You may have heard by now that coming in late May of this year to Eastern Ohio is the emergence of the 17-year cicada. These noisy but harmless insects are a fascination to lovers of science, nature and math.
When we talk about cicadas we often find ourselves talking about numbers. Numbers high: up to 300 per square yard, several million per acre. Numbers prime: seventeen and thirteen. What is the significance of all these?
Scientists believe that periodical cicadas unique life represented in numbers helps to explain the species’ success. The impressive emergences, including mass number of the insects (more than a million per acre in some areas) is the first key to their success. We see this in other species also – massive schools of sardines in the oceans and undulating flocks of starlings in the autumn take to the skies. Do predators eat them? Of course, lots of them! But they can’t possibly eat them all. Many survive to reproduce and create the next generation. Scientists estimate that 15-40% of adult cicadas are eaten by predators.
This is raccoon scat left on a log in May of 2016. It is made up of mostly cicada shells.
If predators could be ready for this emergence, they would hold the advantage. If each generation of a cicada-eater got a windfall of the insect feast, the predators would sustain higher populations and make larger and larger dents in the cicada populations. But another interesting phenomenon represented by math keeps the predators guessing: prime numbers. The periodical cicadas emerge every seventeen years in the Northern half of the Eastern US and every thirteen years in the Southern states.
This cicada nymph is still in the ground, peeking out of it's hole. It's waiting for the right time, but may be eaten by predators such as woodpeckers or skunks in this phase of its life.
Predators to the periodical cicadas include raccoons, opossums, skunks, blue jays, crows and more! Many of these creatures have life spans of 2-6 years. Spend a moment with me thinking about math. Seventeen is a prime number, meaning that it is divisible only by one and itself. So, creatures with a life span of two years would encounter them only every ninth generation. A longer lived creature like a raccoon may encounter cicada emergences only every third or fourth generation. Even people who may encounter four to six emergences in a lifetime, are often caught off guard by the phenomenon. Patterns made with prime numbers higher than five can be difficult to follow.
If this topic interests you, I’d invite you to watch a short explanation made by PBS that explains more on the math of cicadas. Check out “It’s Okay to be Smart” at http://www.itsokaytobesmart.com/post/124795087737/pbsdigitalstudios-periodical-cicadas-only