They’re coming ! … by the millions … and now we know when!

May 16 to May 20!
The emergence/convergence begins!
Brood XIII periodical cicadas emerge in Jo Daviess County

Periodical cicadas emerge every 17 years in northern Illinois, and this is the year!  Scientists say “emerge” because they won’t be flying in.  They’ve been here all along, underground.  Here’s what they’ve been doing all that time and what they’ll be doing when they show up.

In July of 2007, 17 years ago, Brood XIII periodical cicadas hatched from eggs their mothers had laid that spring in the twiggy new growth of trees.

Brood XIII includes not only our home-hatched cicadas but also millions more which are on the same 17-year cycle across northern Illinois and into contiguous parts of Iowa, Wisconsin and Indiana. Scientists number the broods to keep track of them. See the map to get a look at the location of the various periodical cicada broods. Note that some emerge every 17 years, and some emerge every 13 years. In 2024, there will be a very rare emergence of two different broods in the same year. Brood XIX, which is on a 13-year cycle, will also emerge this spring, in southern Illinois.

But let’s get back to that day in 2007 that our own Brood XIII hatched. The tiny nymphs crawled out of their eggs, fell from the tree limbs to the ground and immediately burrowed underneath, feeding first on juices in the shallow roots of grasses. They dug deeper, eventually reaching a depth of about two feet, where the soil temperature is a comfortable 56 degrees year-round.  Here they spent their days and their years, tunneling and feeding on the juices of tree roots. Summers turned to falls, falls to winters, winters to springs, and springs to summers, as meanwhile, the cicada nymphs slowly grew, shedding their exoskeletons, or outer shells, 4 times as the years passed.

But this year will be different. The nymphs will climb toward the surface as the spring temperatures rise. Scientists have only guesses as to why every 17 years certain broods of periodical cicadas are called to emerge. In fact, every now and again, the timing for some of them goes amok, and they hear this calling early or late. But this is rare, and not the story we are telling here.

The final task of emerging nymphs is to build tunnels to the surface. These tunnels end in a myriad of dime-sized holes, usually clustered under trees. If there has been a heavy rainfall, the nymphs might build finger-sized chimneys on top of these holes.

On April 30, cicada expert Gene Kritsky will predict the date in May, plus or minus two days, when the surface soil temperature will have reached 64 degrees, setting off the emergence of masses of nymphs – hundreds of thousands, maybe millions of them. They will mostly emerge at night, blanketing the trunks of trees or other vertical objects. They will dig in with their nymphal legs and shed their final exoskeleton. It will come apart in the back and they will pull out in a gymnastic move worthy of an Olympian.

Over the course of the next few hours, the nymphs – which emerge snowy white with atrophied wings – will harden and transform into black adult insects, above an inch in length with W-shaped wings spanning two to three inches. Only the dramatic red eyes of the nymphs remain about the same in the adults.

Males will emerge first, congregate on trees and in five days, begin singing in their loud, buzzy choruses. A few days later, the females, who can’t sing, will emerge, and mating begins. Singing and mating will continue as females lay their eggs, usually within a mile of where they emerged. Just so you know, cicadas are perfectly harmless. They can’t sting or bite and they’re even healthy to eat, some think tasty!

After about four weeks of this frenzied activity, all the male cicadas will die, followed by the females, once they have laid all their eggs. Their carcasses will now blanket the soil beneath the trees, providing nutrients for the trees as well as wildlife. An entire generation of periodical cicadas will then be extinct, even as the next generation is waiting in the twigs of trees to hatch a few weeks later and begin the next 17 years of their lives underground.

*For more information on periodical cicadas, see A Tale of Two Broods: the 2024 emergence of periodical cicada broods XIII and XIX, by Dr. Gene Kritsky, Professor Emeritus, Mount St. Joseph University.

Article by Kay Weibel.  Photos (c) Gene Kritsky.