The Great Cicada Emergence: Two Broods Align After 221 Years

In the spring of 2024, the United States witnessed a biological event of historic proportions. For the first time since 1803, two massive broods of periodical cicadas emerged from the ground simultaneously. If you live in the Midwest or the Southeast, you likely experienced the deafening hum of trillions of these insects. This rare alignment of life cycles offers a unique opportunity to understand the fascinating biology of the Magicicada genus.

Understanding the Double Emergence

Periodical cicadas are distinct from the annual cicadas you hear every summer. Periodical species live underground for specific prime-number intervals, either 13 or 17 years. In 2024, the 17-year Brood XIII and the 13-year Brood XIX completed their underground development at the same time.

Because 13 and 17 are prime numbers, their emergence cycles rarely align. Mathematically, they only coincide every 221 years (\(13 \times 17\)). The last time these two specific groups surfaced together, Thomas Jefferson was the U.S. President and the Louisiana Purchase was being negotiated. They will not appear together again until the year 2245.

Identifying the Players

To understand the scale of this event, it helps to know exactly which insects are involved:

• Brood XIII (The Northern Illinois Brood): These are 17-year cicadas. They are concentrated heavily in Northern Illinois, extending into Southern Wisconsin, Eastern Iowa, and Northwest Indiana.
• Brood XIX (The Great Southern Brood): These are 13-year cicadas. Their range is much wider, covering a large portion of the Southeast and Midwest, including Missouri, Arkansas, Tennessee, Georgia, Alabama, North Carolina, and Southern Illinois.

Where the Zones Overlap

While the emergence covers a massive geographic area across 16 states, the two broods do not mix everywhere. In most regions, residents saw only one of the two broods. However, a specific “collision zone” exists in central Illinois.

Areas around Springfield and Peoria act as the boundary line where the northern Brood XIII and the southern Brood XIX meet. In these narrow bands, the density of insects can be staggering. Scientists estimate that in peak areas, there can be as many as 1.5 million cicadas per acre.

This geographical proximity provides entomologists with a rare chance to see if the different species within these broods will interbreed. While they are distinct broods, they share similar genetic makeups, and hybridization is scientifically possible, though rare.

The Biology of the Emergence

The synchronization of billions of insects relies on precise environmental triggers.

The Temperature Trigger

Cicadas do not look at a calendar. They rely on soil temperature. When the soil eight inches below the surface reaches approximately 64 degrees Fahrenheit (18 degrees Celsius), the nymphs begin their ascent. They construct “chimneys” or mud turrets at the surface before crawling out to molt.

The Transformation

Once above ground, the nymphs climb the nearest vertical surface—usually a tree trunk, fence post, or patio chair. They shed their exoskeletons, leaving behind the crunchy brown shells that litter suburban lawns. Initially, the adults are soft and white (teneral), but their outer shells harden and turn black within hours. Their eyes turn a deep red, and their wings become rigid and transparent.

The Sound of Survival

The deafening noise associated with cicadas comes exclusively from the males. They possess a vibrating membrane on their abdomen called a tymbal. The sound is a mating call designed to attract females.

Different species produce different songs, and the volume is immense. A chorus of male cicadas can reach 90 to 100 decibels. This is roughly equivalent to a lawnmower or a jet flyover at 1,000 feet. The sheer volume confuses predators like birds, making it difficult for them to isolate a single insect.

Impact on Nature and Neighborhoods

While the sheer number of bugs can be intimidating, periodical cicadas are largely harmless to humans and animals. They do not bite, sting, or carry diseases. They are not toxic, though pets that gorge on them may suffer from stomach upset due to the hard chitin in the insect shells.

Risks to Trees

The primary environmental risk is to young saplings. Female cicadas use a sharp tube called an ovipositor to slice into small tree branches and lay their eggs. This process, called “flagging,” can cause the tips of branches to brown and die.

For mature oak or maple trees, this acts as a natural pruning service. However, for saplings or ornamental trees planted within the last two to three years, this damage can be fatal. Experts at arboretums often recommend wrapping young trees in fine mesh netting (holes smaller than ¹⁄₄ inch) during the 4 to 6 weeks the insects are active.

A Nutrient Boom

Once the mating cycle concludes, the adult cicadas die. This results in a massive biomass drop on the forest floor. As the bodies decompose, they return nitrogen and other essential nutrients to the soil. This acts as a powerful fertilizer that benefits plant growth in the following years. Additionally, the emergence provides a feast for wildlife. Birds, raccoons, fish, and even squirrels see population boosts following a cicada year due to the limitless food supply.

The “Zombie” Fungus

A strange phenomenon observed during these emergences involves a fungus called Massospora cicadina. This pathogen infects the cicada, eating away the rear segments of its abdomen and replacing them with a chalky white plug of fungal spores.

The fungus contains psilocybin (found in magic mushrooms) and cathinone (an amphetamine). It alters the insect’s behavior, causing males to mimic female mating wing-flicks. This lures other males in, spreading the fungus further. These infected insects are often referred to colloquially as “salt shakers of death” because they drop spores as they fly.

Conclusion

The 2024 convergence of Brood XIII and Brood XIX is a reminder of the complex and rhythmic nature of our ecosystem. While they may be loud and messy, these insects are a crucial part of the eastern North American hardwood forest ecosystem. If you missed them this time, you will have to wait centuries for this exact alignment to return, though other single broods will continue to emerge in different years across the region.

Frequently Asked Questions

How long does the cicada emergence last? The entire event typically lasts about four to six weeks from the time they emerge until they die. By early July, the adults are usually gone, and the eggs begin to hatch a few weeks later.

Do cicadas eat my garden vegetables? No. Adult cicadas do not chew leaves like locusts or caterpillars. They feed on plant fluids (xylem) from trees and woody shrubs but do remarkably little damage while feeding. The damage comes solely from egg-laying in branches.

Why do they stay underground for 13 or 17 years? Scientists believe this is an evolutionary survival strategy known as “predator satiation.” By emerging in such massive numbers at prime-number intervals, they overwhelm predators. Birds and squirrels can eat as many as they want, and millions of cicadas will still survive to reproduce.

Can I kill them with pesticides? Using pesticides on periodical cicadas is generally discouraged and ineffective. There are simply too many of them. Spraying chemicals will likely kill beneficial insects and harm birds that eat the cicadas, without significantly reducing the cicada population. Physical barriers like netting are the best defense for trees.

Are there blue-eyed cicadas? Yes, but they are extremely rare. Most periodical cicadas have red eyes. A blue-eyed variation occurs due to a genetic mutation. Finding one is considered a “one in a million” discovery.