Invertebrates
Periodical Cicada
Magicicada septendecim · also called 17-year cicada, periodical cicada, 17-year locust
Periodical cicadas run one of the strangest schedules in the animal kingdom. For 13 or 17 years they are invisible, feeding underground on tree roots. Then, in a single synchronised week, they erupt from the soil in numbers so vast — over a million per acre — that the forest roars, and no predator on Earth can eat them fast enough to matter.
Overview
Periodical cicadas (genus Magicicada) live in the deciduous forests of the central and eastern United States. They are divided into broods, each emerging in a particular year across a particular region; 17-year broods are generally northern and 13-year broods southern. Within a brood, several species emerge together, separated by their songs. In between emergences the only trace of them is the nymphs, buried 20–40 centimetres down, sipping dilute sap from roots and slowly growing through five stages.
The prime-number strategy
The extraordinary features of the life cycle — its length, its synchrony, and the fact that the intervals are prime numbers — are best explained as defences against being eaten:
- Staying underground for years and surfacing all at once means predators cannot stockpile to exploit them.
- The sheer glut means birds, fish, snakes, raccoons, and even domestic pets gorge themselves and still leave the vast majority untouched to reproduce — a tactic called predator satiation.
- A prime interval rarely coincides with the shorter population cycles of predators, or with other cicada broods, so the timing stays clean.
Broods that emerged early, late, or in smaller numbers would have been picked off; only the perfectly synchronised ones persisted.
Not locusts
Colonists called them “17-year locusts,” but they are not locusts and do not eat foliage or crops. The only damage is minor: females slit thin twigs to lay eggs, which can break the tips of saplings. The billions of dead adults are a major fertiliser pulse for the forest floor.
The emergence
When the soil warms to about 18 °C, mature nymphs tunnel up, climb, and moult into adults, leaving empty shells everywhere. For two to four weeks the males form deafening choruses — over 100 decibels — the cicadas mate, and females lay up to 600 eggs apiece before dying. Six weeks later the tiny hatchlings drop, burrow, and reset the clock.
Status
Periodical cicadas have not been formally assessed. Some broods number in the trillions, but each brood is geographically limited, and localised broods have been lost to deforestation and urban sprawl. The great spectacle of a full emergence depends on large blocks of mature forest surviving in place for decades at a time.
Periodical Cicada: common questions
Why do periodical cicadas stay underground for 13 or 17 years?
The long cycle and the specific prime numbers both appear to be anti-predator strategies. Spending most of the life cycle safely underground, and then having the entire population surface in a single brief pulse, means that no predator can build up its numbers to match the feast — the cicadas are gone again before any bird or mammal population can respond. Choosing a prime-numbered interval like 13 or 17 further reduces the chance that the emergence coincides with predators that have 2-, 3-, 4-, or 5-year population cycles, or that two different cicada broods emerge together and hybridise into an intermediate, less-synchronised cycle. Broods that got the timing wrong would have been eaten out of existence.
Are periodical cicadas the same as locusts?
No. Early European colonists called them "17-year locusts" because a mass emergence looked like a biblical plague, but locusts are grasshoppers that swarm and strip vegetation, while cicadas are sap-suckers in a completely different insect order and do not eat leaves or crops. The adults do minor damage by slitting thin twigs to lay eggs, which can flag or break the tips of young trees, but a mature forest shrugs it off, and the huge influx of dead cicadas is a significant pulse of fertiliser for the soil.
How loud is a periodical cicada chorus, and how is the sound made?
A large chorus can exceed 100 decibels — comparable to a chainsaw or a rock concert — and is loud enough to interfere with conversation and to be picked up by weather radar as it rises off the forest. Male cicadas produce it with a pair of ribbed membranes called tymbals on the abdomen; muscles buckle each tymbal in and out hundreds of times a second, and the largely hollow abdomen acts as a resonating chamber. Different Magicicada species sing at different pitches and rhythms, which keeps the broods reproductively separate.
What happens during an emergence?
Over a few nights when the soil about 20 cm down reaches roughly 18°C, the mature nymphs dig to the surface, climb the nearest vertical surface, and moult into winged adults, leaving their split brown shells everywhere. For the next two to four weeks the males sing in deafening choruses, the sexes mate, and females cut slits in twigs and lay up to 600 eggs each. The adults then die en masse. About six weeks later the pinhead-sized hatchlings drop to the ground, burrow in, and begin the next 13- or 17-year countdown.