Cicada "Apocalypse": The Double Brood Emergence

For the first time since Thomas Jefferson was president, the United States is witnessing a rare biological phenomenon. In a synchronized event that has not occurred since 1803, two distinct broods of periodic cicadas are emerging from the ground simultaneously. This guide breaks down exactly what is happening, where the trillions of insects are appearing, and what this historic natural event means for residents in the affected regions.

The Science Behind the Double Emergence

This event is driven by the mathematical alignment of life cycles between two specific groups of periodic cicadas: Brood XIII and Brood XIX. These insects spend the vast majority of their lives underground, feeding on tree roots, only to emerge for a few frantic weeks to mate and lay eggs.

The Broods Explained

  • Brood XIII (The Northern Illinois Brood): This group operates on a 17-year cycle. They last emerged in 2007. They are known for their density, with populations reaching up to 1.5 million cicadas per acre in some wooded areas.
  • Brood XIX (The Great Southern Brood): This group operates on a 13-year cycle. They last emerged in 2011. This is geographically the largest of all periodical cicada broods, covering a massive stretch of the American Southeast and Midwest.

Because 13 and 17 are prime numbers, their cycles rarely align. They co-emerge only once every 221 years. After the 2024 event, these two specific broods will not appear together again until the year 2245.

Triggers for Emergence

The emergence is not triggered by a specific calendar date but by soil temperature. Once the soil eight inches beneath the surface reaches approximately 64 degrees Fahrenheit (18 degrees Celsius), the nymphs begin to tunnel upward. This usually happens after a warm rain, softening the ground for their exit.

Where the Cicadas Are Appearing

While “billions” or even “trillions” of bugs sounds overwhelming, the emergence is not nationwide. It is concentrated in specific regions of the Midwest and Southeast.

Brood XIII Territory

You will find the 17-year cicadas primarily in:

  • Northern Illinois (including suburbs of Chicago)
  • Southern Wisconsin
  • Eastern Iowa
  • Northwestern Indiana

Brood XIX Territory

The 13-year cicadas cover a much wider footprint, appearing in parts of 15 states:

  • Midwest: Missouri and Southern Illinois.
  • South: Alabama, Arkansas, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, and Virginia.

The Overlap Zone

The most intense activity is occurring in a narrow band in Central Illinois, specifically around Springfield and near Peoria. This is the primary location where the geographical ranges of Brood XIII and Brood XIX overlap. In these specific woods, residents might see both broods intermingling, though interbreeding between the two distinct genetic lines is rare.

What to Expect: Noise, Safety, and Trees

The arrival of the cicadas brings a unique sensory experience that can last from four to six weeks.

The Decibel Level

The male cicadas “sing” to attract mates using a vibrating membrane called a tymbal. When millions sing at once, the noise level can be substantial. In areas with dense tree coverage, the sound can reach 90 to 100 decibels. This is roughly as loud as a lawnmower, a motorcycle engine, or a jet flyover at 1,000 feet.

Safety for Humans and Pets

Despite their intimidating red eyes and clumsy flight patterns, periodic cicadas are physically harmless.

  • No biting or stinging: They do not have mandibles to bite or stingers to attack. They feed through a straw-like mouthpart.
  • Non-toxic: They are not poisonous.
  • Pet safety: Dogs and cats often try to eat them. While the insects are not toxic, their hard exoskeletons (made of chitin) are difficult to digest. If a pet eats too many, it may cause vomiting or stomach upset.

Risk to Trees and Gardens

Cicadas do not eat leaves or vegetables. The risk to vegetation comes from the female laying eggs. She uses a saw-like appendage to slice into small tree branches (about the width of a pencil) to deposit her eggs.

  • Mature Trees: Generally safe. They may experience “flagging,” where the tips of branches turn brown and break off. This acts as a natural pruning.
  • Young Saplings: These are at high risk. The physical damage from egg-laying can kill a young tree.
  • Protection: If you have planted fruit trees or decorative trees in the last 2-3 years, cover them with mesh netting with holes smaller than 1⁄4 inch. Secure the netting to the trunk to prevent insects from climbing up.

The Ecological Benefit

While they may be a nuisance to homeowners, this emergence is a massive boon for the ecosystem.

  1. A Free Buffet: Birds, raccoons, opossums, fish, and even copperhead snakes gorge themselves on the cicadas. This abundance of food often leads to higher survival rates for wildlife offspring during an emergence year.
  2. Soil Aeration: As the nymphs tunnel out of the ground, they naturally aerate the soil, improving water filtration.
  3. Nutrient Recycling: When the adult cicadas die after their few weeks in the sun, their bodies decompose. This releases nitrogen and other nutrients back into the soil, acting as a massive dose of natural fertilizer for the forests.

Frequently Asked Questions

How long does the cicada invasion last? The adult lifespan of a periodic cicada is short. Once they emerge, molt, and become adults, they live for about three to four weeks. The entire event, from the first emergence to the death of the stragglers, typically lasts about six weeks. By July, the insects will be gone.

Are cicadas the same as locusts? No. This is a common misconception. Locusts are a type of grasshopper that swarms and devours vegetation (crops). Cicadas are part of the order Hemiptera and are more closely related to aphids. They do not eat crops.

Will they get inside my house? Cicadas are clumsy fliers and do not seek out indoors. However, they may accidentally fly through open windows or doors. They are attracted to the vibration of power tools and lawnmowers, so they may land on you while you are doing yard work.

What happens to the dead cicadas? They accumulate in piles at the base of trees. They can smell unpleasant as they decay. You can rake them into a compost pile or bury them in your garden to utilize the nutrients they provide.