Periodical cicadas spend the majority of their time underground as nymphs. Then, every 13 or 17 years, a whole group of billions of cicadas emerges together. The insects rush out all at once, completing one of the most unusual lifecycles in nature. But why do they do it, and how do they know when to do it?
Why cicadas have this peculiar cycle
Nature often operates with year-long patterns, where many organisms are attuned to the seasonal changes of the environment. Migration is a good example of this. Some predators also have natural cycles that happen every 2 or 3 years. So, cicadas, which are almost defenseless against most predators, found a way to take advantage of these cycles. They deviate from all norms and emerge at prime-numbered life cycles of 13 or 17 years.
These cycles are not only unusually long but also effectively desynchronize them from the cycles of potential predators and competitors. This essentially means no predator species can adjust to their cycles.
The cicadas have another advantage, although it’s not a very elegant one. To ensure enough of them end up surviving, they simply overwhelm with numbers. That’s why cicadas come out in such great numbers at such odd times: so they can evade predators and ensure enough of them survive — the predators can’t consume them all.
But how do these creatures, which spend the majority of their life underground, know when it’s time to emerge en masse? The answer involves a complex interplay of biology, environmental signals, and an inherent biological clock.
How cicadas know when to come out
We should mention that not all cicadas are periodical. There are over 3,000 cicada species, and only some of them are periodical — the others have “normal” yearly cycles.
Researchers have spent a long time trying to figure out how exactly cicadas synchronize themselves. While it’s still not entirely clear, it’s probably linked to environmental signals and a molecular clock. Dr. Chris Simon, molecular systematist at the University of Connecticut’s Department of Ecology and Evolutionary Biology told Entomology Today:
“The year of cicada emergence is cued by what I and others believe to be an internal molecular clock,” she said. “The clock is most likely calibrated by environmental signals that signify the passage of a year, such as the trees leafing out, changing the composition of the xylem fluid on which they feed. The molecular clock keeps track of the passage of years. The accumulation of 13 or 17 years triggers the emergence of fifth instar nymphs. The day of emergence is triggered by accumulated ground temperature. This was demonstrated by James Heath in a study published in 1968.”
Basically, what the study concluded was that the cicadas use their internal clock to know what year it is. This is possibly linked to tree cycles as well, which change their chemistry slightly based on yearly cycles. Then, when the year is right, they follow thermal signals to know when the season is right.
This is also tied into the cicadas’ moulting cycles. They have five moulting cycles, the fifth one being the one they emerge at. To sum it up, cicadas know when to come out thanks to two main factors:
2024’s arrival
The year 2024 is a special one for cicadas. Two major broods, Brood XIII and Brood XIX, will appear this year.
This is even more exciting because they are different kinds of cicadas. Brood XIII cicadas remain underground for 17 years, while Brood XIX remain underground for 13 years. Both broods last emerged simultaneously in 1803.
Even though a large number of cicadas are expected to emerge in a relatively short time, but there's no need for concern. Cicadas are not harmful to humans or pets, unless the pets consume a large quantity, in which case they may experience digestive issues.
Even for most plants, cicadas don’t cause damage. They are not locusts, and the only harm they inflict on plants is when they deposit their eggs in small cuts on the plants. If you live in a cicada area and want to take extra precautions, you can temporarily close your windows and cover your plants.
In general, the cicada emergence is a natural spectacle — one of the unusual phenomena that make nature so impressive and surprising — not something to be feared.
