Over the next few weeks, some of central Illinois will experience a rare phenomenon that only occurs every other decade.
As soil temperatures warm, millions of insects will emerge from the ground in forests, city parks, yards and gardens. They will carefully navigate the terrain and scale a nearby large object (such as a tree trunk) to shed their nymphal skin and enter the world as adult cicadas.
These cicadas are a part of what’s known as Brood X (the Roman numeral 10; there are 15 total broods), which makes its appearance across Indiana, a tiny chunk of Illinois and portions of 15 other eastern states once every 17 years. They are one of many periodical cicada broods that emerge across the Eastern and Midwestern U.S.
at regular intervals of 13 or
17 years. These incredible insects are some of the longest lived on the globe and only occur here in the United States.
You might be asking yourself, “What is a cicada brood?” Well, the simple definition is that it is a group of cicadas that emerge on a common schedule across a geographic area. However, when we look a little deeper, it becomes apparent that a single cicada brood does not comprise just one species, but includes several.
All species of periodical cicada belong to the genus Magicicada, and Brood X typically includes three species (M. cassini, M. septendecim, and M. septendecula) of 17-year cicadas.
“It’s a natural wonder of the world to see insects emerge in these kind of numbers and the sheer biomass of these cicadas is incredible,” says Katie Dana, an entomologist with the Illinois Natural History Survey who studies periodical cicadas, as well as many other species that occur in Illinois and across the Midwest.
For most of their life, periodical cicadas are underground, quietly feeding on tree roots and molting at regular intervals. While the first molt for these cicadas typically happens in their first year underground, subsequent molts generally occur on a four-year schedule in the 17-year cicadas and faster in the 13-year cicadas. So, all periodical cicadas emerge either at 13- or 17-year intervals, based on the timing of these molts.
However, there are always those that break the mold, and the “common schedule” of a particular brood can be confused by some individuals who emerge four years, or one molting cycle, earlier than the rest.
In 2017, there were some early emerging Brood X cicadas, and Dana collected samples in western Indiana for later comparison.
“This year, I am going to be collecting cicadas at the same locations I found them in 2017,” she said. “I plan to compare things like size and morphology, and hopefully, we’ll be doing some genetics work to see if the early emerging individuals
may potentially be setting up a new brood.”
That’s no easy task, as predators love the influx of these large, tasty insects. If too few emerge early, they may be entirely eaten by predators before they can complete their lifecycle and perpetuate a new brood on a different four-year schedule.
Once above ground, cicadas have a short interval to mate and lay eggs before they die. Females lay their eggs in tree-branch tips and tiny nymphs hatch in just a few weeks. They drop to the ground only to burrow down in search of tree roots they can feed upon for years to come.
While some cicada damage may be observed in branch tips that die back after eggs are deposited, the damage is generally negligible. At the ecosystem level, cicada emergence is actually a huge benefit, as these insects release a burst of nutrients from the soil that they have collected over many years.
“Its free fertilizer for plants,” noted Dana, “and even though they have bright colors that suggest they might be poisonous, they’re not, and they actually make a really great meal for a lot of native wildlife.”
There are already reports of Brood X emergence in Indiana, so they should start to appear in their expected range in Illinois, which only covers Clay, Crawford, Edgar and Vermilion counties, at any moment. From early May until about mid-June, they will emerge and belt out their mating call, which can be incredibly loud in areas of high concentration. This timing is much earlier than annual cicada species that emerge later in summer each year.
Dana and other researchers are really interested in exactly where and when these insects emerge across central Illinois, since we are at the edge of their known range. If you are interested in reporting the timing and location of cicada emergence, please download the “Cicada Safari” mobile app at cicadasarfari.org) to report your sightings.
If you observe Brood X cicadas in Illinois outside of the four counties listed above, contact Dana directly at firstname.lastname@example.org or on Twitter (@ONoKatyDidnt). Her lab is interested in following up on any sightings outside the expected range, and you can help contribute to this important research effort.
Ryan Pankau is a horticulture educator with UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his ‘Garden Scoop’ blog at go.illinois.edu/GardenScoopBlog.