Prime-Period Cicadas

Depending on where you are in eastern North America, every 13 or 17 years something magical happens, figuratively and literally. Out of the soil emerge broods, all at once, of Magicicada, otherwise known simply as cicadas.

For most of the lives of these insects, they exist underground, sucking up the juices from deciduous trees. When they are ready, they emerge as adults synchronously. This appearance occurs in a location with incredible periodicity.

Magicicada cassini - Flint Ridge, Ohio, USA, 28 May 2016 - photo by James St. John

After spending more than a decade (or close to two!) in the soil, the crescendo of a cicada’s life occurs in a relative flash. They live in our realm for approximately four to six weeks before they perish. This month in the sun is the window for reproduction. Males sing the songs of their species, hoping to attract a female mate. In aggregate, the males form choruses, a humming harmony with which most people are familiar. This symphony can reach 100 decibels! I find the soaring din to be incredible and calming; others find the noise to be deafening and grating. No matter your perspective, the females love it and the result is a lot of babies.

After mating, the females cut V-shaped slits in the bark of young twigs and insert approximately 20 eggs into each one. The female produces a total of 600 or more eggs. Six to 10 weeks later, the eggs hatch and the nymphs fall to the ground, where they burrow into the soil. From there the 13- or 17-year period begins anew!

If you see slits such a these in a tree, chances are cicada babies are or were incubating there! - photo by lorax

Research data indicate nymphs can drop at up to 1.5 million individuals per acre! This synchronized drop is a form of survival strategy called “predator satiation.” As you can imagine, newborn insects on the ground are easy pickings for all sorts of critters, such as birds, squirrels, or reptiles. Those animals will have their fill – until they are satiated – and many nymphs don’t make it. But with so many all dropping at once, the species is able to survive the feasting.

The remains of a swarm - photo by Karen Kasmauski

Why the incredibly strange 13- or 17-year periods? One idea asserted the periods being in multiples of relatively large prime numbers made it more difficult for predators to adapt their own generations. Predators receive a population boost when a brood emerges. Studies have shown significant bumps when analyzing mole and turkey populations in emergence years. But having a long cycle that happens on prime numbers can help prevent predators from matching life cycles. For example, a 12-year cycle could be severely hampered by any species with a life cycle of two, three, four, six, or 12 years. The prime numbers would go a long way to preventing that overlap.

Despite the sense the above hypothesis makes, scientists now widely consider it a “short-term maintenance strategy.” They believe the greater force at play is the ability to prevent the hybridization of the different broods during cycles of high selection pressure. Essentially, since the glaciers receded and the earth warmed, pressure on cicadas has increased. Keeping the number of broods as high as possible allows for a greater possibility of overall survival. If the broods emerged at the same time, they might cross-mate, decreasing the number of broods. Mathematical models support this theory and it is now the prevailing notion.

US Forest Service map of periodical cicada brood locations - slightly outdated, though still accurate; some broods overlap geographical areas

Periodical cicadas are grouped geographically into broods.  In 1898, entomologist C.L. Marlatt assigned Roman Numerals to the different broods. This system is still used to discuss the groups. The map above shows, roughly, the zones for each brood. Keep in mind, though, the broods can actually overlap, which illustrates the hybridization theory well. For example, the map shows Brood II in New Jersey, which emerged in 2013 and will appear again in 2030. However, I witnessed Brood X’s magic in 2004 in central New Jersey. Various resources are available online to discover the brood that might exist where you live.

2020 is the time for Brood IX to shine. Southwestern Virginia and southern West Virginia are the prime locations to see a swarm of cicadas this year. So, despite the rash of articles making the rounds on the internet and other media, you’ll have to be in a specific region if you want to catch them this year. The crucial point is when soil temperatures about 20 centimeters below the surface reach about 18 degrees Celsius. This trigger arrives in late April or early May in the south and late May or early June in the north.

In general, most insects seem repulsive to many humans. Cicadas are sometimes confused with locusts, which can be overly destructive on the landscape (Africa is currently in the midst of a locust inundation). Our periodical cicadas, however, pose very little threat to humans or plant life. Some twig damage and die-off occur due to the laying of eggs, but in general mature vegetation is unaffected. They get their nutrients below the earth! They don’t sting, are not venomous, and rarely bite. What they are, though, is beneficial to the soil. When they die at the end of their cycles, if they aren’t immediately cuisine for local predators, they decompose, adding a nice dose of nutrients to the soil.

If you have the chance to witness this incredible natural magic, move past the fear or disgust of insects and experience it! If you don’t, you’ll have to wait a long time to get another chance.

Further Reading and Exploration

Cicada Mania – website dedicated to “the most amazing insects in the world”

Magicicada – Brood mapping project

17 Year Periodical Cicadas – Planet Earth BBC Video – Fantastic footage here

Cicada Song and Chorus Database – from Cicadamania

The Bizarre Life Cycle of a Cicada – by Greg Roza

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