By Howard Whiteman
Murray State University
We all know the sound, something we hear all the time in the summer. But in some places, this year is going to be different.
Unless you have been living under a rock like the cicadas among us, you know what is about to happen. For the first time since 1803, two different periodical cicada swarms are going to emerge simultaneously, 11 and 17 years after they were first born. This convergence won’t happen in the U.S. again for 221 years; the last time they were both emerging at the same time, Thomas Jefferson was President.
The 17-year old swarm, called Brood XII, will emerge in Iowa, Illinois, Indiana, and Wisconsin. The 13-year olds, called Brood XIX, will be crawling out next to the XIIs in Indiana and Illinois, but will also be found throughout much of the midwest and south, including Kentucky, Tennessee, Missouri, Arkansas, and Virginia.
Periodic cicadas are insects within the Order Hemiptera (the “true bugs”) and are closely related to leafhoppers and planthoppers. They are called periodic because they always come out at set times—13 or 17 years from birth— rather than annually, like other cicadas and insects.
During this one year of emergence, males are the ones making all of the noise, in an effort to attract females. Females try to find the best partner they can, mate, and then lay their eggs in small tree branches by creating a hole with their sharp ovipositor, or egg-laying structure.
Six weeks later, cicada nymphs hatch and fall to the ground, unhurt. These small, white larvae use their legs to dig into the ground near plant roots, which is an important food source. The nymphs go through five stages, no matter how long they are in the ground, growing larger and shedding exoskeletons. We often see the final exoskeleton on trees, homes, and lawns as they emerge, become adults, and start looking for mates.
Soon after the eggs are laid, the adult cicadas die, much like spawning salmon. Their bodies litter sidewalks, yards, and forest floors, and eventually break down and help feed the plants and the new generations of cicadas living off of them.
But why the mass emergence? There are advantages to swarms. We know prey will seek a refuge when chased by a predator, like when a rabbit dives into a brush pile or a deer runs toward the woods. Any cover can be a refuge.
Some animals use size as a refuge. That is, they grow so big that they functionally become immune to predation. Elephants and whales share this sort of refuge, except against humans.
Numbers can also be a refuge, and this is what cicadas do. At low numbers of prey, predators can quickly gobble them up. As the number of prey goes up, the number that predators can consume goes up as well, but eventually reaches a plateau where they just can’t consume any faster. So, once prey get above that number, prey consumption rate goes way down. Predators still eat lots of prey, but as a percentage of the population the loss is minimal.
Putting all of those numbers of cicadas out at one time also has another advantage. Not only are predators limited by how many they can eat at one time, but they also can’t reproduce fast enough to take advantage of the resource. Birds that eat cicadas might have a great summer this year and rear multiple broods of offspring, but it still won’t be enough to put a dent into the current swarm. Next year all of those new birds will have to look for a new prey source, as the cicadas will be gone. Once again, advantage to the swarm. As you can imagine, when two swarms emerge together, like this year, the advantage is even greater.
Many other species do the same thing, although in different ways. Masting trees, like oaks, are a great example. In some years red oaks are all reproducing at the same time, and in other years it is white oaks; in some years it is neither. Acorns are eaten, but just like cicadas, during masting years only a small percentage is lost, and predators can neither eat nor reproduce more to catch up. Fish, sea turtles, corals, and many other species do similar things.
Nature is funny that way. Predators and prey are in a unique and interesting game, in which they each try to outdo the other. In this case the predators win some battles and eat a lot of prey, but it doesn’t even dent the cicada population, which goes on happily reproducing and waiting 13 or 17 years for the next swarm.
The cool thing is that we get to enjoy this wonderful phenomenon. Whether it is seeing the cicadas or their abundant exoskeletons, listening to their fluctuating chorus of sound, or watching our dogs feast on their dead bodies, they definitely provide entertainment for all.
Enjoy the show
Dr. Howard Whiteman is Commonwealth Chair of Environmental Studies, Director of Watershed Studies Institute, and a professor in the Department of Biological Sciences at Murray State University.