Buzz
In 1996, the cicadas from Brood II, also known as the 'East Coast Brood,' invaded the northeastern United States, only to disappear as suddenly as they arrived, leaving their eggs and shed exoskeletons in their wake. Once the eggs hatched, the new generation of cicada nymphs burrowed underground, where they’ve spent the past 17 years waiting and feeding on tree roots.
This year, when the conditions are right and the soil warms up, they will resurface to molt, become adults, mate, create a lot of noise, and lay their own eggs. Seventeen years is a long wait underground. It’s enough time for Bill Pullman to transition from being a cool fictional president to a not-so-great one. But why do cicadas spend so many years out of view and out of mind?
Not all cicadas take part in this lengthy game of hide-and-seek. The majority of North American species are 'annual cicadas' with unsynchronized life cycles of 2 to 5 years, emerging each summer. Only a few species, organized into broods based on their emergence year, have longer, synchronized life cycles and surface all at once every 13 or 17 years.
The so-called 'periodical' cicadas have puzzled scientists since the 1600s, when a visitor to the American colonies first described their mass emergence in the inaugural volume of Philosophical Transactions, the oldest scientific journal. For a time, one popular theory was that the long life cycles kept different broods from emerging at once, avoiding competition for scarce resources.
Another theory, proposed by Stephen Jay Gould in his 1977 book Ever Since Darwin: Reflections in Natural History, suggests that these cycles protect cicadas from predators and parasites with shorter life spans. He argued:
“They are large enough to outlive the life cycle of any predator, but they also follow prime number cycles (divisible only by themselves). Many potential predators have 2-5 year life cycles. These cycles are not governed by the availability of periodical cicadas (since they peak during non-emergence years), but they may thrive when their cycles align. Consider a predator with a five-year cycle; if cicadas emerged every 15 years, every cycle would align with the predator’s. By cycling at a large prime number, cicadas reduce the chance of such coincidences (every 5X17, or 85 years, in this case).”
In simpler terms, by remaining out of sync with the life cycles of predators, cicadas avoid becoming a predictable, year-round food source. This prevents predators from adapting and specializing in hunting or feeding on them. The challenge, however, is that because cicada emergences are so infrequent, Gould’s theory is hard to test. Nonetheless, mathematical models, like those created by Glenn Webb and Mario Markus, provide some support for his hypothesis.
Although no predators exclusively target periodical cicadas, a variety of animals—such as birds, rodents, snakes, lizards, and fish—will feast on them when they emerge. They are abundant, easy to catch (since cicadas lack real defenses), and the right size for a meal.
Luckily for cicadas, their sheer numbers work to their advantage, at least for some of them. Since so many emerge simultaneously, the number of cicadas in any given area far exceeds the capacity of local predators to consume them. While some are eaten, once the predators are satiated, the rest can focus on mating and laying eggs without being disturbed.
If you live in cicada territory (Brood II will surface in parts of Connecticut, Maryland, New Jersey, New York, North Carolina, Pennsylvania, and Virginia) and wish to assist scientists in predicting and monitoring their emergence, consider purchasing or building a soil thermometer and participating in WNYC’s “Cicada Tracker” citizen science project.
