It was a good question. We were in the territory of Brood X cicadas, set to emerge this year. But these cicadas emerged from their 17-year adolescence four years early. Such a phenomenon is not unheard of: Scientific literature documents early emergences as far back as 1969, when a group of cicadas surprised the Chicago suburbs. But the premature occurrences are usually sparse — the odd male here, a female there. Easy pickings for the birds. As a result, early risers typically cannot establish self-reproducing populations.
But 2017 was different. The chorusing cicadas lasted for four weeks and sounded like a normal emergence. There were so many that the birds could not possibly eat them all, and several entomologist friends in the area confirmed that they laid eggs, and that the eggs hatched later that summer.
The reasons for early emergences are not fully understood, but we expect climate change plays a role. As we prepare for the rest of Brood X to crawl out from their earthly dwellings this year, we may see further evidence of what scientists have been piecing together for years: These bugs are changing dramatically as humans take a toll on the environment.
The same questions we were asking four years ago will be top of mind this spring. In the warmer southern United States, periodical cicadas have no trouble completing development in 13 years. Is climate change allowing 17-year cicadas to grow up faster? Will these four-year-early events continue to increase in number and intensity? Will they affect the size of 17-year broods?
By midcentury, the climate of Connecticut is predicted to be similar to that of North Carolina now. Will the cicadas there move north? Will they once again thrive in New England and southern Wisconsin, where they are currently extinct? These are the questions evolutionary biologists are asking — and to answer them, we have to look to the past.
In the past 10,000 years, periodical cicadas moved from what is now the southeastern United States to occupy an area stretching as far north as Nebraska, southern Michigan and Massachusetts. They returned north as the advancing edge of the deciduous trees recolonized their interglacial range and coniferous forests retreated to what is now Canada and northern New England.
But their movements are very slow. Periodical cicadas are not the best fliers; they do not fly much farther than the width of a football field. Males gather in chorusing trees, and females only move to find mates and lay eggs in the young and vigorously growing trees on forest edges that are likely to be standing in 17 years.
Clearing land for agriculture in the eastern United States knocked back many cicada populations, especially in the 1800s and early 1900s. Reforestation was not able to save the now-extinct Brood XI in New England. Today, further urbanization, as well as climate change, are the most powerful forces facing cicadas, though it’s unclear which of the two impacts the bug’s future more. Highways, asphalt and cement will block migration unless green corridors are pieced together. Climate change will gradually alter tree species composition — but periodical cicadas are generalists, and can lay eggs and feed in many types of trees and shrubs. Periodical cicadas can thrive in suburbs as long as new developments are close to forest edges, so that females can fly from the forest and start new populations where people have planted trees.
This has happened throughout the D.C.-Baltimore area, where there is a mosaic of 17-year cicadas, including Broods II and X. The four-year-early emergence of Brood X may eventually be called Brood VI, because the brood numbers refer to the year of the cicadas’ emergence, in 17-year cycles. With climate change, we may see a patchwork of Broods II, VI and X in the D.C.-Baltimore suburbs.
If the climate continues to warm, and this cycle repeats over and over, all of the 17-year cicadas in this area could eventually become 13-year cicadas. In fact, we have DNA and other evidence that this is what happened to the 13-year periodical cicadas in the upper Mississippi Valley. And there’s no reason why it won’t happen again. Throughout the south, 13-year cicadas could eventually switch to a nine-year life cycle. We saw this happening last year with the aid of citizen scientists, who reported nine-year cicadas all across the range of the largest 13-year brood.
Many in the D.C. area eagerly await this year’s once-in-a-generation phenomenon. Indeed, many citizen scientists are again poised to help map the distributions of periodical cicadas, using a cellphone app, Cicada Safari, to send thousands of photos to entomologists. But as the cicada’s song rings out, residents should hear it as an eerie reminder of how quickly our world is changing — and how life cycles that have been in place for thousands of years are being upended.
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