Earlier this month, millions of Americans looked up to the skies to witness a total solar eclipse. Well, a new cyclical wonder has now arrived 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.
There are 15 cicada swarms in the United States, and in any given year, at least one of them emerges. This spring, Brood XIX, known as the Great Southern Brood, and Brood XIII, known as the Northern Illinois Brood, will appear at the same time.
Cicada watchers spotted the first insects emerging from the ground and reported sightings to apps like iNaturalist and Cicada Safari. The Great Southern Brood appears in the South and Midwest every 13 years and has been spotted in locations from North Carolina to Georgia. The northern Illinois brood, which appears in the Midwest every 17 years, is expected to emerge next month as temperatures rise.
How cicadas are able to emerge en masse after spending so long underground remains largely a mystery. “There's surprisingly little information we know about cicadas,” says Raymond Goldstein, a physicist at the University of Cambridge.
When the cicada chicks crawl up from the ground, the cicadas crawl up trees to mate, and the females lay eggs on tree branches. The hatched larvae fall to the ground and burrow into the soil. Each cicada then spends the next 13 or 17 years of her life underground, then comes above ground to mate and repeat the cycle.
That means trillions of insects must be tracked over time in the soil. You may be detecting annual changes in tree roots. But after 13 or 17 years, how can the cicada accumulate the changes and make them sacred? Scientists cannot say.
Chris Simon, a cicada expert at the University of Connecticut, thinks insect DNA may hold some answers. “Is there a consistent difference between the 13-year cycle and the 17-year cycle?” she asked.
Dr. Simon and his colleagues recently sequenced the cicada genome for the first time. They captured the insect, which is part of a 17-year swarm, in Tennessee in 2021. They also hope to sequence the genes of other groups of insects and compare their DNA.
When the cicadas somehow realize that they're having a special year, they need a way to emerge together. Evolutionary biologists have proposed that cicadas spawn in large numbers as a survival strategy. Enemies such as birds or parasitic wasps can attack only a small portion of them, leaving the rest free to reproduce.
One important signal is the ground temperature. Before the chicks begin to appear, the soil must exceed a threshold of about 64 degrees.
However, simply sensing soil warming is not enough to cause cicadas to emerge en masse. An immature cicada that happens to be several feet underground will experience cooler temperatures than a cicada that is just a few inches below the surface. If cicadas were only concerned with the temperature they feel nearby, they would come out in small groups and be quickly wiped out by their natural enemies.
Dr. Goldstein and his wife, Adriana Pesci, a mathematician at the University of Cambridge, recently became intrigued by this contradiction. “We're sensitive to mystery,” Dr. Goldstein says.
Working with colleague Robert Jack from the University of Cambridge, Dr. Goldstein and Dr. Pesci created a mathematical model of underground cicada swarms based on observations of real insects. They then manipulated various variables in the model to make the simulated cicadas emerge en masse, just like the real thing.
Scientists suspect that cicadas base their decision to go outside not only on rising soil temperatures, but also on the behavior of nearby cicadas. Researchers enabled virtual insects to eavesdrop on each other. If your neighbors are making noise trying to crawl out of the ground, insects are also more likely to appear.
It turns out that this model only works if scientists get cicadas to communicate in this way. A combination of temperature and communication resulted in the emergence of chicks in rapid succession. This is exactly what happens in the real world. Each explosion involved cicadas that were in soil that had not yet reached 64 degrees. After the cicadas left, it took several days for the soil to warm up enough for more cicadas to begin preparing to leave.
Dr. Goldstein acknowledged that he and his colleagues simply added hypothetical communication channels to the model to make it work. There is no direct evidence that cicadas actually hear each other underground.
“No one has ever tried to figure it out,” Dr. Simon says. “That would be very difficult to do.”
Although cicada emergence may seem strange, Dr. Goldstein sees it as part of a broader pattern in biology. Many animals, from flocking birds to herds of wildebeest, must make collective decisions based on noisy and unreliable signals. Even the cells of a developing embryo need to regulate their growth.
“That's the nature of life,” he said.