Parasitization by the Tachinid Fly

If you have had a Monarch chrysalis send out these long stringy silk-like threads and wondered, "What the heck is going on?" then your butterfly is dead because it has been parasitized by the Tachinid Fly.

The Tachinid Fly is a parasitic fly that uses the Monarch caterpillar as a host. What does this mean? The Monarch uses Milkweed for its host and lays its eggs on the Milkweed plant, right? Well, the Tachinid Fly uses the Monarch caterpillar for its host!


You probably want to know what this fly looks like and how it does its horrific deed to the poor butterfly, right? Let's explain…

The Tachinid Fly is one of a large number in the true flies family of Tachinidae. They are a bit larger than a housefly and instead of flying erratically, tend to move much slower. The easiest way to identify them is to look for a bristly or hairy fly with big eyes. Once you spot one you won't likely forget it as they are quite distinctive. There are a number of different Tachinid Flies; some have red eyes and others have elongated abdomens but they all have that same bristliness.


The female Tachinid Fly uses her sense of smell and excellent eyesight to find her host, the unsuspecting Monarch caterpillar, while she is out feeding on nectar from flowers. She slowly walks up to the caterpillar and using a piercing ovipositor, injects a fertilized and incubated egg into the Monarch, usually behind the caterpillar's head. The egg will subsequently hatch the 1st instar of the fly which will begin feeding on the hemolymph of the caterpillar. The caterpillar does not die, just yet. In fact, you wouldn't even be aware that your Monarch has been parasitized by this parasitoid because the caterpillar will continue to molt and go through all its instars with the fly inside, continuing its life!

*Note: The Lespesia archippivora does not 'broadcast' its eggs or lays its eggs out onto leaves. It oviposits them onto its host, the caterpillar OR Monarch egg. It also will use other caterpillars as its host.


Visual Signs of Tachinid Fly parasitization

When do you realize your butterfly is dead? You can see visual signs during the Monarch's pupal stage.

Once your pupa shows signs like those pictured above, it usually won't be long before long, silky 'threads' will appear, followed by the 'dropping' of a seed- or bean-like thing.


In time, the adult Tachinid Fly will eclose from this puparium and the life-cycle will begin anew.


This 'bean' is the next stage in the Tachinid Fly's life-cycle; it is the pupa of the fly and you are seeing what is called the puparium, which is the hardened shell of the third instar.


Recommendation from the Editor:

If you find your Monarch pupa has been parasitized by a Tachinid Fly, destroy it immediately by placing the chrysalis and anything else that comes out into a plastic baggy and freezing it. Keep it sealed when disposing the dead Monarch so that if the fly does eclose it won't be able to escape into your garden.

2010–2013 Sherry Skipper Spurgeon All Rights Reserved.