mariuscomper.uk RO

Periodical cicadas

Seventeen years underground

Under parts of the eastern United States, young cicadas are holding on to tree roots right now. They have been there for years. On a spring night in their seventeenth year (or thirteenth, for some), they will all come up within a few weeks of each other.

The wait

A female cicada cuts slits in a twig and lays her eggs there. Weeks later the young hatch, drop to the ground and dig down until they find a root. There they push their mouthparts into the wood and drink the sap that runs up from the roots, a watery fluid with very little food in it. That diet is one reason the wait is so long.

Underground they moult four times and pass through five stages, growing slowly. Nothing happens on the surface that you would notice.

How do they know what year it is?

They seem to read it from the tree. In 2000, three biologists in California moved fifteen-year-old nymphs of a 17-year brood onto the roots of potted peach trees, and used warmth and light to make the trees drop their leaves and flower twice in one year. Of the eleven nymphs that survived, eight came out a year early. The trees had given them an extra spring to count.

The button under the field runs that experiment. The year is then fixed by the count; the day by the soil. They come up when the ground 20 centimetres down passes about 18 °C, usually on a few warm nights in late spring.

Why all at once?

Coming out together is a defence. In one study of a 13-year brood, birds ate a large share of the cicadas where they were few, and only a small share where they were many: the predators were full long before the cicadas ran out. Under ideal conditions there can be 1.5 million in a single acre.

Adults live a few weeks. They sing, mate, lay eggs in the twigs and die, and the woods go quiet for another 13 or 17 years.

Why 13 and 17?

Both are prime numbers: nothing divides them but one and themselves. Two ideas, neither proven, say that is no accident. A cycle of 12 years would meet anything that peaks every 2, 3, 4 or 6 years, such as a predator whose numbers rise and fall, or a neighbouring brood it could interbreed with, every time it came out. A prime cycle meets them far less often.

Try it. Choose a cycle and which other cycles share its woods. The bars show, for every cycle from 8 to 20 years, how many of those peaks it meets each time it comes out, on average.

Other things that peak every…

Meetings per emergence, cycles of 8 to 20 years

By this measure 11, 13, 17 and 19 do equally well; it does not say why the cicadas settled on 13 and 17. A 2020 modelling study found exceptions to the idea that primes always overlap least. And bird numbers do seem to dip around emergences: a 2013 study found fewer of the birds that eat cicadas during the years they came out.

Which brood came out in your year?

Each group that comes out in the same year in the same area is called a brood, and has a Roman numeral. Twelve 17-year broods and three 13-year broods survive. Type a year: the year you were born, or one coming up. In 2024 two broods, XIII and XIX, came out together for the first time since 1803. The next time will be 2245.

Send it on

They spend 17 years on a tree root, counting the springs.

Sources

Brood years, biology and experiments come from these. The field above is drawn, not to scale, and its emergence is sped up; its years follow the counting rule from the peach-tree experiment.

  1. University of Connecticut, Simon lab, Periodical cicada brood table: the emergence years of each brood. States follow the brood table in Periodical cicadas and are approximate.
  2. T. Sota, Periodical cicadas (review), Ecological Research (2022): 12 surviving 17-year broods and 3 surviving 13-year broods; five nymphal instars feeding on root xylem; adults live about 4–6 weeks.
  3. R. Karban, C. A. Black and S. A. Weinbaum, How 17-year cicadas keep track of time, Ecology Letters 3, 253 (2000): peach trees made to drop their leaves and flower twice a year; of 11 nymphs that survived, 8 came out a year early.
  4. NOAA National Centers for Environmental Information, Brood X cicada emergence: emergence when soil 8 inches (20 cm) down exceeds about 64 °F (18 °C).
  5. The Nature Conservancy, What to know about Brood X: as many as 1.5 million in one acre.
  6. K. S. Williams, K. G. Smith and F. M. Stephen, Emergence of 13-yr periodical cicadas: phenology, mortality, and predator satiation, Ecology 74, 1143 (1993).
  7. W. D. Koenig and A. M. Liebhold, Avian predators are less abundant during periodical cicada emergences, but why?, Ecology 94, 1186 (2013).
  8. J. Yoshimura, The evolutionary origins of periodical cicadas during ice ages, American Naturalist 149, 112 (1997). J. Toivonen et al., Hybridization avoidance and the evolution of prime cycles (2020): exceptions to the idea that primes always overlap least.
  9. Library of Congress, Periodical cicadas: Broods XIII and XIX co-emerged in 2024 for the first time since 1803.
  10. Brood XI: Missouri Department of Conservation, Periodical cicadas, "only small numbers were seen in 1954, and then nothing". Brood XXI: last recorded in 1870.