Tribute to Howard Ensign Evans

While in Fort Collins, Colorado last month I spent a few minutes peering into small holes in the ground.

Why? This hole was not made by ants.

Instead, the hole was constructed by a digger wasp. By looking for wasps in action, I was paying tribute to Dr. Howard Ensign Evans, who spent most of his life studying digger wasps. Dr. Evans was a professor at Colorado State University in Fort Collins for many years, after working at Kansas State University and Harvard. Dr. Evans passed away this month (July 18) in 2002. I’d like to think that perhaps this wasp was the descendant of a wasp that he had once watched.

In general, digger wasps, such as this one, build burrows under ground as a place to raise their young. The female wasps capture various arthropods to feed to their offspring, and the type of prey taken depends on the species of wasp. Potential prey ranges from bees to spiders.

One she has captured a prey item, the wasp carries it into the burrow. There she lays an egg on the paralyzed arthropod. The larval wasp hatches out and consumes the arthropod meal over a period of days. When it is finished, it pupates and later emerges as an adult wasp.

You can see a digger wasp constructing the start of her nest in this video.

Over the years Howard Evans studied many, many different species of these wasps and carefully recorded all the different variations in their behavior. Some species only brought in one prey item and left it. Others brought in a series of prey over time. Some closed their burrows each time they went hunting, others left the entrances open. Some had simple tunnels for nests, others had more complex branching structures.

Most of these solitary wasps carry their prey under their bodies with their legs. Evans studied the wasps of the genus Clypeadon that grab worker harvester ants as their prey, sting the ants and then carry them back to their burrow attached to the end of their abdomen. As you would imagine, that looks quite unusual. Previous workers had thought the ant was carried on the sting, but Evans showed the wasp actually has a structure on the tip of its abdomen that is used for transporting the ant.

Over time, Evans put together a careful map of how all these behaviors were related, and how they had become modified. He produced a great body of work that is still being used and cited today.

In addition to being an accomplished scientist, Howard Evans was also a prolific nature writer. I discovered some of his more popular works, such as Wasp Farm and Life on a Little Known Planet, early in my career and they definitely influenced my interest in the family Hymenoptera. I’m sure many other budding entomologists have had a similar experience.

So, it seemed fitting to spend a few minutes watching a digger wasp and remembering, as a tribute to Dr. Evans.

See a more extensive biography at Howard E. Evans (February 23, 1919–July 18, 2002) by Mary Jane West-Eberhard

Alexander, B.A. 1985. Predator-prey interactions between the digger wasp Clypeadon laticinctus and the harvester ant Pogonomyrmex occidentalis. Journal of Natural History. 19:1139-1154.

Have you read any of Howard Ensign Evans books?

The Sand Wasps: Natural History and Behavior by Howard Ensign Evans and Kevin M. O’Neill, published by Harvard University Press, 2007.