It's a Double Brood of Periodical Cicadas
A double brood of cicadas will emerge in the Midwest and Southeast in late April through the middle of May. We’re talking about trillions of red-eyed, deafening insects. They’re actually pretty good Samaritans. Find out what to expect: where they’ll be, how long they’ll stay, how they make that noise, and whether they are harmful to plants.
When Will the Cicadas Emerge?
A ‘Double Brood’—Brood XIII and Brood XIX—will co-emerge, an event that happens only once every 221 years! The last time this occurred was 1803, the birth year of Ralph Waldo Emerson, Thomas Jefferson’s third year as president of the United States, and the year of the Louisiana Purchase.
Expect to see these critters arriving from late April to the middle of May 2024 and sticking around just long enough to finish growing, molting, and mating—around four to six weeks.
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Where Will the Cicadas Be?
The two broods are predicted to emerge in 17 midwestern and southeastern United States. Experts have noted that while they are co-emerging, the broods won’t necessarily cohabitate.
- Brood XIII, also known as the Northern Illinois Brood, is a 17-year brood found mostly in northern Illinois. It also appears in central Illinois, northern and central Missouri, southern Wisconsin, eastern Iowa, and northwestern Indiana.
- Brood XIX—the Great Southern Brood—is a 13-year brood that visits northern Louisiana, southern Arkansas, Alabama, Mississippi, northern Georgia, western South Carolina, North Carolina, eastern Tennessee, northern Arkansas, southern Missouri, southern Illinois, and western Kentucky.
Although geographically adjacent in Illinois, the broods won’t overlap much, and it’s unlikely that cicada density will double anywhere.
Are All Cicadas Periodical? (What Does Periodical Mean?)
Periodical organisms emerge simultaneously after a set number of years—they’re extremely rare! But not all cicadas are periodical: Emerging every year and in far fewer numbers, annual cicadas are predominantly green, while periodical cicadas have red eyes and red wing veins. They’re both around an inch long.
Although annual and periodical cicadas are easy to tell apart visually, the different species of periodical cicadas are not. These include the seven species of the genus Magicicada, which emerge every 13 to 17 years in North America (hence the term 13- or 17-year brood).
Under that genus, there are three species groups: Decim, Cassini, and Decula. Within those groups, there are three species of 17-year broods and four of 13-year broods. Each brood is named in reference to the year it emerges. For example, Brood I last emerged in 2012, Brood II in 2013, Brood III in 2014, and so on. There are 12 broods of 17-year cicadas and three broods of 13-year cicadas.
Why And How Do Cicadas Emerge?
Cicadas can tell time! Sort of.
The 13 to 17 years living underground help cicadas avoid predators. Basically, they emerge in big enough numbers that predators like birds, raccoons, and mice can have a veritable feast while still leaving plenty of cicadas to perpetuate the species.
Female cicadas lay eggs, and after they hatch, baby cicadas—nymphs—burrow underground and feed on the xylem in tree roots. (The xylem is a vascular tissue that transports water and nutrients from the soil throughout the tree—tree sap!) While underground, the cicadas seem to count the seasonal cycles of the trees. When the right year rolls around, they await a soil temperature above 18ºC or 64.4ºF, and then they emerge.
The cicadas may also “talk” to each other to make this decision. There are variations in soil temperature impacted by elevation, tree cover, and sun exposure. The sunnier the area, the earlier the cicadas swarm. Because of this, it could take a whole brood up to a month to emerge in full. But interestingly, nymphs reared in a lab for the last nine months of their growth still emerged simultaneously. Whatever cues they have must be set in advance.
Are Cicadas Sleeping Before They Emerge?
Cicada nymphs aren’t necessarily sleeping underground: They’re eating and growing.
But they can’t move much, so the trees may kill them with roots in a number of ways: over-growing callus, smothering them with gums, or poisoning them. Since the cicada nymphs can’t move, they can’t escape.
How Do Cicadas Make That Loud Noise?
First of all, not all cicadas sing! Only male cicadas make that characteristic cicada sound and don’t expend much energy to do so. In fact, they make the loudest sounds in the insect world, possibly hurting predators’ ears when they roar in unison. But unlike crickets or katydids, which rub their legs together to make noise, the secret to a cicada’s song is its tymbals. Each male cicada has one on either side of its abdomen—thinner, even, than human hair. Cicadas rapidly vibrate these tiny membranes to produce sound, and the tymbals act like two speakers that can perfectly overlap sound waves, pumping up the volume. A single tymbal can’t achieve this.
While the songs might all sound the same to us, cicadas sing multiple, species-specific songs in order to attract females to mate. Female cicadas signify readiness to mate by rubbing their wings together and making a clicking sound. If you’re curious, you can tell male and female cicadas apart by flipping them over. Females have a groove on their abdomen known as the ovipositor (organ used to lay eggs), while males do not.
Are Cicadas Harmful?
Yes and no.
Cicadas aren’t harmful to humans—they don’t bite or sting—nor are they toxic to pets if eaten in small quantities. That said, if you have a shellfish allergy, you may want to avoid a cicada snack yourself!
Though they won’t harm your garden, cicada nymphs are technically tree parasites because they feed on tree root xylem. Yet, this hasn’t been shown to impact tree growth. If anything, because they feed on and digest xylem, cicadas are great for the earth and other organisms. Xylem itself is not very nutrient-dense, but when cicadas die, they leave behind bombs of nutritious carcasses that increase the biomass of soil bacteria and fungi, fertilizing everything. Studies have even shown a flush of tree growth in the two to five years following cicada emergence, as well as a correlation with reduced numbers of and damage created by other above-ground herbivores.
On the other hand, cicadas can damage trees—and even kill saplings—by cutting egg nests into their branches. Growers and nurseries cover young trees with netting or simply take care not to plant saplings during years of periodical cicada emergence. But for the most part, the damage to mature trees is minor. Adult cicadas probably feed on trees once they’re out of the ground, too, but it’s unclear how much they feed.
In conclusion, cicadas may be loud, but they’re not really pests. If anything, they give a lot back to the environment. And hey, who isn’t guilty of talking a little too much and a little too loudly around their crush… Just me?
Lucy Mutz
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