Monarchs and tachinid flies

A monarch butterfly chrysalis is usually a brilliant color spotted with golden dots.

Photo of a chrysalis on a tree

However, sometimes something goes wrong — and one day, we spotted a chrysalis that was beginning to turn brown.

At the bottom of the container that the chrysalis was in inside of our monarch enclosure in Pollinator Paradise were three brown, egg-shaped items.

Photo of tachinid fly pupae

It turns out that this caterpillar had been parasitized by a tachinid fly, which ended up killing the monarch in its pupal stage as a chrysalis.

There are more than 1,300 species in the family Tachinidae in North American alone, all of which are types of parasitic flies. Most tachinids attack caterpillars and beetles, but some also affect other insects and arachnids.

What typically happens to a monarch is that a tachinid fly will lay an egg on a monarch caterpillar. It will hatch and the larvae will bore into its host, where it completes its larval development. At about the time when the caterpillar begins to change into a chrysalis or after it has changed, the fly maggot will emerge as a pupae and will drop. It no longer needs its host, and the host dies.

One monarch can host several tachinid larvae or pupae.

 

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