Why Do Cicadas Come Out Every 17 Years? Periodical Broods

What happens during those 17 years?

A periodical cicada is not asleep underground. It hatches from an egg laid in a tree twig, drops to the ground and burrows down to a root. For the next 17 years the nymph sucks xylem sap, the watery fluid that carries water and minerals up from the roots. Sap is so poor in nutrients that the nymph grows very slowly, molting through five stages and moving to larger roots as it grows.

In spring of the final year, the mature nymphs tunnel up toward the surface, emerge in the evening, climb a tree and molt into winged adults. The adults live only about four to six weeks, so periodical cicadas spend about 99.5% of their lives underground.

The unusual part is not the long life, since many insects take years to develop. It is the precision: in a given area, nearly every cicada emerges in the same few weeks of the same year.

Safety in numbers: predator satiation

The main benefit of a synchronized emergence is predator satiation. In the densest areas, more than 1.5 million cicadas can emerge per acre, or about 370 per square meter. Birds, squirrels, raccoons, foxes, turtles, fish and even snakes gorge on them, but they quickly become too full to eat more, and most cicadas survive long enough to mate.

Predators cannot simply breed more young to take advantage, because the feast lasts only a few weeks and then vanishes for 17 years. A cicada that emerged in the wrong year would face hungry predators alone and almost certainly be eaten. This is why “straggler” cicadas that come out early or late usually have little chance of breeding.

The emergences ripple through the whole ecosystem. Moles thrive on the large nymphs in the year before an emergence, wild turkeys benefit from the abundance of adults, and egg-laying damage to twigs can reduce the nut crops that eastern gray squirrels depend on.

Why 13 and 17? The prime-number idea

Both 13 and 17 are prime numbers, divisible only by 1 and themselves, and scientists have long wondered whether that matters. One idea concerns predators with shorter population cycles. A predator with a three-year cycle that benefits from one emergence will be out of step at the next one, 17 years later, because 17 is not a multiple of 3. A cicada with a 15-year cycle, by contrast, would meet a peak of three-year or five-year predators every time it emerged.

A second idea is that prime-numbered cycles reduce crossbreeding between broods. Broods with 13- and 17-year cycles emerge together only once every 221 years (13 × 17), while cycles such as 12 and 15 would overlap far more often. A computer model published in 2009 by Jin Yoshimura and colleagues found that prime-numbered cycles outlasted others when populations were small and mates were scarce, conditions that may have occurred during the Ice Age.

Genetic studies suggest that 13- and 17-year cycles have evolved several times within the last few million years, and that the split between the 17-year Magicicada septendecim and its 13-year relative happened about 530,000 years ago. None of these explanations is proven, and they are not mutually exclusive.

How do cicadas count the years?

Cicada nymphs have no calendar, but they appear to count the seasonal cycles of their host trees. Each spring, as trees leaf out, the sap in their roots changes in content and flow, and nymphs seem to register these yearly pulses.

The best evidence came from an experiment in California published in 2000. Richard Karban and colleagues moved 15-year-old nymphs of a 17-year brood onto the roots of peach trees that had been manipulated to flower and leaf out twice a year. Many of the cicadas emerged after 16 years instead of 17, one year early, suggesting they had counted the extra tree cycles.

Once the count is complete, the final trigger is temperature. Mature nymphs wait near the surface until the soil about 20 cm (8 in) down warms to about 17.9 °C (64 °F), which happens from late April in the South to early June in the North.

Which broods emerge next?

Periodical cicadas are grouped into broods, numbered with Roman numerals since 1907. Fifteen broods survive today: 12 with a 17-year cycle and 3 with a 13-year cycle.

The famous Brood X last emerged in 2021, and Broods XIII and XIX emerged together in 2024 for the first time since 1803.

Stragglers and shifting cycles

The timing is not perfect. Stragglers often emerge exactly four years early in 17-year broods or four years late in 13-year broods, and sometimes one year off. If enough stragglers survive together, a new brood can form, and some 17-year populations may have switched to 13-year cycles this way in the past.

Different species of periodical cicadas are thought to have formed partly because broods separated in time stopped interbreeding. This makes them a classic example of how timing alone can split one population into two.

Do any other cicadas have cycles?

Most cicadas are annual cicadas. Their nymphs take anywhere from about two to ten years to develop, but generations overlap, so some adults appear every summer. In North America the familiar “dog-day” cicadas of July and August are annual cicadas.

Only a few other cicadas show regular mass emergences. A cicada in northeastern India, nicknamed the World Cup cicada, emerges every four years, and one species in Fiji emerges every eight years. Neither comes close to the 17-year record of North America’s periodical cicadas. When a brood does emerge, the deafening chorus is explained in Why are cicadas so loud?, and whether all those insects can hurt you is covered in Do cicadas bite?