Trogus Wasps In Black Swallowtail Chrysalides

Life is like a box of chrysalis- you just don't know what you're going to get.
                - Forest Gump If he had studied entomology

Found in the Bill Roston Butterfly House- two Black Swallowtail caterpillars that had formed their pupa (chrysalis in butterflies, cocoons in moths) and something was wrong.  Dr. Chris Barnhart sent these pictures with the following note.

Black Swallowtail chrysalis

"Found a pretty dramatic parasite in the Bill Roston Butterfly House yesterday.  Two black swallowtail chrysalides didn’t look right so I opened them up- here are pictures:

WARNING some viewers may find these disturbing…."

Note long Ovipositors!

One of the predators that cause the most problems for caterpillars are wasps.  Many wasp species, particular of the superfamily Ichneumonidae (ick-new-MON-ni-dee), lay their eggs in larva (grubs, caterpillars, etc of all types), usually a particular species for each type of wasp.  

Immature wasp larva

Ichneumonids have a long ovipositor which looks like a formidable stinger.  Some species can use them to bore through several centimeters of wood straight into a grub.  This scary appearing appendage is of no danger to you unless you happen to be its favorite caterpillar.


Once imbedded in the caterpillar, grub or other larval host, the parasite larva eats the host's non-vital organs such as fat.  It needs its host to stay alive until it is ready to emerge, chewing its way out of the caterpillar which usually dies afterward.

Trogus Wasp adult emerged

This particular wasp is a Trogus species, likely Trogus pennator.  Like all the other Ichneumonids, it has no common name.  It attacks swallowtail caterpillars except for the Pipevine Swallowtail Battus philenor.  Pipevine contains aristolochic acid which the caterpillar and the adult butterfly retains, causing it to be toxic to attacking predators.  


Researcher Karen Sime discovered that Trogus wasps which sampled Pipevine caterpillars with their antennae flew off without depositing eggs.  They had the same response to their normal prey caterpillars when they were painted with aristolochic acid.

If you haven't had your full dose of ICK! see this National Geographic Video
 but remember, we warned you!