Cicada Double Brood Event Explained

In the spring of 2024, the United States witnessed a biological event that had not occurred since Thomas Jefferson was president. Two distinct groups of periodical cicadas, Brood XIII and Brood XIX, emerged from the soil simultaneously. This rare co-emergence brought trillions of buzzing insects to the surface in a synchronized display of nature that happens only once every 221 years.

The Players: Brood XIII and Brood XIX

Periodical cicadas are distinct from the annual cicadas you might hear every summer. Periodical species spend most of their lives underground feeding on tree roots and emerge only after completing a specific 13-year or 17-year life cycle. The 2024 event is special because the cycles of two massive broods aligned perfectly.

Brood XIII (The Northern Illinois Brood)

Brood XIII operates on a 17-year cycle. These insects last appeared in 2007. They are concentrated primarily in the Midwest. Their territory covers parts of:

  • Northern Illinois
  • Eastern Iowa
  • Southern Wisconsin
  • Northwestern Indiana

Brood XIX (The Great Southern Brood)

Brood XIX follows a 13-year cycle. They last emerged in 2011. This is the largest geographic brood of 13-year cicadas in the United States. Their range is much wider and covers the Southeastern and Midwestern states, including:

  • Alabama
  • Arkansas
  • Georgia
  • Kentucky
  • Louisiana
  • Mississippi
  • Missouri
  • North Carolina
  • South Carolina
  • Tennessee
  • Virginia

The Mathematics of the Convergence

The rarity of this event comes down to simple mathematics involving prime numbers. Because 13 and 17 are both prime numbers, they rarely share a common multiple. To find out how often these two specific life cycles sync up, you multiply 13 by 17. The result is 221.

This means Brood XIII and Brood XIX only emerge together every 221 years.

  • Last Occurrence: 1803
  • Current Occurrence: 2024
  • Next Occurrence: 2245

While other combinations of broods overlap more frequently, the sheer size and geographic proximity of these two specific groups make this event historically significant.

The Overlap Zone: Where the Swarms Meet

For most of the country, residents saw either one brood or the other. However, there was a specific geographic ā€œoverlap zoneā€ where both broods emerged near each other.

The epicenter of this convergence was in central Illinois, particularly around the Springfield area. Entomologists were particularly interested in this region to see if the different species would interbreed. While the broods are genetically distinct, the close proximity in these narrow bands of Illinois provided a unique opportunity for scientific study regarding hybridization between 13-year and 17-year species.

Understanding the "Predator Satiation" Strategy

You might wonder why these insects emerge in such overwhelming numbers. The strategy is known as ā€œpredator satiation.ā€

Cicadas have no natural defenses. They do not sting, they do not bite, and they are not poisonous. They are slow, clumsy fliers that are high in protein. To survive, they rely on numbers.

When trillions emerge at once, every predator in the ecosystem eats until it is full. Birds, squirrels, raccoons, fish, and even household pets will gorge themselves on cicadas. Because there are simply too many insects to eat, the remaining millions survive to mate and lay eggs, ensuring the next generation.

What the Emergence Looked and Sounded Like

For residents in the affected zones, the experience was intense.

The Sound

Male cicadas produce a mating call by vibrating membranes on their abdomen called tymbals. A single cicada is loud, but a swarm can be deafening. In areas with dense populations, the sound levels reached 90 to 100 decibels. This is roughly equivalent to the noise of a lawnmower or a jet flyover at 1,000 feet.

The Visuals

The emergence began when soil temperatures reached 64 degrees Fahrenheit roughly 8 inches underground. The nymphs crawled out, climbed vertical surfaces like trees or walls, and shed their exoskeletons. These brown, crunchy shells were left behind by the billions, accumulating at the base of trees and on patio furniture.

Once the adults emerged, they displayed their characteristic red eyes and orange-veined wings. They spent roughly four to six weeks above ground with a singular goal: mating.

Ecological Impact and Homeowner Tips

While the noise can be a nuisance, the double brood event provides significant ecological benefits.

Nutrient Boom: After the cicadas mate and die, their bodies decompose. This releases a massive amount of nitrogen and other nutrients back into the soil. Biologists view this as a ā€œfertilizer bombā€ that boosts plant growth in the forests for years following an emergence.

Tree Safety: Cicadas do not eat leaves. However, female cicadas use a sharp, tube-like organ called an ovipositor to slice into small tree branches to lay their eggs. This process, called ā€œflagging,ā€ can cause the tips of branches to turn brown and break off.

  • Mature Trees: Established trees can handle this pruning without issue.
  • Saplings: Young trees are at risk. Homeowners were advised to wrap young trees in fine mesh netting (holes smaller than 1ā„4 inch) to prevent females from laying eggs in the tender branches.

Pesticides: Spraying pesticides is generally ineffective and discouraged. The number of cicadas is too high for chemicals to make a dent, and spraying introduces toxins that can harm the birds and animals feasting on the insects.

Frequently Asked Questions

Do cicadas bite or sting? No. Cicadas mouthparts are designed only for drinking plant sap. They cannot bite or sting humans or pets. They may mistake your arm for a tree branch and try to probe it, but they are harmless.

Is it safe for dogs to eat them? Generally, yes. Cicadas are not toxic. However, the hard exoskeletons are difficult to digest. If a dog eats too many, it may suffer from vomiting or constipation. It is best to limit their intake.

What happens to the cicadas after the event? Adult cicadas live only for a few weeks. After mating and laying eggs, the adults die. The eggs hatch six to ten weeks later, and tiny nymphs fall to the ground and burrow into the soil. They will remain there, feeding on roots in the dark, until their 13 or 17-year cycle is complete.

Will this happen again next year? No. While there are other broods that emerge in different years (for example, Brood XIV is due in 2025), this specific double emergence of Brood XIII and Brood XIX will not happen again until the year 2245.