Each color on this map represents a brood of 13-year or 17-year cicadas, denoted by University of Connecticut researchers observing active cicada choruses. Broods XIII (brown) and XIX (orange) will emerge in 2024. Click on any point to see which brood it belongs to. Source: University of Connecticut, used with permission.

The key feature of Magicicada biology is that these insects emerge synchronously in huge numbers – as high as 1.5 million per acre. This increases their chances of accomplishing their key mission aboveground: finding mates.

Dense emergences also provide what scientists call a predator-satiation or safety-in-numbers defense. Any predator that feeds on cicadas, whether it’s a fox, squirrel, bat or bird, will eat its fill long before it consumes all of the insects in the area, leaving many survivors behind.

While periodical cicadas largely come out on schedule every 17 or 13 years, often a small group emerges four years early or late. Early-emerging cicadas may be faster-growing individuals that had access to abundant food, and the laggards may be individuals that subsisted with less.

If growing conditions change over time, as is happening now with climate warming, having the ability to make this kind of life cycle switch and come out either four years early in favorable times or four years late in more difficult times becomes important. If a sudden warm or cold phase causes a large number of cicadas to come out off schedule by four years, the insects can emerge in sufficient numbers to satiate predators and shift to a new schedule.

As glaciers retreated from what is now the U.S. some 10,000 to 20,000 years ago, periodical cicadas filled eastern forests. Temporary life cycle switching in diverse locations has formed a complex mosaic of broods.

Today there are 12 broods of 17-year periodical cicadas in northeastern deciduous forests, where trees drop leaves in winter. These groups are numbered sequentially and fit together like a giant jigsaw puzzle. In the Southeast and the Mississippi Valley there are three broods of 13-year cicadas.

Because periodical cicadas are sensitive to climate, the patterns of their broods and species reflect climatic shifts. For example, genetic and other data from our work indicate that the 13-year species Magicicada neotredecim, which is found in the upper Mississippi Valley, formed during a previous interglacial period about 200,000 years ago.

As the environment warmed, 17-year cicadas in the area emerged successively, generation after generation, after 13 years underground. Eventually, they permanently shifted to a 13-year cycle.

But it’s not clear whether cicadas can continue to evolve as quickly as humans are altering their environment. Although periodical cicadas prefer forest edges and thrive in suburban areas, they cannot survive deforestation or reproduce successfully in areas without trees.

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