by A cicada synonymous with its brood is a cicada with chance.
The emergence of the insect’s synchronization is an evolutionary strategy, scientists say. Birds, raccoons and other predators can only eat so much of them. So the more cicadas that evolve together, the better the chance that more of their genes will survive to reproduce and pass on.
“They have the safety-in-numbers strategy,” said Chris Simon, a professor of ecology and evolutionary biology at the University of Connecticut who studies the insects.
In contrast, the rare cicadas that lose track of time and emerge without their relatives are called “stragglers”. Most of the small groups catch up with stragglers and do not survive to reproduce.
“Natural selection favors individuals who stay, because those who don’t stay get eaten,” Simon said.
This summer, the number of periodical cicadas is expected to be very large, as two broods arrive at the same time. The last time these two appeared together was in 1803. Thousands of insects are predicted to emerge. Users of the Cicada Safari app, which is designed to report cicada sightings and help scientists track the insects, have seen more than 1,000 sightings in Georgia and hundreds in North Carolina and Alabama.
Period cicadas fall into two brood categories, or age classes: those that take 13 years to emerge and those that take 17 years. The temperature seems to rise when they emerge, but how exactly they set their internal clocks or signal when to emerge from the ground together remains a mystery.
In addition, the scientists say they observed some changes in the insects’ rhythms, which led to hypotheses that elevated temperatures may be rewiring the internal clocks of some periodical cicadas.
Gene Kritsky, an entomologist and cicada expert at Mount St. Joseph University in Ohio, said that as average temperatures have risen higher due to global warming, emergence dates have shifted earlier in the calendar year.
“Cicadas are insects of the climate,” he said, adding, “They’re emerging now almost 10 days to two weeks earlier than they did in 1940.”
John Cooley, a University of Connecticut cicada researcher who maps cicada hatcheries, said he expects the bugs’ range to shift northward as the climate and plant species they prefer shift northward.
He also noted an increase in reports of stragglers, a trend that is interesting to researchers.
“If you look at the data, we definitely have more straggling reports now than ever before,” Cooley said. “That could be because there are more stripes than before or because we have the internet and if you see this weird bug in the yard, you can send it in.”
Simon said Brood XIII, emerging this year, produced a record number of stragglers in 2020.
“This time four years ago, there were so many that they were out for the whole four weeks,” she said. “They didn’t eat completely. They were able to sing and lay eggs. So they could be creating a new population.”
“They are definitely responding to climate warming and the length of the growing season,” Simon added.
Simon has a theory about the possible role of climate change: She thinks that rising temperatures are extending the growing seasons of the plants cicadas feed on, crowding out the development of underground insects. Many other “stragglers” may soon emerge as a result. Eventually, she said, the entire population will adapt and change with the times.
Simon predicted that 17-year cicada broods will transition to 13-year cicadas. And 13-year cicadas could emerge every nine years.
If the theory is true, it would be another example of how climate change is disrupting the regular closures that govern the natural world.
Periodical cicadas, which are harmless to humans, are distributed throughout the Eastern and Midwestern U.S. Unlike annual cicada species, which reappear each year, the emergence of cicadas like those that come out in the Southeast now. There are 12 broods that emerge on 17-year cycles and three broods with 13-year cycles in the United States.
During their years underground, periodical cicadas spend their time feeding on plant roots. Once above ground, they engage in song-filled mating rituals, trying to lay eggs before making lunch for a bird or raccoon.
Scientists are still trying to figure out exactly how cicadas choose a certain period of days to emerge together.
Although temperature clearly plays a role, ground temperature is rarely uniform in cicada habitat, and the insects are often placed at different layers in the soil. A study this year said there is no clear explanation for the success of cicadas’ coordination and suggested scientists should investigate whether they can communicate underground.
“No one has ever studied this,” Simon said.
This article was originally published on NBCNews.com
