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Cicadas Are Right on Time…But Why?

By Cheryl Simon Silver

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This cicada is part of Brood X, which is emerging this week in the Eastern United States.
After 17 years offstage, cicadas in the Eastern United States are poised for their brief moment of insect prime time. Over the course of several days, untold millions of the red-eyed root suckers will emerge from their subterranean abodes, and during a glorious four-to-six week interval they will mate, lay their eggs, and die.

One of the largest insect “emergences” on Earth will leave behind a layer of carcasses and shells, nicely aerated soil, and trees pruned by female cicadas as they slash twigs neatly to insert their eggs. Then, the cicadas, known as Brood X (as in ten), will return in 2021.

Entomologists call the 17-year cycle a nearly perfect convergence of environmental cues and genetic programming. Although scientists are still trying to identify the cues that influence how “periodical” cicadas develop and tell them when to emerge, it’s clear that the process involves temperature and other environmental factors.

Brood X, for example, senses when the soil temperature underground has warmed to an ideal 64 degrees F.

The insects have “inborn molecular clocks” that are ticking based on environmental cues, says Chris Simon, who studies ecology and evolutionary biology at the University of Connecticut in Storrs. “The soil temperature only tells them what day to come out, not what year.”

Many scientists who study cicadas focus on the insect’s juvenile development, which is the longest of any insect. Once the eggs hatch, the nymphs drop down from the twigs in which they lived as eggs. They quickly burrow down at least two feet, find a succulent root, and start sucking nutrient-rich plant juices. They pass through five nymphal stages.

Not all cicadas destined to emerge in the same year mature at the same rates, however. Some spend up to six years in the fifth stage, waiting for their peers to catch up.

“The whole key to their life style is in the juvenile stages.” says Simon. “What people who work on clock genes—circadian rhythms, life span, and developmental timing—want to know about is the development that pushes them from juveniles to adulthood.”

The scientific literature on cicadas suggests that cicada nymphs live too deep to detect changes in light or annual fluctuations in temperature. Some researchers believe that seasonal changes in plant hormones or amino acids in xylem fluids may help the insects to tick off the passing years.

Periodical cicadas are a North American phenomenon. Three broods emerge in 17-year cycles and four broods emerge after thirteen years. Some entomologists think the insects may have developed their timetables in response to conditions such as climate or underground crowding. Essentially, they hedge their bets about when their chances of survival are best.

In fact, their main defense is their massive numbers. The insects may look fearsome, but they’re actually defenseless, tasty snacks for creatures including arthropods, birds, reptiles, and mammals.

Evolution has favored large groups of the insects because cicadas that emerge in small numbers in odd years are quickly devoured before they have a chance to reproduce. Emergence en masse means that their predators can be satiated, and the remaining brood members can go on to reproduce.

If the cicadas were to emerge too frequently, the population of predators might increase. But no predator can expand its population based on a feast that happens every 17 years. Cicadas that emerge after thirteen years also skirt the build-up in some predator species that occurs after major emergences.

As other scientists ponder cicada genetics, Keith Clay of Indiana University in Bloomington studies cicada ecology and their effects on forests. His team is embarking on a long-term research project to try to answer the question: “If there were no cicadas would it make a difference to the environment?”

This week they are spreading nets over large blocks of young forest habitat. The nets will prevent the nymphs from burrowing into the ground. Then it will be possible to measure the effect the root-sucking nymphs have on tree growth, and compare the cicada-free areas with unprotected, similar forest tracts.

The answer should emerge before the next generation of Brood X. “If they’re having a negative effect on tree growth, we should be able to detect it before 17 years have passed,” says Clay.

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