The bugs that come out every 17 years are periodical cicadas, members of the genus Magicicada. These remarkable insects spend most of their lives underground as nymphs, only emerging in massive numbers for a brief, noisy, and fascinating adult phase. Their synchronized emergence is one of the most unusual natural phenomena in North America, captivating scientists and the public alike.
What Are Periodical Cicadas?
Periodical cicadas are a group of cicada species native to eastern North America, best known for their unique developmental cycles of either 13 or 17 years. Unlike the more common annual cicadas, which appear every summer, periodical cicadas remain underground for well over a decade before emerging in enormous broods. The 17-year cicadas, in particular, are famous for their vast synchronized swarms.
There are seven recognized species of periodical cicadas: four with a 13-year lifecycle and three with a 17-year lifecycle. The 17-year species are Magicicada septendecim, Magicicada cassini, and Magicicada septendecula. Each emergence is known as a “brood,” and these broods are tracked and mapped by scientists.
The 17-Year Life Cycle: How It Works
The periodical cicada’s life begins when eggs are laid in tree branches by the previous generation. After about six to ten weeks, the tiny nymphs drop to the ground and burrow several inches below the surface. There, they feed on sap from tree roots and undergo several molts as they slowly grow. This underground phase lasts for 17 years for the 17-year species.
When the time is right—typically when the soil temperature eight inches below the surface reaches about 64°F (18°C)—the mature nymphs dig their way to the surface, often synchronously over a few nights. They then climb nearby vegetation or structures, molt one last time, and emerge as winged adults. The adults live for about four to six weeks, during which they mate, lay eggs, and start the cycle anew.
Why 17 Years? The Mystery of Their Timing
The reason for the 17-year cycle is not fully understood, but several theories exist. The most widely accepted hypothesis is that the long, prime-numbered cycles help periodical cicadas evade predators. Because predators cannot rely on cicadas as a regular food source, few develop life cycles synchronized with the cicadas. The prime number intervals (13 and 17) also make it less likely for different broods to overlap, which could dilute their survival strategy.
This strategy, known as “predator satiation,” means cicadas emerge in such overwhelming numbers that predators cannot possibly eat them all. Even with heavy predation, enough cicadas survive to reproduce successfully.
Broods and Geographic Distribution
Periodical cicadas are organized into broods, each emerging in a specific region and year. Each brood is assigned a Roman numeral; for example, Brood X (Brood Ten) is one of the largest and most famous, last emerging in 2021 and due again in 2038. Other broods, such as Brood XIII (Illinois area) or Brood II (Mid-Atlantic), follow their own 17-year schedules.
- Brood X: Covers parts of the Midwest, Mid-Atlantic, and Southern U.S.
- Brood XIII: Found mainly in northern Illinois and nearby states.
- Brood XIV: Appears in parts of Kentucky, Ohio, and surrounding areas.
Each brood’s emergence is highly localized, so while one region may be swarming with cicadas, others may not see them at all that year. Maps of brood locations and schedules are available from organizations like the U.S. Forest Service and university entomology departments.
What Happens During an Emergence?
When a brood emerges, the event is impossible to miss. Billions of cicadas surface almost simultaneously, blanketing trees, shrubs, and even buildings. The males produce a distinctive, loud mating call—sometimes reaching over 100 decibels—to attract females. The chorus can be deafening in heavily infested areas.
Adult cicadas do not bite or sting and are harmless to humans and pets. They do not eat leaves or flowers; instead, they sip small amounts of plant sap. The primary concern for gardeners and orchardists is that female cicadas lay eggs by slicing into small branches, which can damage young trees. Mature trees are generally unaffected.
After mating and egg-laying, adults die off, leaving behind their shells and millions of eggs, which hatch into nymphs and fall to the ground to begin the cycle anew.
Ecological Impact and Benefits
While their sudden appearance can be startling, periodical cicadas play an important role in forest ecosystems:
- Soil Aeration: Their burrowing helps aerate the soil, benefiting plant roots.
- Nutrient Cycling: The bodies of dead cicadas decompose, enriching the soil with nitrogen and other nutrients.
- Food Source: Cicadas provide a feast for birds, mammals, reptiles, and even fish during mass emergences.
- Pruning Effect: Egg-laying can prune weak branches, helping some trees by removing diseased or less vigorous growth.
These benefits help explain why forests and wildlife often flourish in the years following a major cicada emergence.
Common Misconceptions About Periodical Cicadas
Periodical cicadas are often confused with locusts, but they are not the same. Locusts are a type of grasshopper that can form destructive swarms, while cicadas are related to aphids and leafhoppers. Another misconception is that cicadas are dangerous; in reality, they pose little risk to humans, pets, or mature plants.
Some people worry about the noise, but the cacophony, while impressive, is temporary—usually lasting only a few weeks. Afterward, the cicadas vanish for another 17 years.
Frequently Asked Questions
- Are 17-year cicadas the same as annual cicadas?
No. Annual cicadas emerge every summer and belong to different genera (such as Neotibicen). They have green or brown coloring, are larger, and do not appear in synchronized swarms. Periodical cicadas are smaller, black-bodied with red eyes, and emerge en masse only every 13 or 17 years. - Do cicadas damage crops or gardens?
Cicadas do not eat leaves or fruits, so they rarely harm crops. The main risk is to young or newly planted trees, which can suffer from egg-laying damage. Wrapping young trees in netting during emergence helps protect them. - Can cicadas bite or sting humans?
No. Cicadas lack biting or stinging mouthparts and are harmless to people and pets. They may land on you out of curiosity, but they do not attack or pose any health risk. - Where will the next 17-year cicada emergence happen?
The location of the next emergence depends on the brood. For example, Brood XIII is expected in northern Illinois and surrounding states in 2024. Maps showing future brood locations are published by entomology departments and the U.S. Forest Service. - What should I do if cicadas are emerging in my area?
Enjoy the spectacle! If you have young trees, use fine mesh netting to protect them. Otherwise, cicadas are harmless and require no control. Many people take the opportunity to observe or photograph these fascinating insects.
Cicada Emergence Around the World: How Unique Are They?
While many countries have cicadas, periodical cicadas with synchronized 13- or 17-year cycles are unique to North America. Other cicada species found worldwide typically have much shorter, overlapping life cycles and do not emerge in such overwhelming numbers. This makes the North American periodical cicada phenomenon a truly rare natural event.
Role in Human Culture
Periodical cicadas have fascinated people for centuries. Native American legends, colonial records, and modern media have all remarked on their sudden, mysterious appearances. In some areas, cicadas are even celebrated with festivals, recipes, and educational events. Their predictable emergence has made them a subject of scientific study and public curiosity for generations.
Alternatives and Lookalikes: Not All Noisy Summer Bugs Are Cicadas
It’s easy to mistake other insects for periodical cicadas. Annual cicadas, which are larger and greenish, emerge every year and produce a different song. Grasshoppers and katydids, which also make noise in summer, are unrelated and follow very different life cycles. If you see a black-bodied insect with red eyes appearing in huge numbers, it’s almost certainly a periodical cicada.
Conclusion: Nature’s Timed Spectacle
The emergence of 17-year cicadas is one of nature’s most impressive and puzzling displays. These insects’ long, synchronized life cycles, mass appearances, and ecological roles make them a subject of enduring fascination. While their presence may be noisy and startling, they are a harmless and important part of the natural world, offering a rare glimpse into the wonders of biological timing and adaptation.