Tuesday, June 15, 2010

The Tachina or Tachinid Fly Strikes Again

As usual, I'll give you the good news first.  Another chrysalis formed early this morning, and 3 more caterpillars are ready to pupate.  In addition, 3 more are almosted finished eating and will be ready to pupate within the next 24 hours.

Next, the bad news.  A couple of days ago, one of our caterpillars made its chrysalis on one of our Milkweed flower pots.  Yesterday, I noticed that one spot on it was turning a slight brownish color.  I thought maybe it had gotten injured or something but would be ok.  However, this morning, it is clearly dead.  (see photo below)

(Silky string from chrysalis to the ground / Tachinid fly larvae kill a Monarch chrysalis)

The Tachinid fly lays eggs on the caterpillar.  The fly larvae eventually kill the Monarch caterpillar around the time it tries to pupate.  Or, sometimes they wait until the caterpillar has become a chrysalis, as shown in the above photo.  The silky string connected from the chrysalis to the ground is the tell tale sign that the Tachinid fly has struck.

This is probably the 6th or 7th caterpillar of mine that has died due to this parasitic fly.  At the nursery where I'm currently getting my Milkweed plants, the plants are out in the open, and the flies can openly get to the developing caterpillars there.  So, my wife suggested that in the future when we buy more plants, that we remove all caterpillars and put them on other plants.  However, any eggs we could leave, and these would not have been infected with the fly larvae yet.  I think it's a marvelous idea.  This will help us to get the most out of our Milkweed leaves too.  You just hate to invest all of that Milkweed and then get nothing out of it.

I don't mean to say that the Tachinid Fly is all bad.  It preys on other garden pests such as Japanese Beetles, Squash Bugs, Stink Bugs and more.  However, since it preys on my beautiful Monarch caterpillars, it is a neusance to me.  Hopefully, keeping my babies in an enclosure will do the most help.

One note: Please be careful not to use any poisons or pesticides to get rid of pests such as these flies because it will kill your butterflies too.  The Milkweed sap they consume is poisonous, and therefore, the Monarch is full of poison already.  It's body just can't tolerate any more.  So, look for natural ways to control pests and predators.

Perhaps these observations will help you too.  Be sure to keep your caterpillars in a protective enclosure at all times.  In my next Blog entry, I'll show you a pavilion that I have that has worked great for me.

Happy Monarching!

Caleb & Janae Warren


  1. I have tons of those flies this year, and we have had the same problem with our monarchs. We have "raised" monarchs for years, and this is the first. We lost 9 out of 11 in the first batch, 2 chrysalis; 3 out of 6 in the next batch. Wondering if I need to "spray with chemicals" in order to stop the flies from coming and save the plant for next year. Any suggestions from anyone?

  2. The much maligned and misunderstood tachinid fly strikes again. First, Lespesia archippivora, the one that feeds on monarch butterflies, is only one species in a family of thousands (1300 species in our area). They parasitize a huge variety of species, many of them pests, such as stink bugs, tent caterpillars, earworms, and many others. In fact they are used as biological controls. The one that feeds on monarchs is also a biological control of armyworms. So, paradoxically, it helps monarchs by allowing to reduce pesticide use. I empathize with your feelings if the ones you are lovingly raising fall victims to this parasitoid. It won't do any good to kill them, though. No chemicals, please. http://bugguide.net/node/view/264987

  3. This comment has been removed by the author.

  4. I have only been able to have 2 Monarchs this year out of 140 caterpillars total, due to this fly. I hung a fly trap but there must be a way to eradicate it from my yard without poisoning my milkweed. Please give us suggestions.