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Cicadas: Inescapable Yet Elusive

If you live east of the Rockies in the United States, there’s a decent chance that you’ve witnessed one of nature’s weirder happenings: the emergence of trillions of cicadas.

The United States is home to 15 broods of cicadas, each of which emerges every 13 or 17 years. At least one brood comes out of the ground most years, but you have to be at the right place at the right time to see one. It wasn’t until I was 46 that I witnessed my first emergence in 2013.

Brood II was scheduled to emerge that year in the northeastern United States, and the town where I live in central Connecticut was smack-dab in the zone. And yet I could barely hear them from my house near the center of town. I had to drive a few miles north to visit friends who lived in the woods to really experience the madness of cicadas.

The forest had become a surreal entomological carnival. Male cicadas were singing, females responding by flicking their wings, and pairs were mating. The trees were studded with the ghostly husks of cicada molts. (When cicada nymphs emerge from the ground, they climb into the trees, anchor themselves to the bark, and split open their skin. The winged adult form emerges.) It was easy to catch the red-eyed insects for close inspection. They seemed drunk. From time to time, a cicada would pop off its tree and lazily fly overhead, only to be snatched by a bird. The ground was littered with cast-off cicada body parts.

I made that experience the subject of my very first column for the New York Times. I have not experienced another cicada emergence firsthand. But this year two broods are coming out at once across a wide swath of the United States. With so much interest in cicadas this spring, my editors asked me to write about them again.

I thought that in 11 years since I wrote my first piece, scientists would have solved a lot of cicada mysteries. They have indeed learned a lot. But I was surprised at how much still remains unknown.

The basic question of why cicadas are locked in these two cycles is still pretty hard to answer, for example. Is it important that 13 and 17 are prime numbers? Could it be that primes offer some evolutionary advantage? Some scientists have suggested that a prime cycle keeps cicadas out of sync with the population cycles of their predators. Others have suggested that they are less likely to emerge at the same time as other broods, and thus less likely to hybridize.

Both ideas sound plausible, but both have problems. For one thing, the United States is not the only place where cicadas have long life cycles. India and Fiji are home to their own broods. But they have cycles in four and eight years, which are not prime. Maybe the primeness of American cicada cycles is a red herring.

You might think that with trillions of cicadas emerging on a regular basis, scientists would have ample opportunity to run the experiments required to resolve such mysteries. While cicadas can become inescapable, they’re fundamentally elusive. They spend almost their entire life cycle in the ground, then join us in the air for just a few weeks before they die. You can’t breed a swarm of cicadas on a rotting banana the way you can with Drosophila flies, and so it’s hard to run laboratory experiments on them.

This elusiveness extends to the parasites and symbionts of cicadas. Spores of a fungus called Massospora lurk in the soil, waiting to make contact with cicadas emerging from the ground. They then fill up much of the insect’s body, and spread from one cicada to another as they mate. (They even cause males to behave like females to increase their spread.) The spores leak out of the cicadas after they die, and wait for years for the next generation to pick them up. The biology of Massospora remains murky, because the only place the fungus can grow is in the body of an adult cicada.

For now, cicada science is rife with fascinating ideas in need of testing. And it will probably remain that way for a long time. For this year’s emergence, I took a look at one fascinating idea about how cicadas know how to emerge all at once. It may involve collective decision-making. You can read more about it here.

And if anyone has questions about cicadas, please don’t hesitate to reach out. I’ll be happy to respond in my next newsletter, with whatever insight science can offer!

It’s In the Air

One of the most intriguing scientific chapters in the pandemic was the conflict over whether Covid-19 was airborne. That led the World Health Organization to bring together experts to hash out a common language that can be used to talk about how diseases spread. It has taken them over two years to agree on that language—a sign of how contested this topic is. Here’s my story on their report.

I will have A LOT more to say about life in the air in a few months. Stay tuned!

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