Friday, June 10, 2016

Ichneumon Wasp Trogus pennator

The parasitoid ichneumon wasp Trogus pennator.

     The natural world is intricately connected and isn't always what some people might consider nice in how it works. Take for example the colorful Ichneumon Wasp pictured above, Trogus pennator. It is about an inch long and the the iridescence of its wings can make the colors look variable. This pretty wasp is harmless to us (its coloration is thought to perhaps mimic stinging wasps to protect them from predators), but certain caterpillars need to take care when she's around.

Trogus pennator wasp searching for Tiger Swallowtail caterpillars ona Tuliptree leaf. Notice how the iridescence can alter the perceived color of the wings.

     All 6 members of the Genus Trogus in North America are parasitoids of swallowtail butterfly caterpillars. "Trogus" comes from the Greek for "gnawing." "Penna" is Greek for "feather wing" and the wings are indeed attractive. Ichneumon wasps like Trogus in the subfamily Ichneumoninae all need caterpillars as hosts for their young. Adult females find certain caterpillars and inject a single egg into them. The caterpillar continues to feed and grow as though healthy, but that is an illusion. It is walking dead, doomed to die when it pupates with the Ichneumon emerging from its zombie host. That is why they are called parasitoids rather than parasites. Both terms imply an organism feeding and harming the host, but parasitoids always kill the host, where parasites do not always result in death. 
     Trogus pennator is no different. Adults may feed on nectar or honeydew, but the females need caterpillars in order to reproduce. They hunt for Swallowtail Butterfly caterpillars (Family Papilionidae), apparently being able to use any of our native ones as hosts. These wasps seem to show a distinct preference for searching on the host plants of swallowtail caterpillars, somehow being able to identify them visually or perhaps even chemically. They always seem to hunt in such swallowtail host plants as pawpaw (Zebra Swallowtail host plants), tuliptree (as in the photos where Tiger Swallowtail caterpillars would be feeding), cherry (Tiger Swallowtail hosts), sassafras (Spicebush Swallowtail hosts), spicebush (Spicebush Swallowtail hosts), snakeroot (our native Pipevine Swallowtail host), pipevines (where planted since not a locally native plant for Pipevine Swallowtails), and members of the carrot family (Black Swallowtail host plants). They also then seem to target leaves with damage, like what a feeding caterpillar would leave.
     When a swallowtail caterpillar is found, she injects a single egg and the zombie-like life of the caterpillar begins. The wasp larva feeds inside, but does not destroy the vital organs, allowing the caterpillar to continue to grow. Eventually the swallowtail caterpillar forms its chrysalis. But what emerges is the adult wasp rather than a swallowtail butterfly. This endoparasitoid exits through a small circular hole in the side of the chrysalis, leaving just an empty skin behind. The wasp then looks for a mate and starts the life cycle all over again.  
     While they may kill swallowtails, they do not eradicate them. Keep in mind that the only things the larvae of these wasps can feed on are swallowtails. It would make poor evolutionary sense to kill off all of the only host you can survive on. So they do kill some, but not all, as they have evolved together to coexist. That involves behavioral adaptations by the swallowtails as well such as choosing low places to lay eggs which are less attractive for wasps to search or feeding at night such as the Zebra Swallowtail caterpillars do when the wasps (and birds) would be sleeping. 

An empty swallowtail butterfly chrysalis. Notice the circular exit hole where the wasp emerged after consuming the host.

     So this may seem cruel, but these are just all parts of the nature puzzle. People who know me know that I like to use a puzzle analogy for the natural world. It's a puzzle because certain pieces fit together (such as the swallowtails needing certain plant caterpillar hosts and Trogus needing swallowtail caterpillars). As a naturalist, I try and solve as much of the puzzle as I can, knowing this will be life long learning. But as any puzzle maker knows, you need all the pieces or you can't really complete the puzzle. As the great naturalist Aldo Leopold is credited with saying "the first rule of intelligent tinkering is not to lose the pieces." Unfortunately we are losing pieces through extinction and loss of habitat all the time. 
     We also know that you can't take a piece from another nature puzzle where it fit perfectly well and stick it into another puzzle. That causes other pieces to no longer fit well. This is what happens when we introduce invasive exotic species into new habitats. I love trying to learn how the pieces fit together, even if they are sometimes a bit gruesome in the way they do so. The life cycle of this colorful wasp is just one example of the fascinating puzzle of a natural world we all live in. 


  1. Have you read the book Tropical Nature? The puzzle involving oropendulas, giant cowbirds, bees, and bot flies is a stunning demonstration of the complicated relationships that can result from natural selection. Ben Jesup

  2. Thanks for another great post, Alonso. The capital region is incredibly lucky to have you!