How do cicadas know when to come out?

NEW YORK: Earlier this month, millions of Americans looked up at the sky to witness a total eclipse. Now, another cyclical marvel has arrived, this time at our feet. Trillions of noisy, red-eyed insects called cicadas are emerging from the earth after more than a decade of feeding on tree roots. The United States is home to 15 cicada broods, and in most years at least one of them emerges. This spring, Brood XIX, known as the Great Southern Brood, and Brood XIII, or the Northern Illinois Brood, are emerging simultaneously.

Cicada watchers have spotted the first insects coming out of the ground, reporting their sightings to apps such as iNaturalist and Cicada Safari. The Great Southern Brood, which emerges across the South and the Midwest every 13 years, has been seen at sites scattered from North Carolina to Georgia. The Northern Illinois Brood, which appears every 17 years in the Midwest, is expected to appear in the next month, as temperatures there warm.

How cicadas manage to rise en masse after spending so long underground remains largely a mystery. There's surprisingly little information about cicadas that you'd like to know, said Raymond Goldstein, a physicist at the University of Cambridge.

Once a brood climbs out of the ground, the cicadas crawl up trees to mate, and the females lay eggs in tree branches. After hatching, the young insects drop to earth and burrow into the soil. Then, each cicada spends the next 13 or 17 years underground before emerging to mate and repeat the cycle.

That means that trillions of insects have to track the passage of time in the soil. It's possible that they detect annual changes in tree roots. But how can cicadas add up those changes to divine when 13 or 17 years have passed? Scientists cannot say.

Chris Simon, a cicada expert at the University of Connecticut, suspects that some answers will be found in the insects DNA. Is there a consistent difference between something that has a 13-year cycle and a 17-year cycle? she asked.

Dr. Simon and her colleagues recently sequenced the genome of a cicada for the first time. They caught the insect, which belonged to a brood with a 17-year cycle, in Tennessee in 2021. They hope to sequence the genes of insects from other broods as well, and compare their DNA.

Once cicadas recognize somehow  that they've reached their special year, they need a way to emerge together. Evolutionary biologists have proposed that cicadas come out in vast numbers as a survival strategy. Their enemies, such as birds and parasitic wasps, can attack only a small fraction of them, leaving the rest free to reproduce.

One crucial signal is the temperature of the ground. The soil needs to pass a threshold of about 64 degrees before broods start to appear.

But cicadas cannot surface together simply by sensing the warming soil. An immature cicada that happens to be a couple feet underground will experience cooler temperatures than one just a few inches below the surface. If cicadas paid attention only to the temperature they felt nearby, they would come out in small groups and be quickly wiped out by predators.

Dr. Goldstein and his wife, Adriana Pesci, a mathematician at Cambridge, recently became intrigued by this paradox. We're attuned to mysteries, Dr. Goldstein said.

Working with their Cambridge colleague Robert Jack, Dr. Goldstein and Dr. Pesci created a mathematical model of an underground cicada brood based on observations of real insects. Then, they played with the different variables in their model to get the simulated cicadas to emerge together like real ones.

The scientists speculated that cicadas base their decision to come out not just on the rising temperature of the soil, but also on the actions of neighboring cicadas. The researchers allowed their virtual insects to eavesdrop on each other. If their neighbors were getting noisy as they prepared to climb out of the ground, the insects were more likely to emerge as well.

It turned out that the model worked only if the scientists let the cicadas communicate this way. The combination of temperature and communication caused broods to emerge in a rapid series of bursts which is exactly what happens in the real world. Each burst included cicadas that were in soil that had not yet reached 64 degrees. Once they left, it took a few days for the soil to warm enough to prompt more cicadas to prepare to leave.

Dr. Goldstein acknowledged that he and his colleagues simply added a hypothetical communication channel to their model to make it work. They have no direct evidence that cicadas actually listen to each other underground.

Nobody's ever tried to figure that out, Dr. Simon said. It would be very difficult to do.

As bizarre as cicada emergences may seem, Dr. Goldstein sees them as part of a broader pattern in biology. Many animals, from flocking birds to herds of wildebeests, have to make collective decisions based on noisy, unreliable signals. Even cells in a developing embryo have to coordinate their growth.

Its the essence of life, he said.