Cicadas are large and noisy insects that can appear in intimidatingly large swarms with millions of individuals. Despite this, they are actually harmless.
There are roughly 3,000 species of cicadas, and they are divided into two broad groups based on their life cycle. Annual cicadas are the less well-known type. They are green with brown eyes and large wings, and they produce a humming or clicking noise sometimes called “the song of the summer.” Annual cicadas can be found across the US from the Mid-Atlantic to the Southwest. They emerge every year in late summer.
Periodical cicadas have the distinction of being among the longest-lived insects in the world, for they emerge only every thirteen or seventeen years. They emerge by the billions and are sometimes erroneously called locusts after the Biblical insects.
Periodical insects are grouped into broods based on their life cycle and years of emergence. There are approximately 30 broods, but the exact number fluctuates as broods die out and new ones emerge. Currently, Broods I through XVII emerge every 17 years and live mainly in the northeastern United States, while Broods XVIII through XXX emerge every 13 years and live mainly in the South.
There can be billions or trillions of cicadas in a given area during an emergence. Scientists believe this be a survival strategy called “predator satiation” or “predator saturation.” The vast numbers of cicadas ensure that some will survive to reproduce even if birds, humans, and other hungry predators eat many of the others.
Scientists hypothesize that their prime number life cycles add another layer of protection by being too irregular for any predator to predict and depend on. The year of a brood’s emergence is simply a very lucky break for predators.
Female periodical cicadas deposit their eggs, which are shaped like grains of rice, on tree branches. The female’s ovipositor creates a groove in the branch that covers the egg and gives the hatchling access to the tree’s fluids. Newly hatched cicadas look like white ants. They drink fluids from the branch before falling to the ground.
The young cicadas then tunnel underground and feed on the fluid from tree roots. They spend the next 13 or 17 years in a state of arrested development. At the appointed time, the cicada nymphs crawl out of their burrows and up their home tree, where they shed their skins. They then wait for their new exoskeleton to harden and their wings to unfurl and fill with liquid.
Male periodical cicadas are infamously noisy and use their songs to attract females. The adult cicadas soon die after mating and laying a new batch of eggs. They live for only a few weeks.
Scientists know that soil temperature is one factor that determines the time of cicadas’ emergence. They also know that cicadas undergo five stages of development underground and that cicadas that develop early patiently wait for their fellows.
They used to believe that cicadas had an internal clock, but researchers at the University of California found that cicadas get their cues from the trees they are feeding on. In spring, the amount of sugar in tree sap markedly increases, and cicadas can detect that increase.
This particular cue isn’t foolproof. If an early spring is followed by a freeze and a second leafing of trees, the cicadas will interpret that as two complete cycles. If it happens often enough, the cicadas may emerge a few years early. If a large number of cicadas emerge early, a new brood could develop.