Dual Cicada Emergence: A Historic Natural Event 221 Years in the Making

In the spring of 2024, the United States witnessed a biological event of staggering proportions that had not occurred since Thomas Jefferson was president. Two distinct broods of periodical cicadas, Brood XIX and Brood XIII, emerged from the soil simultaneously. This rare synchronization brings trillions of these noisy insects to the surface at once, creating a spectacle that defines the season for millions of residents across the Midwest and Southeast.

The Science Behind the Co-Emergence

Periodical cicadas are unique to eastern North America and are famous for their incredibly long lifespans. Unlike annual cicadas, which appear every summer, periodical cicadas spend almost their entire lives underground in a nymph state. They emerge only after completing a specific cycle of either 13 or 17 years.

The 2024 event is mathematically significant because of how these prime number life cycles align.

  • Brood XIII (The Northern Illinois Brood): This group operates on a 17-year cycle.
  • Brood XIX (The Great Southern Brood): This group operates on a 13-year cycle.

Because 13 and 17 are prime numbers, they rarely coincide. To find out how often they emerge together, you multiply the two numbers. 13 times 17 equals 221. Therefore, these two specific broods only sync up every 221 years. The last time this happened was in 1803, and the next time won’t be until the year 2245.

Mapping the Swarm: Where They Appear

While both broods are emerging at the same time, they do not cover the entire country. They have distinct geographical footprints, though there is a small, fascinating area of overlap.

Brood XIX: The Great Southern Brood

Brood XIX is geographically the largest of all 13-year cicada broods. Its reach is extensive, covering a wide swath of the Southeastern and Midwestern United States. If you live in or visit the following states, you are likely hearing their song:

  • Alabama
  • Arkansas
  • Georgia
  • Illinois (Southern portion)
  • Indiana
  • Kentucky
  • Louisiana
  • Mississippi
  • Missouri
  • North Carolina
  • Oklahoma
  • South Carolina
  • Tennessee
  • Virginia

Brood XIII: The Northern Illinois Brood

As the name suggests, this brood is much more concentrated in the upper Midwest. The population density here can be intense, with reports of up to 1.5 million cicadas per acre in some wooded areas. The states affected include:

  • Illinois (Northern portion)
  • Indiana
  • Iowa
  • Wisconsin

The Convergence Zone

The most scientifically interesting aspect of this dual emergence is the narrow band where the two populations might interact. Central Illinois, specifically around the Springfield and Peoria areas, serves as the meeting ground. Here, the 17-year northern cicadas and the 13-year southern cicadas are emerging within miles of each other. While widespread hybridization is unlikely due to subtle differences in mating calls, the sheer volume of biomass in this region is historic.

The Life Cycle and the "Alien" Invasion

The emergence begins when the soil temperature 8 inches underground reaches approximately 64 degrees Fahrenheit (18 degrees Celsius). This usually triggers a mass exit during late April, May, or early June, depending on the latitude.

The process is consistent and visually striking:

  1. Exit: Nymphs dig their way out of the ground, often leaving distinct holes in lawns and forest floors. They look for vertical surfaces like trees, fences, or patio furniture.
  2. Molting: Once they secure a spot, they shed their exoskeletons. These crunchy, brown shells are left behind by the billions, often piling up at the bases of trees.
  3. Maturation: The adult cicada emerges white and soft but quickly hardens and darkens. The adults of the genus Magicicada are distinct from annual cicadas. They have black bodies, striking red eyes, and orange-veined wings.
  4. The Song: Males use vibrating membranes on their abdomens called tymbals to create their famous buzzing sound. This chorus can reach 90 to 100 decibels, which is roughly as loud as a lawnmower or a motorcycle engine.

Ecological Impact: The "Predator Satiation" Strategy

It might seem counterintuitive for a species to be so loud, clumsy, and defenseless. Cicadas do not bite, they do not sting, and they fly poorly. Their survival strategy is purely numerical. This is known as predator satiation.

By emerging in the trillions, cicadas overwhelm their predators. Birds, raccoons, opossums, fish, and even copperhead snakes gorge themselves on the insects. Because there are simply too many cicadas to eat, the majority survive long enough to mate and lay eggs, ensuring the next generation.

Nutrients for the Soil

Once the mating frenzy concludes after four to six weeks, the adult cicadas die. Their bodies decompose, returning a massive amount of nitrogen and other nutrients to the soil. This acts as a significant fertilizer event for forests, often resulting in a growth spurt for trees in the years following an emergence.

Protecting Your Property

While cicadas are harmless to humans and pets, they can pose a risk to vegetation. Female cicadas cut slits into small tree branches to lay their eggs. This process can damage the tips of branches, causing them to turn brown and wither. This phenomenon is called “flagging.”

What to do:

  • Mature Trees: Large, established trees can withstand this damage easily. It acts as a natural pruning.
  • Young Saplings: Newly planted trees or fruit trees are at risk. If you have saplings with trunks less than 1.5 inches in diameter, cover them with fine mesh netting (cheesecloth or specialized cicada netting).
  • Avoid Pesticides: Spraying insecticides is ineffective and harmful. Cicadas are too numerous to control with chemicals, and you risk killing beneficial pollinators or the birds that eat the cicadas.

Looking Ahead

By late June or early July 2024, the noise will fade. The eggs laid in the tree branches will hatch into tiny nymphs six to ten weeks later. These nymphs will fall to the ground, burrow into the soil, and tap into tree roots to feed on sap. There they will wait in the darkness.

Brood XIX will not return until 2037. Brood XIII will wait until 2041. And as for the simultaneous emergence of both? That spectacle will not return to the United States until the year 2245.

Frequently Asked Questions

Are periodical cicadas dangerous to my pets? Generally, no. Cicadas are not toxic and do not bite or sting. However, the exoskeletons can be hard to digest. If a dog eats too many, it may cause an upset stomach, vomiting, or blockage. It is best to monitor your pets while they are outside during the peak emergence.

Why are they so loud? The sound is a mating call produced exclusively by the males. They congregate in “chorus centers” to attract females. The louder the group, the more likely they are to attract mates. The sound intensity drops at night when the insects rest.

Is this a plague of locusts? No. Although they are sometimes colloquially called “locusts,” cicadas are a completely different insect order. Locusts are related to grasshoppers and eat plant leaves, often destroying crops. Cicadas feed on fluid (sap) from xylem tissue and do not chew leaves or destroy vegetable gardens.

Can I eat them? Yes. Cicadas are edible and are considered a delicacy in many cultures. They are high in protein and low in fat. Many chefs describe the taste as similar to soft-shelled crab or nutty asparagus. However, people with shellfish allergies should avoid eating cicadas, as they share a biological relation to shrimp and lobsters.