Signs of Spring 7: The Cicadas Are Coming!

Photo by K. Schulz Flickr

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Every seventeen years the quiet parks and forests of Western Pennsylvania explode with sound. Over a four or five week period in May and June periodical cicadas (also called “seventeen year locusts”) emerge from their underground nurseries in densities of tens of thousands to several millions of individuals per acre and begin to generate a roar that blocks out not only most of the other sounds of nature but also the usual crushing din of human existence!

Pay attention, everyone! 2019 is a Cicada Year!

It is the male cicadas that are making all of the noise. Their sound producing organs are on the undersides of their rounded abdomens, and their buzzing is intended to attract female cicadas so that they can reproduce. The male cicadas’ bodies are hollow and act as resonance chambers to amplify the buzzing which can be as loud as ninety decibels (the equivalent of the roar of a nearby power mower or a small chain saw). The ability to generate noise of this magnitude has earned the periodical cicada the title of the “world’s loudest insect.”

There are over 1500 described species of cicadas in the world, but only seven are classified as “periodical.” The life cycle of a normal, “annual” cicada (like our yearly “dog-day” cicadas, for example (see Signs of Fall 1, September 6, 2014)) can span several years and typically includes extensive nymph stages in which the cicada lives underground feeding on the fluids of tree and other plant roots. In periodical cicadas this underground portion of the life cycle is stretched out to intervals of thirteen to seventeen years! These adult “magical” cicadas (their genus name is Magicicada!), then, spend their allotted month in the open air buzzing and reproducing after more than a decade and a half of dark, subterranean existence!

Photo by M. O’Donnell Flickr

Adult periodical cicadas have stout, black to brown bodies that are just over one inch long.  They have two pair of membranous wings that are tipped in orange. The front wings are twice as long as the hind wings and have an open span of about three inches. The head is dominated by a pair of large, bulging, red eyes. They are slow flyers and are easily taken by a wide range of predators.

There are seven species of periodical cicadas all of which are found exclusively in the eastern United States from the Great Lakes down to the Gulf of Mexico. The three species in the northern portion of this range tend to have seventeen year life cycles while the four species in the southern portion tend to have thirteen year cycles. There is considerable overlap in the ranges of these different types but little potential for interbreeding because of the asynchrony of the emergence of their adult forms.

In both the northern and southern ranges the cicada species form communities that have synchronously timed emergences. These cicada communities are called “broods.” These Cicada Broods were first described in the Nineteenth Century, and there is some controversy as to how many broods there actually are. Most authorities, though, agree that there at least twelve broods of seventeen year cicadas and thirteen broods of thirteen year cicadas. The broods are dynamic communities influenced by changes in climate and habitat. A number of broods have died out since their initial descriptions while others have come into relatively recent existence.

Photo by J. Sturner Flickr

The name “locust” is unfortunately used to refer to these periodical cicadas. “Locust” is an ancient, Biblical name for the grasshopper. The plague of locusts that beset the Egyptians in Exodus consisted of swarming clouds of voracious, plant consuming grasshoppers that decimated hundred of square miles of crops and forage. Early European settlers in North America seeing the unexpected emergence of thousands upon thousands of these cicadas thought that they were observing a plague of Biblical proportions and so named the insect “locust.”

The adult periodical cicadas, though, feed only moderately on plant fluids and do very little damage to trees or other vegetation via their feeding. They are also unable to bite/sting or otherwise hurt a human being! Limb scarring from egg laying and larvae emergence can open some trees up to infections, but that too is usually without very much serious damage except in very young trees or in delicate, ornamental tree species like dogwoods. Blocking access of the gravid females to tree limbs (by cheese cloth coverings etc.) can lessened potential cicada damage to vulnerable trees. Even the nymphs feeding on fluids from the roots of their host trees do not seem to greatly affect the overall health or rates of growth of the trees.

The following is a scenario for the upcoming emergence of the Brood VIII periodical cicadas. Brood VIII is the synchronized community of periodical cicadas found throughout the counties of Western Pennsylvania:

In June, 2002 female cicadas gathered in wooded areas that were filled with the incessant songs of the males. The loudest songs and the largest gatherings of singing males attracted the greatest number of receptive females. After mating, the female cicadas used their saw-like, posterior, abdominal appendages (their “ovipositors”) to dig under the bark of limbs of oak or hickory or dogwood trees. Into each of these gashes they laid one or two dozen tiny eggs. Each female then moved on to another limb and then another and another until they had deposited their six hundred eggs into roughly forty different sites.

Photo by J. Gallagher Wikimedia Commons

By August, all the adult cicadas were all dead. The eggs that hadn’t been eaten by birds or ants, or rotted by fungi, or destroyed by the summer heat hatched into tiny, ant-sized larvae that fell unnoticed to the ground. The larvae then burrowed six to eighteen inches into the forest soil where, among the tree roots that will sustain them, they began a slow, steady growth and metamorphosis that would last the next seventeen years.

In April 2019, the nymphs, now nearly fully grown, begin to dig their way back out of their soil home. In May, they will pause about eight inches below the soil surface, waiting for the just the right weather to stimulate their emergence out through their soil turrets and mounds. A nice, warm rain is often the trigger that brings the soil temperatures to 64 degrees and initiates the cicada’s final climb up into the open air. Once up on the soil surface, the cicadas undergo a four or five day metamorphosis into their short-lived, flying adult forms.

In June, the rolling, buzzing, and some say, maddening, chorus of the seventeen year cicada will once again fill the countrysides and suburbs of Western Pennsylvania. Those individual cicadas fortunate enough (mostly via dumb luck and sheer force of numbers) to escape predation by birds, snakes, spiders, skunks, fish, moles and even dogs and cats, will reproduce and set up, for 2036, another generation and another extension of their “magical” life cycle.