Cicada Double Brood Phenomenon

The spring and summer of 2024 marked a biological event of historic proportions in the United States. For the first time since 1803, two distinct broods of periodical cicadas—Brood XIII and Brood XIX—emerged from the ground simultaneously. This rare synchronization resulted in trillions of flying insects appearing across the Midwest and Southeast, creating a natural spectacle that will not occur again for another 221 years.

The Science Behind the Co-Emergence

Periodical cicadas are unique to North America and are famous for their incredibly long, prime-number life cycles. Unlike annual cicadas, which appear every summer in smaller numbers, periodical cicadas spend either 13 or 17 years underground as nymphs.

The 2024 phenomenon involved 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, their emergence cycles rarely align. To find out how often they co-emerge, you multiply the two cycles (13 x 17), which results in 221 years. The last time these two specific broods surfaced together, Thomas Jefferson was President of the United States.

Mapping the Invasion: Who Saw What?

While the emergence was massive, the two broods did not mix in a single “super swarm” across the entire country. Instead, they largely occupied adjacent geographical territories, with a narrow band of overlap.

Brood XIII Territory

The 17-year Brood XIII is smaller in geographic range but extremely dense in population. Residents in these areas saw up to 1.5 million cicadas per acre. The primary locations included:

  • Northern Illinois: Including the Chicago suburbs.
  • Southern Wisconsin: Extending to areas around Lake Geneva and Madison.
  • Eastern Iowa: Along the Mississippi River counties.
  • Northwest Indiana: Bordering Illinois.

Brood XIX Territory

The 13-year Brood XIX covers a much larger geographic footprint across the Southeast and Midwest. This brood appeared in parts of 15 states, including:

  • Missouri and Southern Illinois
  • Arkansas and Tennessee
  • Georgia, Alabama, and South Carolina
  • North Carolina and Virginia

The Overlap Zone

The most scientifically interesting region was central Illinois, specifically around Springfield and Champaign. This is the rare geographical boundary where the southern edge of Brood XIII met the northern edge of Brood XIX. While hybridization between 13-year and 17-year broods is rare due to slight differences in mating songs and genetics, entomologists from the University of Connecticut and other institutions flocked to this zone to study potential interactions.

The Trigger: When Do They Surface?

Cicadas do not use a calendar; they use a thermometer. The emergence is triggered when the soil temperature at a depth of roughly eight inches reaches 64 degrees Fahrenheit (18 degrees Celsius).

Depending on the latitude, this occurred at different times:

  • Late April: Sightings began in warmer southern states like Georgia and South Carolina.
  • Mid-May: The emergence peaked in Missouri and Southern Illinois.
  • Late May to Early June: The colder soil in Wisconsin and Northern Illinois delayed the emergence until late spring.

Once they emerge, a warm rain often encourages the final molting process. The insects climb vertical structures—trees, fences, and walls—shed their exoskeletons, and transform into winged adults.

Understanding the "Cicada Apocalypse"

The sheer number of insects can be overwhelming, but understanding their biology helps demystify the event.

The Noise

The most defining characteristic of a cicada emergence is the sound. Male cicadas possess an organ called a tymbal, which they vibrate to create a mating call. When millions of males sing at once, the chorus can reach 90 to 100 decibels, comparable to a lawnmower or a jet flyover at 1,000 feet.

Ecological Impact

Despite the noise and the mess, cicadas are beneficial to the ecosystem:

  • Soil Aeration: The tunnels created by emerging nymphs aerate the soil and improve water filtration.
  • Tree Pruning: Female cicadas cut slits into small tree branches to lay eggs. This causes the tips of the branches to die and fall off (flagging), which acts as a natural pruning process that can spur future growth in mature trees.
  • Nutrient Cycling: After the adults mate and die (a process lasting about 4 to 6 weeks), their decomposing bodies return massive amounts of nitrogen to the soil.

The “Zombie” Fungus

One of the stranger aspects of the 2024 emergence was the prevalence of Massospora cicadina. This is a fungal pathogen that infects periodical cicadas. It eats away the insect’s abdomen and reproductive organs, replacing them with a plug of fungal spores. The fungus creates hyper-sexual behavior in the insects, causing males to mimic female mating wing-flicks to attract other males, thereby spreading the spores further.

Protecting Your Property

While periodical cicadas are harmless to humans and pets (they do not bite or sting), they can damage young plants.

  • Young Trees: Trees planted within the last 2 to 3 years are at risk. The egg-laying process can damage weak branches. Experts recommend wrapping young trees in mesh netting with holes smaller than 1⁄4 inch.
  • Pesticides: Chemical control is generally ineffective and discouraged. The number of cicadas is too high for pesticides to work, and spraying kills beneficial insects while potentially harming birds that feast on the cicadas.
  • Pets: Dogs often view cicadas as “sky raisins” and will eat them. While the insects are not toxic, their hard exoskeletons are difficult to digest. Eating too many can cause vomiting or intestinal blockage.

Looking Ahead

Once the 2024 season concluded in late June, the adults died, and the new generation of nymphs hatched from the tree branches, dropped to the ground, and burrowed into the soil.

The offspring of Brood XIX will not emerge again until 2037. The offspring of Brood XIII will remain underground until 2041. As for the simultaneous emergence of these two specific broods? That alignment will not happen again until the year 2245.

Frequently Asked Questions

Are periodical cicadas the same as locusts? No. This is a common misconception. Locusts are a type of grasshopper that eats vegetation and destroys crops. Cicadas are hemipterans (true bugs) that drink plant sap. They do not strip leaves or destroy crops.

Can the two broods mate with each other? It is possible but rare. They are different species groups (Brood XIII is mostly Magicicada septendecim, M. cassini, and M. septendecula, while Brood XIX contains M. tredecim). Even if they do crossbreed, the offspring often have mismatched life cycles that cause them to emerge at the wrong time and die without mating.

Why are their eyes red? Periodical cicadas typically have bright red eyes, black bodies, and orange-veined wings. This distinguishes them from the annual “dog-day” cicadas, which usually have green or black eyes and appear later in the summer. Occasionally, a rare blue-eyed periodical cicada is found, which is a genetic mutation (one in a million).

What happens to the dead cicadas? They decompose rapidly. While they can create a smell similar to rotting meat if they pile up, they are excellent fertilizer. You can shovel them into your garden beds or compost pile to enrich the soil.