Cicada-geddon 2024: Two Broods Emerge at Once

Spring 2024 marked a biological event so rare that the last time it occurred, Thomas Jefferson was the President of the United States. For the first time in 221 years, two distinct broods of periodical cicadas—Brood XIII and Brood XIX—emerged from the soil simultaneously. This phenomenon, dubbed “Cicada-geddon,” brought trillions of buzzing insects to the surface across the Midwest and Southeastern United States.

A Historical Alignment of Cycles

Periodical cicadas are famous for their prime-number life cycles. Unlike annual cicadas that appear every summer, these insects spend over a decade underground feeding on tree roots before emerging en masse to mate.

The 2024 event was mathematically special because it involved the synchronization of two massive 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 cycles rarely align. The last time these two specific broods emerged together was in 1803. They will not synchronize again until the year 2245. This creates a unique window for entomologists and nature enthusiasts to witness a density of biomass that few people ever see in a lifetime.

The Geography of the Emergence

While both broods emerged at the same time, they did not cover the exact same territory. However, their ranges bumped against each other, creating a massive corridor of noise and activity.

Brood XIII Range

The 17-year Brood XIII is concentrated in the Midwest. The heaviest emergence occurred in:

  • Northern Illinois: This includes suburbs of Chicago and areas extending toward the Wisconsin border.
  • Southern Wisconsin: Areas around Lake Geneva and Madison saw significant activity.
  • Eastern Iowa: Parts of the state bordering the Mississippi River.
  • Northwest Indiana: A smaller pocket appeared here.

Brood XIX Range

The 13-year Brood XIX covers a much larger geographic footprint. It spans across 15 states in the Southeast and Midwest. Major hotspots included:

  • Missouri and Arkansas: These states saw some of the densest populations.
  • The Southeast: Georgia, Tennessee, Alabama, North Carolina, and South Carolina all experienced the emergence.
  • Southern Illinois: This is where the story gets interesting.

The “Overlap Zone”

The most fascinating aspect of the 2024 event was the narrow band in central Illinois where the two broods nearly touched. While they generally stay separated, areas around Springfield and Peoria acted as the convergence point. Researchers from the University of Connecticut and local institutions flocked to these zones to study potential interbreeding between the 13-year and 17-year species.

The Physical Reality: What Trillions Look Like

When scientists say “trillions,” it is difficult to visualize. In the hardest-hit areas, density levels reached up to 1.5 million cicadas per acre.

For residents in these zones, the experience was intense. The sheer number of insects meant that sidewalks, driveways, and patios were often covered. The insects are large, measuring about an inch and a half long, characterized by:

  • Sturdy, black bodies.
  • Striking red eyes.
  • Translucent wings with orange veins.

The Deafening Soundtrack

The defining feature of this dual emergence was the noise. Male cicadas sing to attract mates using a structure on their abdomen called a tymbal. When millions sing at once, the sound can reach 90 to 100 decibels. This is roughly as loud as a lawnmower, a motorcycle engine, or a jet plane flying overhead at 1,000 feet. In heavily wooded suburbs of Chicago and St. Louis, the sound was constant from late morning until dusk.

Ecological Impact and the "Flagging" Phenomenon

Despite the intimidation factor, cicadas are harmless to humans. They do not bite, sting, or carry disease. They are not locusts and do not chew leaves. However, they do leave a mark on the local flora.

Damage to Trees

Female cicadas lay their eggs in the new growth of trees. They use a sharp ovipositor to slice into small branches about the width of a pencil. This process can cause the tips of the branches to turn brown, die, and break off. This is known as “flagging.”

While this acts as a natural pruning process for mature oaks and maples, it poses a severe threat to:

  • Young saplings planted within the last 2-3 years.
  • Fruit trees and ornamental shrubs.
  • Blueberry bushes and grapevines.

Experts at the Morton Arboretum in Illinois advised homeowners to wrap young trees in fine mesh netting (1⁄4 inch holes or smaller) rather than using pesticides.

The Nutrient Boom

Once the mating cycle concludes after 4 to 6 weeks, the adult cicadas die. Their bodies accumulate at the base of trees and decompose. This releases a massive amount of nitrogen and other nutrients back into the soil. This “fertilizer bomb” often results in a growth spurt for local plant life the following year. Additionally, the tunnels dug by the emerging nymphs help aerate the soil, improving water filtration.

Predators and the Food Web

The survival strategy of the periodical cicada is called “predator satiation.” They emerge in such overwhelming numbers that predators can eat their fill without making a dent in the overall population.

During the 2024 emergence, local wildlife feasted.

  • Birds: Robins, turkeys, and cardinals gorged themselves to the point where many stopped visiting bird feeders entirely.
  • Mammals: Squirrels, raccoons, and foxes added the insects to their diet.
  • Fish: Anglers reported that fish in local lakes were so full of cicadas that fell into the water that they stopped biting lures.
  • Copperheads: In the southern range of Brood XIX, copperhead snakes are known to congregate at the base of trees to eat the molting nymphs.

Scientific Opportunities

This event provided a rare laboratory for scientists. Apps like Cicada Safari, created by Mount St. Joseph University, allowed regular citizens to snap photos and upload location data. This crowdsourced data helps researchers map the boundaries of the broods with higher precision than ever before.

Researchers were specifically looking for “stragglers” (cicadas emerging off-cycle) and studying how climate change might be affecting the timing of the emergence. Soil temperature acts as the trigger; once the ground reaches 64 degrees Fahrenheit at a depth of 8 inches, the emergence begins. In 2024, a warming spring caused the insects to appear weeks earlier than they did historically.

Frequently Asked Questions

Are these cicadas dangerous to my pets? generally, no. Cicadas are not toxic. However, they have a hard exoskeleton that is difficult to digest. If a dog or cat eats too many, it can cause vomiting or stomach upset. It is best to monitor pets when they are outside during the peak emergence.

Why do they have red eyes? The red eyes are a genetic trait of the Magicicada genus. Occasionally, a rare mutation occurs resulting in blue or white eyes. During the 2024 emergence, a blue-eyed cicada found in Illinois made national headlines and was donated to the Field Museum.

How long does the event last? The entire spectacle is relatively short. From the first emergence to the death of the adults, the cycle lasts about 4 to 6 weeks. By late June 2024, most of the noise had subsided, leaving behind only the shells and the eggs that will hatch into the next generation.

Will they come inside my house? Cicadas do not seek out shelter in homes. They are clumsy fliers and want to be on trees. If one gets inside, it is likely an accident. They cannot breed indoors and will die quickly without access to tree fluids.

Is it safe to use pesticides? Entomologists strongly advise against using pesticides. Chemicals are generally ineffective against such massive numbers. Furthermore, spraying kills the birds and beneficial insects that help control the cicada population naturally. Physical barriers like netting are the only effective method for protecting plants.