I’ve probably used the term predator satiation more often during the past couple of weeks than I have during the entire rest of my life. Students of ecology know this as an antipredator adaptation in which prey occur at such high population densities that they overwhelm predator populations.¹ This ‘safety in numbers’ strategy reduces the probability that any given individual will be consumed, thereby ensuring that enough individuals survive to reproduce. With St. Louis currently experiencing the appearance of Brood XIX of periodical cicadas, I’ve gotten lots of questions recently from many coworkers and friends wanting to know more about these cicadas. Often the first question is “What is their purpose?” My standard reply begins with a statement that they, like all living organisms, are the products of natural selection, which then presents an opportunity to explain how natural selection might result in such massive, temporally synchronized, multiple-species populations. A few eyes have glazed over, but I think most have found my answer interesting, often even leading to further questions about where they lay their eggs, what is their life cycle, why are they so loud, how do they “do it” and select mates, etc. Of course, as an entomologist with a strong natural history orientation, I’m always anxious to introduce people to ecological concepts, and right now the periodical cicada is providing a conspicuous, real-life example of such.
¹ Also called “predator saturation,” although this term might be misconstrued to mean that it is the predators that are over-abundant.
A few weeks ago, right at the beginning of their emergence in the St. Louis area, my friend Rich Thoma and I observed predator satiation in action. While hiking one of the trails at Shaw Nature Reserve, we heard the unmistakable shriek and cellophane-sounding wing flapping of a just-captured male cicada. Tussling on the ground ahead of us was the cicada in the grasp of a Polistes carolina/perplexus wasp, which was repeatedly stinging the hapless cicada on the underside of the abdomen. The shrieking and wing-flapping grew less frequent as the stinging continued, until at last the cicada lay quiet. As we approached, the wasp spooked and flew off, but we knew it would be back—we parked ourselves in place while I setup the camera, and before long the wasp returned. It took several minutes of searching from the air and on the ground before the wasp finally relocated her prey, but once she did she began voraciously devouring it. As the wasp was searching, we hypothesized that our presence had altered the visual cues she had memorized when flying off, resulting in some confusion when she returned, and thus the long period of time required to relocate her prey.
We watched for awhile—first the eyes were consumed, then the legs. As it consumed its prey, Rich remarked that he bet he could pick up the wasp and not get stung—likely the entirety of its venom load had been pumped into the cicada. Both of us declined to test his hypothesis. We also wondered if the wasp would butcher the cicada after consuming part of it and bring the remaining pieces back to the nest. We had seen a European hornet do this once with a band-winged grasshopper, consuming the head, then cutting off the legs from the thorax and flying away with it before returning to collect the abdomen as well. No butchering took place this time, however, the wasp seemed content to continue eating as much of the cicada as possible—a satiated predator if there ever was one!
Copyright © Ted C. MacRae 2011