May 29, 2012

A Serious Pest to Monarchs: The Tachinid Fly

Elation ensued when this monarch caterpillar was spotted last week on milkweed in the front yard.  Earlier in the month monarch butterflies were spotted visiting the milkweed and depositing their eggs.   

Imagine my delight when I discovered the caterpillar had situated itself in the window by the front door to transform itself into a chrysalis and then a butterfly.

What luck.  The perfect view of the transformation from inside and outside!  I was overjoyed.

Until I came back the next morning to photograph the process and found this...

There is a serious predator in the garden this spring.  The tachinid fly is a parasitic fly that uses the monarch caterpillar as a host.  This bristly, big eyed, slow moving fly stealthily approaches the unsuspecting caterpillar and injects it's fertilized and incubated egg into the caterpillar.  The egg then hatches inside the caterpillar and the first instar begins to feed on the monarch slowly killing it from the inside out.  Monarch caterpillars are not the only victims, other caterpillars fall prey to this fly as well.

Jenny of Rock Rose solved the mystery.  We were chatting Saturday at the monthly Austin bloggers Go-Go and I explained that when the caterpillar began to dry out a silky string was hanging from it's head.  She did a little research and forwarded a link to me about the tachinid fly.

What really bums me out is that I didn't dispose of the caterpillar the way I should have and will most likely have more of the pest in my garden.  I learned a valuable lesson.  When something strange happens, no matter how busy you are, investigate.


  1. If I remember right some researchers were using some species of tachinid fly as a method of pest control for worms in crops. And it seems some were going to be used in fire ant control. Sorry to hear bout your loss. I haven't had any larva yet on my milkweed.

  2. I'm so sorry to hear about your Monarch caterpillar. I planted some Milkweed to try help the Monarch population. Thank you for posting this and educating others, like myself.

  3. and learn:) I wouldn't have know that either. But next time you'll be ready:)

  4. I read Greggo's comment above, so it seems it is not only the GMOs which kill milkweed is the enemy of monarchs, there still is that biocontrol fly. Crops versus butterfly! Where do we go?

  5. Fascinating...I hope to see monarchs soon and will be on the lookout if this happens.

  6. red in tooth and claw maybe but what a disappointment for your nature study.

  7. Sorry to hear about your monarch. At least now you know what to look out for and how to handle tachnid flies and the dead caterpillar, too. Do the tachnid flies have any beneficial aspect, gardenwise?

  8. So sad! I read that only 2% of the caterpillars actually make it to adults. Ironically when the parasitic wasp lays its eggs on the tomato horn worm gardeners are happy and remove the caterpillar from the plant but keep it alive to ensure that the eggs hatch. What a gruesome death to be eaten alive from the inside! Nature is so very cruel sometimes! I hope you have other monarch caterpillars that will be more successful!


Piglet sidled up to Pooh from behind.
"Pooh!" he whispered.
"Yes, Piglet?"
"Nothing," said Piglet, taking Pooh's paw. "I just wanted to be sure of you."
~A.A. Milne

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