monarch chrysalisThe Question: Can you help solve a mystery with my wife’s first butterfly garden for monarch butterflies? She has had a chrysalis hanging on a watering can but it changed from jade to completely black and now we see it has a 3mm round hole in it. We opened it with a scissor and there in only a remnant left on the inside. We have not seen a successful exit yet. What could possibly have happened? Is this the result of a predator?
Submitted by: John, Florida, USA

The Short Answer: John didn’t attach a picture, but the 3mm (1/8) hole he describes is exactly what is left after a parasitoid fly larvae exits a monarch chrysalis (Danaus plexippus). A parasitoid insect lays its eggs in, on, or near a host, and the larvae then consume the host, often while it is still alive. Tachinid flies, primarily Lespesia archippivora, are major parasitic predators of monarchs, with one study showing that tachinid flies killed 13% of monarch caterpillars and chrysalises. Monarchs often contained three, and sometimes as many as ten, fly larvae. Some people who raise monarch butterflies advocate for killing the fly larvae by squashing any caterpillars or chrysalises that shows signs of parasitism, such as chrysalises that begin to turn dark. I suspect this isn’t particularly effective because it would make such a small dent in the tachinid fly population. I’m not sure there’s much that could be done other than keeping the caterpillars and chrysalises under netting. It might be that you have to just accept that 10-20% of monarch caterpillars won’t turn into butterflies.

For more information and pictures of a tachinid fly exiting a chrysalis, go to:

More Information: There are about 8,000 species of flies in the family Tachinidae, all of which are parasitoids. Their primary prey are butterflies and moths, but there are tachinid species that prey on other types of insects and even some that prey on scorpions and spiders.

The idea of a fly larva eating a monarch caterpillar from the inside out seems pretty awful, and descriptions of this kind of parasitism always make people shudder, especially when the victim is a charismatic creature like the monarch butterfly. But as creepy as the lifestyles of parasites seem, anyone interested in the natural world has to accept that parasites are a major part of every ecosystem. In fact, biologists estimate that parasites make up at least half of all animal species on earth. But studies of animal abundance versus size show that parasites are always less common than other similarly sized creatures, suggesting that being a parasite is actually a tough way to make a living.


Oberhauser, K, Gebhard, I, Cameron, C, et al. (2007). Parasitism of monarch butterflies (danaus plexippus) by lespesia archippivora (diptera : Tachinidae). The American midland naturalist, 157(2), 312-328.

Hechinger, R F, Lafferty, K D, Dobson, A P, et al. (2011). A common scaling rule for abundance, energetics, and production of parasitic and free-living species. Science, 333(6041), 445-448.

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