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Hornworm Caterpillar: Friend or Foe?

This year we expanded our garden and tried to be creative in how we used our space.  Several old wooden crates were transformed into tomato planters.    

One day I happened to notice what I thought was a green tomato that looked like it had a fungus on it.  I was in a hurry to get inside and I did not carefully examine the tomato, I just made a note to myself to tell Greg about it when he got home.  

That evening Greg called me from outside.  He said I had to come see this.

What I thought was a tomato was in fact a dreaded hornworm caterpillar.  What I mistook for fungus was actually little rice like pods hanging off its body.

Immediately we began to research.  What looked like grains of rice are actually the cocoons of the braconid wasp, which lays its eggs under the skin of the caterpillar.  The pupae emerge and spin the cocoon and several days later emerge as wasps.

While injecting its eggs, the wasp also injects the caterpillar with a neurotoxin so that it stops eating and basically just hangs out letting the larvae mature on its fluids.  

I posted all this on my Instagram account real time.  I had some great conversations with people wanting to know if the caterpillar was hurting (according to my research it is not, but how can one ever really know?) and wondering why I just did not pluck the caterpillar from the plant and kill it. 


I was hoping that the video would be larger.  Watch the upper right side of the caterpillar. 
 A wasp slowly emerges.

Once the caterpillar is infected, it is rendered harmless to the tomato plant.  The wasps are actually beneficial insects.  The wasps eat aphids and as they host on hornworm caterpillars, they keep their population down.  By letting the wasps hatch, the continuation of the braconid population is advanced, and I have organic pest control!  


  1. Ugh, ever since I was in 4-H as a kid, those hornworm caterpillars have grossed me out. Other caterpillars don't, but those ones are so big and juicy. Ick!

    Fascinating info on the wasps!

    1. This was a fascinating topic to research. The night we discovered them we were up until midnight watching videos and reading information about them. Every year we learn something new about nature and gardening.

  2. i had so much fun watching this story unfold on Instagram! Glad you posted here! It was definitely an experience I am sure I will never see again...

    1. Instagram is really amazing. At times I feel like I am doing twice the work by posting much of what I Instagram but I want to add more and be able to print a book at the end of the year. If/when Instagram becomes more functional for printing and archiving, I will probably leave blogging.

      I am glad you enjoyed following along!

  3. I agree with Melissa. I loved watching it on IG. I even sent a picture you took to my brother, the herb guy? ;), and he had heard of them but loved the pic. I need some of those wasps. We have millions of wooly aphids. They look like dancing white fairies. Keilee tried to take a picture but they were too small. We did research when we saw them too. I have always thought they were white fuzz drifting around. It is SO awesome to be able to watch something in real time and research it.

  4. Last weekend we were trimming trees and bushes in the front yard and we found one of these. I had found those caterpillars before eating up all my tomatoes. So I just assumed it was the same and those were her eggs or something of the sort. We got rid of it. I will be posting my picture on my monthly learning log and link it to your wonderful explanation about it. TFS!


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