At this very instant, in backyards and forests across the eastern U.S., one of nature’s greatest spectacles is underway. Although it may lack the epic majesty of the wildebeest migration in the Serengeti or the serene beauty of cherry blossom season in Japan, this event is no less awe-inspiring. I’m talking about the emergence of the Brood X cicadas.

Every 17 years the billions of constituents of Brood X tunnel up from their subterranean lairs to spend their final days partying in the sun. This generation got its start back in 2004, when Facebook existed only at Harvard University and Friends aired its last episode. The newly hatched cicada nymphs fell from the trees and burrowed into the dirt. They have been underground ever since, feeding on sap from the rootlets of grasses and trees and slowly maturing. All of that preparation has been leading up to this moment when they surface in droves—up to 1.4 million cicadas per acre—to molt into their adult form, sing their deafening love song and produce the next generation before dying just a few weeks later.

To early European settlers in North America, the sudden appearance of these insects in large numbers brought to mind the locusts of biblical infamy. But whereas locusts are grasshoppers that form giant swarms and travel long distances, devouring crops on a devastating scale, cicadas belong to an entirely different order of insects. They do not swarm and are poor fliers, typically traveling no more than several hundred feet. Moreover, they pose little threat to plants because they do not eat plant tissues. Females do make incisions in twigs for their eggs, which can weaken saplings but not mature trees and shrubs.

Nearly 3,400 species of cicadas exist worldwide. But periodical cicadas that emerge en masse once every 17 or 13 years are unique to the eastern U.S. The 17-year cicadas live in the North, and the 13-year cicadas are found in the South and the Mississippi Valley. The three species of 17-year cicadas—Magicicada septendecim, M. cassinii and M. septendecula—form mixed-species cohorts called broods whose members arise like clockwork on the same schedule. The broods are identified by Roman numerals. Brood X is the largest of the 12 broods of 17-year cicadas, which emerge in different years.

Series of maps shows the geographical ranges of 12 17-year broods and three 13-year broods of cicadas in the U.S.
Credit: Daniel P. Huffman and John Cooley
Illustrations show the three 17-year cicada species—Magicicada cassini, Magicicada septendecula and Magicicada septendecim.
Credit: Cherie Sinnen

The periodical life cycles of these cicadas, with their long developmental phases and synchronized emergences, have long captivated scientists. Most other cicadas studied thus far have life cycles of three to five years, says Chris Simon of the University of Connecticut. Their nymphs grow at different rates depending on genetic and environmental factors, and they stage their exit from underground once they reach a certain body size and level of development. As a result, the offspring of any one female come out in different years, she explains. Periodical cicadas, in contrast, stay belowground for a fixed amount of time, regardless of when they reach full size, and then emerge together.

Exactly how periodical cicadas came to have these unique life history patterns is an area of active research. DNA analyses suggest an approximate time line of their evolution. The last common ancestor of all living Magicicada species branched into two lineages around 3.9 million years ago during the Pliocene epoch. One of these branches itself diverged 1.5 million years later during the Pleistocene. The three resulting lineages ultimately gave rise to the seven species of 13- and 17-year cicadas alive today. Why these cicadas settled on 13- and 17-year schedules is unknown. One hypothesis holds that having long, prime-number cycles might boost their odds of survival by offsetting their emergence from predator-population booms that occur more frequently and on composite-number cycles. But the two other known periodical cicadas—one in Fiji and the other in India—emerge at eight- and four-year intervals, respectively.

Researchers have proposed that periodical cicadas evolved from nonperiodical cicadas by trading a size-based emergence schedule for an age-based one and extending the development period. Climate change probably helped drive this shift. Periodical cicadas are sensitive to temperature—it determines the length of the growing season. During the Pleistocene, cooling temperatures would have slowed juvenile development on average but increased the variation in the growth period, making the timing of adult emergence in ancestral cicadas even more variable than before. With the resulting reduction in the density of adult cicadas emerging in any given year, mating opportunities would have dwindled. Under such conditions, switching from a size-based emergence strategy to an age-based one in which the insects remain underground for a long time and then surface simultaneously would increase the adult population density at emergence and thus their opportunities to find mates and reproduce.

Emerging simultaneously in huge numbers also overwhelms predators. Consequently, even after the birds, mammals and fish have sated themselves on the plump, defenseless insects, plenty of cicadas remain to produce the next generation.

Climate change also shaped the distribution of the broods. As North America’s ice sheets advanced and retreated over the past 20,000 years, the deciduous forests that cicadas inhabit shrank and expanded. Broods evolved in response to those cooling-warming cycles. Gene Kritsky of Mount St. Joseph University in Cincinnati, Ohio, points to Brood X in the western part of his state as an example. Twenty thousand years ago the ice sheets extended to just north of where Cincinnati is today. Because the land was covered in ice, there were no forests, and thus no cicadas, in western Ohio back then. Around 14,000 years ago, however, the ice sheet retreated north. “Forests came in, and periodical cicadas came with them,” Kritsky explains. Ohio hosts three other 17-year cicada broods, each of which occupies its own region of the state. “The distribution in Ohio of 17-year cicadas matches the physiographic regions created by the ice ages,” he observes.

Periodical cicadas have been able to adapt to climate change in part because they have some plasticity in their life-cycle length: they can accelerate or decelerate their emergence schedules by four-year increments. But this flexibility does not assure their long-term survival. Brood XI has been extinct since around 1954; others are waning. The main threat is habitat loss, according to Kritsky. In 1919 the U.S. Department of Agriculture predicted the demise of Brood X as a result of deforestation.

Mapping periodical cicada emergences helps scientists gauge how the broods are faring. Researchers have asked the public to report sightings for decades—in the old days via postcard and later by phone and e-mail. Now they are crowdsourcing data with an app that Kritsky and his colleagues developed, called Cicada Safari, that allows people to submit pictures and videos of any cicadas they encounter and view a map of the Brood X emergence in real time as it unfolds. “In 1902 the USDA based its map on just under 1,000 postcards it received,” Kritsky says. This year, through the app, “we’re hoping to get 50,000 photographs.” A fitting send-off for the Brood X class of 2021.