects in the garden that set up a beneficial feeding situation for their young by laying eggs with a live host. I’ve watched wasps (tarantula hawks and cicada killers) hunt and dig holes and drag their victims all summer. Here we have a beneficial fly looking for the very worms that demolish whole plants if they are left unchecked (and especially if there are several at work at once).

I cut the leaf and its affixed worm off of the plant and with tweezers yanked these white larvae off of the caterpillar to get a look at the stunted size of the caterpillar and to see what the body looked like where they had clung to it. If they did indeed emerge through the hide of this worm it must have been something to see.

I’m pretty certain there are a couple more hornworms over in my cherry tomatoes. I moved one early yesterday (with quite vivid white lines this time: I have plenty of hornworm photos so didn’t snap that one, but he was very prettily marked) and there are spiky limbs with no leaves nearby that he may not have eaten all by himself. I’ll be looking for caterpillars with unusual lumps emerging and see if I can get some photos. Emerging larvae must make it look like a insect analog of a pineapple or ugly fruit–a really strange appearance at that point (of course, the fruit are supposed to look like that).

That has to hurt. One can find sympathy for that kind of travail in the garden. And be glad it isn’t you (different kind of fly).

3 Comments leave one →
  1. October 20, 2012 5:02 am

    Braconid wasps do this, not Tachinid flies.

    • Maggie Dwyer permalink*
      November 26, 2012 4:37 am


  2. Janel permalink
    August 21, 2010 4:16 pm

    I have a worm that’s doing this right now!! It’s a terrible thing to watch.. I wish I could get good pictures but I don’t have a nice enough camera. My mother thought it was ‘giving birth’ through the skin. I told her it looked like a parasite. The little larva look like someones ‘popping pimples’ all over the worm. We have a big one so there are about a hundred or more larva around it. The poor thing is taking big labored breaths (which reminded us of contractions-leading to the birth idea).

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