Dual Cicada Emergence: The Historic 2024 Double Brood Event

For the first time since Thomas Jefferson was president in 1803, two distinct populations of periodical cicadas are emerging from the ground simultaneously. This event, often dubbed “Cicada-geddon” or the “Great Eastern Brood,” involves Brood XIII and Brood XIX appearing together across the Southeastern and Midwestern United States. It is a rare biological phenomenon that creates a chaotic, deafening, and fascinating spectacle of nature involving trillions of insects.

The Players: Brood XIII and Brood XIX

Periodical cicadas are different from the annual cicadas you hear every summer. Periodical species spend the vast majority of their lives underground, feeding on tree roots, only to emerge in massive synchronized waves to mate and die. The 2024 event is special because of the specific alignment of two massive groups:

  • Brood XIII (The Northern Illinois Brood): This group operates on a 17-year cycle. They last emerged in 2007. They are primarily concentrated in northern Illinois, southern Wisconsin, eastern Iowa, and northwest Indiana.
  • Brood XIX (The Great Southern Brood): This group operates on a 13-year cycle. They last emerged in 2011. Their range is much wider, covering parts of Alabama, Arkansas, Georgia, Illinois, Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, and Virginia.

These two distinct life cycles align only once every 221 years. The math is simple but rare: 13 and 17 are both prime numbers. To find when they sync up, you multiply them together (\(13 \times 17 = 221\)). The last time they co-emerged was 1803, and the next time will not be until the year 2245.

The Geography of the Emergence

While residents across 16 states will see cicadas, the main event is occurring in Illinois. This is the only state where the ranges of Brood XIII and Brood XIX significantly overlap.

Entomologists have identified a narrow band in central Illinois, specifically around Springfield and extending toward Champaign and Decatur, where both broods may be present in the same woods. In these convergence zones, the sheer density of insects is staggering. Estimates suggest up to 1.5 million cicadas per acre.

For residents in Chicago, the emergence is dominated by the 17-year Brood XIII. Meanwhile, St. Louis and Nashville are experiencing the 13-year Brood XIX.

The Biology of the Invasion

The emergence begins when the soil temperature 8 inches underground reaches exactly 64 degrees Fahrenheit (17.8 degrees Celsius). Once this trigger occurs, usually in late April or May depending on the latitude, the nymphs burrow to the surface.

The Lifecycle on Surface

Once above ground, the nymphs shed their exoskeletons to become winged adults. This leaves behind the crunchy, brown shells found on trees, fences, and patio furniture. The adults are distinct from annual cicadas; they have jet-black bodies and striking, blood-red eyes.

Their sole focus is reproduction. The males congregate in trees and “sing” to attract females. This song is produced by vibrating a membrane called a tymbal on their abdomen. When millions of males sing at once, the noise level can exceed 90 to 100 decibels, comparable to a lawnmower or a jet flyover at 1,000 feet.

The “Zombie” Fungus Threat

A strange subplot to this emergence involves a fungal pathogen called Massospora cicadina. This fungus lies dormant in the soil and infects the cicadas as they emerge.

The infection eats away the cicada’s abdomen and reproductive organs, replacing them with a chalky plug of fungal spores. Scientists compare the visual to a “salt shaker of death.” The fungus also pumps the insect with psilocybin (the chemical in magic mushrooms) and cathinone (an amphetamine). This causes the cicadas to become hyper-active and hyper-sexual. Infected males will even mimic female wing flicks to attract other males, effectively spreading the spores to new hosts.

Ecological Impact

While they can be a nuisance to humans due to the noise and physical presence, periodical cicadas are a vital ecological event.

  1. A Feast for Predators: Everything eats cicadas. Birds, squirrels, raccoons, fish, and even domestic dogs gorge themselves. This “predator satiation” strategy ensures the species survival; there are simply too many insects for predators to eat them all.
  2. Tree Pruning: Female cicadas cut slits into small tree branches to lay their eggs. This can damage the tips of branches, causing them to turn brown and droop (known as “flagging”). While this acts as a natural pruning for mature trees, it can be fatal to young saplings.
  3. Nutrient Cycling: Once the mating frenzy ends after 4 to 6 weeks, the adults die. Their decomposing bodies return massive amounts of nitrogen and other nutrients to the soil, resulting in a plant growth boom the following year.

Guidelines for Homeowners

If you live in the emergence zone, there is no need to panic. Cicadas do not bite, sting, or carry diseases harmful to humans. They possess no mandibles to chew and only have a straw-like mouth for drinking plant fluids.

  • Protect Young Trees: If you have planted trees in the last 2 to 3 years, you should cover them with mesh netting (holes should be 1⁄4 inch or smaller). Do this before the emergence begins.
  • Avoid Pesticides: Spraying is ineffective due to the sheer number of insects and will only harm beneficial pollinators and the birds that eat the cicadas.
  • Pet Safety: While cicadas are not toxic, the hard exoskeleton (chitin) is difficult to digest. Dogs that eat too many may suffer from vomiting or blockage. Monitor your pets when they are outside.
  • Shellfish Allergies: Interestingly, cicadas are arthropods and are closely related to shrimp and lobsters. The FDA has warned that individuals with severe shellfish allergies should avoid eating cicadas, as the proteins are similar.

Frequently Asked Questions

How long does the cicada emergence last? The entire event lasts about 4 to 6 weeks from the time they emerge until they die. By late June or early July, the adults will be gone, and the eggs will begin to hatch.

Why are they so loud? The sound is a mating call produced exclusively by males. The loudness is a competitive advantage; the louder the chorus, the more likely they are to attract females. The chorus is most intense during the warmest parts of the day.

Will they eat my vegetable garden? Generally, no. Cicadas are not interested in tomatoes, peppers, or flowers. They are exclusively interested in woody shrubs and trees where they can lay their eggs. However, heavy females landing on delicate plants might physically break stems.

What happens after they die? The carcasses will accumulate at the base of trees. They will smell as they decompose. You can rake them into a compost pile, where they make excellent fertilizer, or bury them in the garden.

Can I stop them from coming into my yard? No. If the trees in your yard had cicadas laying eggs 13 or 17 years ago, the nymphs are already in your soil waiting to come up. There is no repellent or barrier that stops the emergence.