Nearly every year in the United States, millions of periodical cicadas emerge from over a decade
of underground transformation.
In 2021, Brood X takes its turn.
Brood X is one of the largest groups of cicadas that emerge every 17 years.
Their powerful chirps can be heard in 15 states.
Periodical cicadas are grouped by broods—
defined by the year and how often they emerge.
Insects in 12 broods emerge once every 17 years,
while three groups, mostly in the southeastern United States, come out every 13 years.
Within these broods, there are three types of cicadas—Decim, Decula and Cassini.
For decades, researchers have been fascinated by how these lifecycles form—
and how they might change in the future.
In 1988, researcher Chris Simon published
the first evidence of 17-year cicadas switching to a 13-year lifecycle.
We were just sampling the entire range of the largest brood of 13-year cicadas.
And as we got into the northern part of that distribution, we noticed that they were slightly smaller,
that their abdomens were more black instead of orange.
We didn't think too much about it until we got back to the laboratory.
We realized that the DNA of the Northern ones
that were smaller and blacker, it actually matched the mitochondrial DNA of the 17-year cicadas
rather than the 13-year cicadas.
It immediately came to me that they had to be
17 years cicadas that recently switched their life cycle to 13 years.
Recently in geological time, that is—
the shift likely happened about 200,000 years ago.
The boundary between these different species of 13-year cicadas occur at the border
between northeast and southeast deciduous forests.
Simon believes that a warmer climate in the northern Mississippi river valley caused this lifecycle shift.
Over this warming period, cicadas would come more often,
four years early. But it happened so many times, and so often, that they gradually became genetically assimilated.
So rather than being a plastic trait, it's now a genetic trait.
These findings suggest that lifecyles are
a plastic trait cicadas can alter their lifecycles in response to changing environments.
All periodical cicadas, we think, have the opportunity to come out four years early or
four years late. They're growing at all different rates underground.
In order to become synchronized and periodical, they had to develop a waiting period.
So by waiting in these four-year windows, they’ll have more individuals to come out with them.
It allows evolutionary flexibility to an organism that otherwise has this very rigid, programmed life cycle.
Emerging in greater numbers at the same time
gives the insects a better chance of surviving—and reproducing.
Simon had repeatedly seen reports about cicadas emerging early.
In 2017, she surveyed the four-year early emergence of Brood X cicadas
in the Washington, D.C. area.
We've never seen such massive early emergences with singing for the entire four weeks.
We drive around, and we listened for the sounds of the different species.
And then we record them here on our automatic data logger.
This year we've come back, just to make sure
that the main range of the brood is also appearing in exactly the same places—and they are.
Everywhere that we saw four-year early in 2017, we're now seeing even more dense populations of Brood X
It is possible that enough cicadas survived
and reproduced in 2017 that they will come out in 2034—although another four-year early
emergence of Brood X could happen at the same time.
Researchers are still uncertain how cicadas keep track of when these four-year windows are coming up.
Simon plans to study whether cicadas’ epigenetics— changes to gene expression—
could be driving this capability.
If we have continued climate change, which we're expecting,
and a continued warming, then we would expect
more and more individuals to come out four years early.
If Simon’s hunch is right, we may not have to wait another 17 years to meet Brood X once again.