Here’s a question we get a lot: “what is the purpose of cicadas?” It can be a loaded question, but I think people just want a concrete answer to justify the magnitude of the unusual (why only once every 17 years, why so many) or annoying (inconvenience, noise, ornamental tree damage) aspects of the 17 year cicadas. Every living thing has reason for existing, a niche to fill, a role to play, a purpose — let’s consider how cicadas fit it to the big picture.
It helps to consider perspective when considering purpose. I’ll break purpose down into 4 groups, for this: critters, fungi, trees and people. Critters first, because their relationship with cicadas is the easiest to explain.
The cicada’s purpose in terms of critters:
Cicadas provide a link in the food chain between trees and critters, which I’ll define as any animal that will eat a cicada. Critters love cicadas, and a 17 year cicada emergence is the single greatest feast of their lives. It’s like 17 years of Christmas, Thanksgiving and birthday parties rolled into one incredible month.
Trees feed off the sun and nutrients in the soil, cicadas feed off the trees, critters eat cicadas, and alpha predators (wolves, foxes, bears, cats, game fish, people) eat critters. The massive release of food and energy that comes from a cicada emergence results in an explosion of critter populations, which in turn results a boon for alpha predators as well.
The cicada’s purpose in terms of fungi:
I’m not a fungi expert, but I’m pretty sure different species of fungi have a grand time digesting dead cicada bodies once they’ve died and begin to rot (I’m sure the same is true for bacteria, and microscopic critters). Fungi, of course, become another link in the food chain.
There is one fungus, the Massospora cicadina fungus, that really love cicadas. The Massospora cicadina spreads via cicada mating, and destroys the cicadas entire abdomen in a matter of days. If you’re a Massospora cicadina, from your perspective, the cicadas purpose is to provide you with nourishment and a home. Gruesome, but true.
The cicada’s purpose in terms of trees:
Periodical cicadas are parasites of trees, more specifically of deciduous trees (leaves fall off in the fall) native to the region in which the periodical cicadas exist (maples, oaks, ash, etc.). The term parasite has negative connotations, but in the grand scheme of things parasites can benefit their hosts, or other species by keeping their hosts in check.
Cicadas provide trees a service by pruning the weak branches of a tree. Cicadas lay eggs in the branch, weak branches wither and die (“flagging”), and the tree benefits from that by not having to waste energy on a weak or diseased branch.
Cicadas also do the trees a service by dying, and releasing a vast amount of nutrients back into the soil. When the cicadas die, it’s like dumping bags of fertilizer around the roots of the trees. The extra nutrients should result in an spurt in tree growth and seed production the following spring, which would result in and increase in tree populations (and acorns, which critters love to eat).
A small percentage of small, weak trees will die during each emergence, particularly non-native species (like imported ornamentals). This can be frustrating for people concerned with the landscaping on their property, but in terms of trees in general, it’s not as bad as it seems. The fertilizing and pruning cicadas perform will actually benefit the older trees in such a way that will encourage them to produce more seeds the following year. Any loss of trees will be balanced by gains the following years. Also cicadas may do native trees a favor by weakening or killing non-native ornamental trees, which compete for the native tree’s food.
The cicada’s purpose in terms of people:
Cicadas are a food source. Many people around the world eat cicadas, and not just “on a dare”, but as a delicacy or staple food. Cicadas have made more than one appearance on Bizarre Foods with Andrew Zimmern for instance. S Native American peoples also ate cicadas too — cicadas are a global and historical taste sensation.
Cicadas provide people, including me, with a hobby. There’s a lot worse things you can do with your time.
Cicadas provide artists and musicians with inspiration. There are bands and albums named after cicadas, and many songs inspired by cicadas.
Cicadas provide memories. If you think about it, we people don’t have all that many milestone experiences in our lives: we have our first day at school, graduations, we get our first car, weddings, we buy your first house, children are born, loved ones pass away, special vacations, and maybe we experience a flood, fire or other unfortunate but remarkable event. A periodical cicada emergence is remarkable because it not only places a memorable milestone in the time line of our lives, it places a series of them; a series of milestones, 17 years apart, and not only within our lives, but linking our historical time lines to the time lines of your children, and grandchildren. Gene Kritsky calls cicadas the insects of history, and I think you can understand why.
Recently, cicadas were discovered to have microscopic structures on their wings that destroy bacteria. This is discovery is being used to inspire medical advancements, such as antibacterial cornea replacements. Amazing.