The Great Cicada Emergence: A Double Brood Phenomenon

In the spring of 2024, residents across the United States witnessed a biological event that had not occurred since 1803. Two distinct broods of periodic cicadas—Brood XIII and Brood XIX—emerged from the soil simultaneously. This rare alignment brought trillions of flying insects to the surface, creating a natural spectacle that scientists and enthusiasts had anticipated for decades.

Understanding the Double Brood Alignment

Periodic cicadas are unique to North America and are famous for their extremely long life cycles. Unlike annual cicadas, which appear every summer, periodic cicadas spend most of their lives underground feeding on tree roots. They emerge only after completing a cycle of either 13 or 17 years.

The 2024 event was mathematically significant because it involved the synchronization of two specific groups:

  • Brood XIII (The Northern Illinois Brood): This group operates on a 17-year cycle. They were last seen in 2007.
  • Brood XIX (The Great Southern Brood): This group operates on a 13-year cycle. They were last seen in 2011.

Because 13 and 17 are prime numbers, these two specific broods only align once every 221 years. The last time they appeared together, Thomas Jefferson was the President of the United States, and the Louisiana Purchase was being negotiated. The next time these two specific broods will co-emerge is the year 2245.

The Geography of the Swarm

While the emergence involved trillions of insects, they did not cover the entire country. The emergence was concentrated in the Midwest and the Southeast, creating a massive “Y” shape across the map.

Brood XIII Locations

The 17-year brood appeared primarily in:

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

Brood XIX Locations

The 13-year brood covered a much larger geographic footprint, stretching across the Southeastern United States:

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

The Overlap Zone

The most intense activity occurred in a narrow band in central Illinois, specifically around Springfield and Champaign. This was the primary region where the ranges of the two broods overlapped. In these areas, residents experienced an incredibly high density of insects, with counts reaching up to 1.5 million cicadas per acre.

The Biological Trigger and Lifecycle

The emergence is not triggered by a specific calendar date but by soil temperature. When the soil eight inches underground reaches approximately 64 degrees Fahrenheit (18 degrees Celsius), the nymphs begin to dig their way to the surface.

Once above ground, the process moves quickly:

  1. Molting: The nymphs climb vertical surfaces like trees, fences, and walls. They shed their exoskeletons to become winged adults.
  2. Hardening: Their bodies are initially soft and white (teneral stage) but harden and darken within a few hours.
  3. Mating: The males begin “singing” to attract females. After mating, the female cuts slits into young tree branches to lay her eggs.
  4. Death: The adult cicadas live for only four to six weeks. After reproducing, they die and pile up at the base of trees.

The eggs eventually hatch into tiny nymphs, which fall to the ground and burrow into the soil, restarting the 13 or 17-year clock.

The Sound of Billions

One of the defining characteristics of this emergence was the noise. Male cicadas produce sound using an organ called a tymbal, situated on the side of their abdomen. They vibrate this organ rapidly to create a buzzing song.

During the peak of the 2024 double emergence, noise levels in some areas exceeded 90 to 100 decibels. This is comparable to the sound of a lawnmower, a motorcycle engine, or a jet plane flying over a nearby neighborhood at takeoff. The noise is a critical mating tool, as males congregate in “choruses” to attract females.

Ecological Impact and Benefits

While the sheer number of insects can be a nuisance to humans, the emergence provides substantial ecological benefits.

  • Predator Satiation: The survival strategy of periodic cicadas is safety in numbers. They emerge in such massive quantities that predators (birds, raccoons, squirrels, and fish) become full and stop eating them. This ensures that enough cicadas survive to reproduce.
  • Soil Aeration: As the nymphs tunnel to the surface, they naturally aerate the soil, improving water filtration and root health for plants.
  • Nutrient Cycling: When the adult cicadas die, their decomposing bodies return massive amounts of nitrogen and other nutrients to the soil. This acts as a widespread, natural fertilizer event that boosts plant growth in the following years.

Risks to Vegetation

Cicadas do not eat leaves, vegetables, or flowers. However, they can damage young trees. The female cicada uses a sharp, needle-like appendage called an ovipositor to slice into small branches (about the width of a pencil) to lay eggs.

For mature trees, this results in a natural pruning known as “flagging,” where the tips of branches turn brown and break off. The tree usually recovers quickly. However, for saplings or fruit trees planted within the last two or three years, this damage can be fatal. During the 2024 emergence, experts at arboretums and extension offices advised homeowners to wrap young trees in fine mesh netting (holes 1⁄4 inch or smaller) to prevent females from accessing the branches.

Frequently Asked Questions

Do cicadas bite or sting? No. Cicadas do not have jaws for biting or stingers for stinging. They feed on plant fluids (xylem) through a straw-like mouthpart. They are harmless to humans.

Are cicadas dangerous to pets? Cicadas are not toxic. However, dogs often find them entertaining to catch and eat. Because the insects have a hard exoskeleton, eating too many can cause gastrointestinal upset or blockage. It is best to limit your pet’s consumption.

What is the “Zombie Cicada” fungus? A fungus called Massospora cicadina affects a small percentage of the population. It infects the abdomen of the cicada, causing the back end of the insect to fall off and be replaced by a plug of white fungal spores. The fungus produces a stimulant similar to psilocybin, causing the infected cicada to remain active and spread spores to other insects despite missing half its body.

When will the next double brood emergence occur? While different combinations of broods align occasionally, the specific alignment of Brood XIII and Brood XIX will not happen again until the year 2245. However, the next major single-brood emergence will be Brood XIV in 2025, appearing in parts of the Ohio Valley and Appalachia.