Parasitic Wasps

The three-sided log shelter at the trail overlook was a welcome sight for sore legs. High on a bluff above the Mississippi River, naturalists on a field trip stopped to catch our breath. The view was breathtaking. Eagles soared over the vast channels of the Mississippi River, and through binoculars we could see turkey vultures riding the air currents over bluffs on the opposite side.

We admired the view of water, rocks, trees, and birds for a little while, but soon got distracted by other things. While Richard Louv, author of “Last Child in the Woods,” praises nature as an antidote to attention deficit disorder, I find that it actually causes my attention deficit. I hardly have time to admire or investigate one thing, when another critter or question pops up. Sometimes, when something catches my eye, I change topics mid-sentence. That didn’t happen before I became a naturalist.

When our guide paused during his stories about the history of the river, a fellow easily- distracted naturalist piped in, “What are these holes in the ground?” We all stopped to look, and found that the packed-dirt trail was dotted with half-inch diameter holes. What could make such a small hole in such hard dirt? The guide continued with his spiel, but all eyes were on the ground. “Pssst …there’s something in this one.” I crawled over with my camera to peer in the hole, and found a little head, with two antennae, and two big compound eyes looking up at me. 

“I just saw one go in this hole!” called someone else. “Here’s one in the grass! And it has something!”  Perched on the dangling seed head was a small, black wasp with a few yellow stripes. Its feet clasped tightly to a drab gray insect every bit as big as the wasp itself.

Ah ha! The mystery was solved. These holes were nurseries dug by parasitic wasps. More than 100,000 species of wasps are parasites, and they each do a version of what this wasp was about to do. They stun or otherwise subdue their prey, often using coma-inducing chemicals injected with their ovipositor; hide their prey in a safe spot, if it wasn’t already there; and lay eggs in or on their prey, using their long, piercing ovipositor.

When the wasp larvae hatch, they have a fresh meal. Most parasitic larvae even eat their prey in a set pattern–nonessential parts first—to keep it alive and fresh as long as possible. Body fat is the appetizer. The main course includes the prey’s digestive organs, and its heart and nervous system are saved for dessert. Some wasps even pupate right inside the hollowed out shell of their buffet.

Once the wasp larvae metamorphose into adults, their diet changes to nectar, a much less gruesome fare.

Almost every pest insect species has at least one wasp species that preys upon it or parasitizes it, making wasps critically important for agricultural pest control. In fact, nearly all insects are attacked by one or more insect parasites, and some parasitic wasps are parasitized by other wasps.

Panoramic views of the Mississippi River almost seem tame alongside such drama in the diminutive world of wasps!

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI, at 13470 County Highway M. The current exhibit, “Deer Camp: A Natural and Cultural History of White-tailed Deer,” opened in May 2013 and will remain open until April, 2014.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.