Monarch Caterpillar Fly Parasite

 Monarch Butterflies are one of the best known butterflies in Minnesota.  Many people grow milkweed and protect milkweed plants, on which the Monarch caterpillars feed.

But in nature there are many battles every species needs to constantly fight to have its population stay healthy and survive to reproduce the next generation.

For the Monarch a parasitic tachinid fly is one of the battles it must face.

 This particular tachinid fly lays its eggs directly onto the skin of the Monarch caterpillar.  It will lay up to eight or more eggs on one caterpillar.


The eggs hatch and burrow into the insides of the  caterpillar, feeding on non-vital tissue, so the caterpillar can stay healthy enough to keep eating, providing more food for these parasites.


The fly larvae, which are small white maggots, continue to eat inside the caterpillar until they are ready to pupate, the stage of metamorphosis in which they will prepare to turn into adult flies.  To do this the maggots have to leave the caterpillar and drop to the ground.

 The process of the fly larvae leaving to pupate in the dirt on the ground often happens shortly after the caterpillar has changed into a chrysalis itself, the Monarch's pupa stage.




After the fly maggots eat an exit hole in the caterpillar or chrysalis, and drop to the ground, the Monarch caterpillar or chrysalis always dies.  Its vital organs finally having been eaten by the maggots before they left.







Below are eight pupae of the parasitic tachinid fly shortly after they emerged from a Monarch chrysalis 11 days ago.

 The white maggots turned into these brown pupae very quickly after they emerged from the chrysalis.


They stayed like this for ten days, and began to emerge as adult flies yesterday.

By this morning six of the eight had emerged as adult flies.  These pictures give an idea of what they look like, and the shape and color patterns on their bodies.

The two gray antenna on the front of the face seem slightly different than other flies.

The adult flies are about the size of an average house fly, and it would be difficult at first glance to notice any difference.

The pattern on the back seems a little more pronounced than some flies.

The face down below would only be appealing to another tachinid fly.