How Is a Cicada Like an Oak Tree? (And Why You Should Care)

Daniel Janzen, DiMaura Professor in Biology, on why cicadas (and wildebeests, salmon, and oak trees) act that way.

Thursday, July 8, 2021

By Susan Ahlborn

Daniel Janzen, DiMaura Professor in Biology

The last time the 17-year cicadas emerged from the ground, Facebook was just being introduced at Harvard and the iPhone didn’t exist. It’s an extreme case of long time no see. To learn more about the reclusive insects, we talked to Daniel Janzen, DiMaura Professor in Biology. Janzen studies tropical ecology and the biodiversity of tropical ecosystems and is known for his pathbreaking research on how insects interact with nature.  

The life of a cicada, he reports, actually begins in the treetops. The female uses a saw-like blade at her rear called an ovipositor to cut a slit in a twig, into which she pushes her eggs. After a few weeks, the nymphs hatch, fall out of the tree, and burrow into the ground. There each one inserts its proboscis into a root, which will provide it with nourishment and water.

Roots, though, are not very nutritious. “They're not like foliage or fruits or seeds, things that you can grow up on fast,” says Janzen. Because of this, various species of cicadas grow underground for years. Being buried provides protection, so they don’t develop any chemical defenses, like a bad taste or smell. “The nymph is basically a hamburger, from the viewpoint of any predator,” Janzen says. Some are found and eaten along the way, but thousands just live under our feet for years. Their genetic programming tells them in what year to emerge to make their final molt to an adult, and the weather tells them at what point during that year.

How did they develop this long, synchronized cycle? Somewhere along the way, says Janzen, possibly in a year with a lot of rain and nutrition, a big bunch of cicadas emerged at once. “So they all come out at the same time,” says Janzen. “And the predators eat them, but because there's so many, a lot escape. And those who escaped were genetically programmed by selection to be synchronized the next time.”

With so many cicadas at once, everything that eats them—carnivores, herbivores, your dog, some humans—has a great year. “All their predators are filling up their stomachs, having the maximum number of babies, doing very, very well,” says Janzen. “But the next year, all those babies are grown up, and there's no periodical cicadas to eat. That means any cicada who was late by one year gets clobbered. But those who hit that 17-year period survive. It's called selection for synchrony by predator satiation.”

Wildebeest, or gnus, in Africa take the same approach. “They all drop their calves within a couple of days of each other in the yearly cycle,” says Janzen. “Every lion, every leopard, everybody stuffs themselves with wildebeest newborn calves. But those synchronized with the herd’s timing have the best chance of surviving.”

Salmon do the same thing, and so do oak trees in a forest setting, storing up energy and then producing a lot of acorns every three or five years. (Lone oak trees in the sun often fruit every year because they can store so much energy that they are triggered by spring every year.) Oaks are helped out by squirrels, who can remember to within a foot where they buried an acorn months earlier, and then use their noses to do the rest. If squirrels didn’t bury acorns, deer, passenger pigeons, and insects would eat them, or they’d rot on top of the ground. And because the oak produces so many acorns, some of those buried are left behind to grow into trees following a massively synchronized year.

“That’s the price mom oak tree pays for a seedling: a lot of acorns for a lot of squirrels to eat and bury,” Janzen says. “An oak tree lives 200 years, and in an intact forest (with all its competitors) it makes a seed crop every two or three or four years. Think how many acorns that is to get one more oak tree. So now we're dealing with astronomical percentages required for population replacement. Extremely low percent. But it works.”

Janzen spends a lot of time thinking about survival. He’s currently researching in the Area de Conservacion Guanacaste in Costa Rica, a national park he and his research partner and spouse, Winnie Hallwachs, also a researcher in the Department of Biology, helped to create beginning in 1985. The preserve is more than 400,000 acres returning to their natural state after 400 years of Western agriculture. It’s also a teaching site and an ecotourism destination for thousands.

Guanacaste is part of their goal of spreading bioliteracy, the ability to understand how humans interact with and need nature. “Bioliteracy means finding out what is in the forest, what its name is, where it is, what it does, and then to put that on the web, so that anybody—schoolchildren, a middle-age mechanic in a garage, I don't care who they are—can connect to some of the things that could matter to them in the wild world." To encourage bioliteracy, he and Hallwachs have set out to DNA barcode the world.

All these living organizms that synchronize for survival, Janzen points out, have been disrupted: Many rivers are now dammed, and agriculture means less food for the few baby salmon that are spawned in the landscape crossed by their streams. In the 1920s wildebeest numbered in the millions; their number at one point was down to just 10 percent of that. And there are many fewer trees—and roots for cicadas to feed on. 

No matter who and where you are in the world, Janzen says, you need to “be willing to think about the fate of not only insects, but the fungi and all the others, the mites, the ticks, the spiders, the birds, the lizards, the snakes. Everybody in the forest. To be willing to think about them as being a reasonable member of the global society. Or we’ll end up living in a shopping mall.”