For about four weeks, wooded and suburban areas will ring with the hissing and buzzing of cicadas. After mating, each female will lay hundreds of eggs in pencil-sized tree branches.
Then the adult cicadas will die. Once the eggs hatch, new cicada nymphs fall from the trees and burrow underground, starting the cycle again.
There may be 3,000 to 4,000 species of cicadas in the world, but the 13- and 17-year-old periodic cicadas of the eastern United States appear to be unique in combining long periods of juvenile development with synchronized and massive emergences of ‘adults.
These events raise many questions for both entomologists and the public. What are cicadas doing underground for 13 or 17 years? What are they eating? Why are their life cycles so long? Why are they synchronized? And is climate change affecting this wonder of the insect world?
We study periodic cicadas to understand questions about biodiversity, biogeography, behavior and ecology – evolution, natural history and the geographic distribution of life. We have learned a lot of surprising things about these insects: for example, they can time travel by changing their life cycle in four year increments. It is no coincidence that the scientific name of the 13 and 17 year old periodic cicadas is Magicicada, short for “magic cicada”.
Natural History
As a species, periodic cicadas are older than the forests they inhabit. Molecular analysis has shown that about 4 million years ago, the ancestor of the current Magicicada species divided into two lineages. Some 1.5 million years later, one of these lineages split again. The resulting three lineages form the basis of the modern periodic cicada species groups, Decim, Cassini and Decula.
Early American settlers first encountered periodic cicadas in Massachusetts. The sudden appearance of so many insects reminded them of the biblical plagues of locusts, which are a kind of grasshopper. This is how the name “locust” became incorrectly associated with cicadas in North America.
In the 19th century, notable entomologists such as Benjamin Walsh, CV Riley, and Charles Marlatt developed the astonishing biology of periodic cicadas. They established that unlike locusts or other grasshoppers, cicadas do not chew leaves, decimate crops, or fly in swarms.
Instead, these insects spend most of their lives out of sight, growing underground and feeding on plant roots as they go through five juvenile stages. Their synchronized emergences are predictable, occurring on a clockwork calendar of 17 years in the North and 13 years in the South and the Mississippi Valley. There are several regional age classes, called broods.
The five stages of the periodic underground juveniles of the cicada. Between each stage, the juvenile cicada moults to become bigger. The actual size of the fifth instar nymph is 0.83 inches. Chris Simon / CC BY-ND
Safety in numbers
The key feature of Magicicada the biology is that these insects emerge in large numbers. This increases their chances of accomplishing their key mission on the surface: finding partners.
Dense emergencies also provide what scientists call a predator satiety defense. Any predator that feeds on cicadas, whether it is a fox, a squirrel, a bat or a bird, will eat their fill well before consuming all the insects in the region, leaving many survivors behind.
While periodic cicadas largely emerge on schedule every 17 or 13 years, often a small group emerges four years earlier or later. Early emerging cicadas can be faster growing individuals who have had access to abundant food, and the laggards can be individuals who survived on less.
If growing conditions change over time, it is important to have the capacity to make this kind of life cycle change and come out either four years earlier in good times or four years later in more times. difficult. If a sudden phase of heat or cold causes a large number of cicadas to make a one-time mistake and fall off the schedule after four years, the insects may emerge in sufficient numbers to satiate predators and move to a new schedule.
Periodic cicada broods, identified by Roman numerals, emerge in 13- or 17-year cycles across the eastern and midwestern United States. University of Connecticut / CC BY-ND
Time of count for brood X
As glaciers retreated from what was now the United States about 10,000 to 20,000 years ago, periodic cicadas filled the eastern forests. The temporary switching of the life cycle has formed a complex mosaic of broods.
Today there are 12 broods of 17-year-old periodic cicadas in the northeastern deciduous forests, where trees drop their leaves in winter. These groups are numbered sequentially and fit together like a giant puzzle. In the Southeast and the Mississippi Valley, there are three broods of 13-year-old cicadas.
Because periodic cicadas are sensitive to climate, the patterns of their broods and species reflect climate change. For example, genetic and other data from our work indicate that the 13-year-old species Magicicada neotredecim, which is in the upper Mississippi Valley, formed shortly after the last ice age. As the environment warmed, the region’s 17-year-old cicadas emerged successively, generation after generation, after 13 years underground until they were permanently switched to a cycle of 13. years.
Member of Brood X laying eggs in 2004. Chris Simon / CC BY-ND
But it’s not clear whether cicadas can continue to evolve as rapidly as humans change their environment. Although periodic cicadas prefer forest edges and thrive in suburban areas, they cannot survive deforestation or breed in treeless areas.
Indeed, some broods have already disappeared. At the end of the 19th century, one brood (XXI) disappeared from northern Florida and Georgia. Another (XI) has been missing in northeastern Connecticut since around 1954, and a third (VII) in upstate New York has grown from eight counties to one since mapping began in the midst of the 1800s.
Climate change could also have far-reaching effects. As the American climate warms, longer growing seasons can provide a greater food supply. This could eventually turn more 17-year-old cicadas into 13-year-old cicadas, just as past warming has changed Magicicada neotredecim. Large-scale early emergencies took place in 2017 in Cincinnati and the Baltimore-Washington metro area, and in 1969, 2003, and 2020 in the Chicago metro area – potential harbingers of this kind of change.
Researchers need high-quality, detailed information to track the distribution of cicadas over time. Citizen scientists play a key role in this effort because periodic populations of cicadas are so large and their adult emergencies only last a few weeks.
Volunteers who wish to help document the emergence of Brood X this spring can download the Cicada Safari mobile app, provide snapshots, and follow our research in real time online at www.cicadas.uconn.edu. Don’t miss this opportunity – the next opportunity won’t present itself until Broods XIII and XIX appear in 2024.
John Cooley is a Assistant Professor of Ecology and Evolutionary Biology, University of Connecticut.
Chris Simon is a Professor of Ecology and Evolutionary Biology, University of Connecticut.
Disclosure statement: John Cooley has received funding from the National Science Foundation and National Geographic. Chris Simon receives funding from the National Science Foundation.
Republished with permission from The Conversation.
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