This year, 2021, an unusual biological event will take place in North Georgia: the emergence of the 17 year periodical cicadas. These insects will have spent the last 17 years underground, sipping sap from tree roots. They are the offspring of cicadas that called, courted and laid eggs in 2004 and then died. How many will emerge and where is not precisely known, but there could be millions or more.
Where should you go to experience this emergence? Forested areas in the top tier of counties in Georgia.
Where should you go to experience this emergence? Forested areas in the top tier of counties in Georgia.
The CicadaMania website has lots of information and is updated frequently. (link)
Temperature is a big factor determining the emergence. A correspondent in northern North Carolina tells me that they have emergence holes with immature cicadas waiting inside.
What are Cicadas?
Most people encounter cicadas in two ways: 1) a clamorous, droning noise coming from trees during the dog days of summer, and 2) empty, brown shells clinging to tree trunks. Many have not made the connection between the two. The ugly shells are the exoskeletons left behind when the adult cicada crawls out of them in its last molt. The noise is caused by groups of male cicadas gathering in the same tree, each producing a courtship song to attract a mate. The individual calls blend together to make a deafening, continuous roar that attracts female cicadas.
Recordings. Radiolab has a podcast interview with a cicada researcher that begins at 9:50. (link)
Cicadas in Georgia. Georgia has two distinct kinds of cicadas. Those that appear every year are called annual cicadas; those that appear every 13 or 17 years are called periodical cicadas.
Annual cicadas. Annual cicadas emerge from the ground every year, usually in the dog days of July and August. Because of the time of year they make their appearance they are often called "dog day" cicadas, but annual cicadas is an equally appropriate name.
In Georgia there are around 15 species of annual cicadas. They are mostly colored with shades of green, brown and black and have dark eyes.
Periodical cicadas. Periodical cicadas appear at intervals of 13 or 17 years, in April and May or June. They are mostly black in color, have bright red eyes and orange wing veins. The adults emerge by the millions. There are three species of 17 year cicadas that emerge together. Similarly, there are three species of 13 year cicadas that emerge together. (Recently, a fourth species of 13 year cicada has been recognized. It has not been found in all the Broods).
Cicadas in Georgia. Georgia has two distinct kinds of cicadas. Those that appear every year are called annual cicadas; those that appear every 13 or 17 years are called periodical cicadas.
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| Annual cicada (one of 15 species in Georgia) Note the dark eye color and the green wing veins; the green, brown and black pattern on the thorax and abdomen. (photo courtesy of Don Hunter) |
In Georgia there are around 15 species of annual cicadas. They are mostly colored with shades of green, brown and black and have dark eyes.
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| Periodical cicada; note the red eyes, orange wing veins and black body. (Katja Schulz from Washington, D. C., USA, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons) |
Periodical cicadas. Periodical cicadas appear at intervals of 13 or 17 years, in April and May or June. They are mostly black in color, have bright red eyes and orange wing veins. The adults emerge by the millions. There are three species of 17 year cicadas that emerge together. Similarly, there are three species of 13 year cicadas that emerge together. (Recently, a fourth species of 13 year cicada has been recognized. It has not been found in all the Broods).
Life cycle: A female cicada lays her eggs in slits she makes in the terminal branches of trees. After a week or so the eggs hatch and the young nymphs fall to the ground and burrow into the soil. (The nymphs look like miniature cicadas, but lack wings and genitalia.) When the tiny nymphs find the fine roots of grasses, they insert their sharp pointed mouthparts and begin sucking sap. As they grow they must periodically shed their exoskeleton. The exoskeleton has limited ability to stretch and it must be replaced in order to keep up with the growth of the nymph. The process of replacing it is called molting and the nymphs will molt a total of five times. As the nymphs get larger they abandon the grass roots and seek out tree roots to feed from.
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| A cicada nymph before the last molt. The brown exoskeleton will become the "cicada shell" when it is left behind after the adult emerges. (USDAgov, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons |
The last molt takes place above ground when the cicada nymph digs out of the soil and crawls up a nearby tree trunk. The exoskeleton of the nymph splits down the back and the adult cicada crawls out. The newly emerged adult is pale and soft bodied, with small, sack-like wings. It pumps body fluids into the wings, stretching them to their adult size. The wings and adult exoskeleton take a few hours to harden and darken. Then the cicada is ready to find a mate and reproduce.
Annual cicadas spend 2 to 5 years as nymphs underground. Periodical cicadas spend 13 or 17 years as nymphs.
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| Periodical cicada emergence tunnels. Some are open, with the nymph still inside. Others are still closed, capped by soil pushed upward by the nymph. (photo courtesy of C. Beane; taken 4/27/21 in Elkin, NC) |
When cicada nymphs are ready to emerge they abandon the tree roots they have been feeding on and dig their way to the surface. They will only emerge when the soil temperature is warm enough. They sit inside their tunnel, waiting for a warm day, to emerge, climb a tree and molt for the last time. The nymphal exoskeleton (the "shell") is left clinging to a tree after the adult cicada struggles out, as seen in the first part of this short David Attenborough video. (link)
Where periodical cicadas are found. Periodical cicadas are only found in the eastern United States, from the New England states to Georgia and extending as far west as eastern Nebraska, Kansas, Oklahoma and Texas. Those with the 17 year periodicity are found in the northern part of that range; the 13 year cicadas are found in the southern states, except Florida, which does not have any
Periodical cicada broods. Periodical cicadas do not emerge simultaneously over their entire geographical range. Instead, the emergence in any single year is in a smaller geographical area within the overall range. All the periodical cicadas that emerge in the same year are said to belong to the same "Brood." There are 17 possible Broods of 17 year cicadas; 13 possible Broods of 13 year cicadas. Some Broods occupy large areas, others have a more restricted distribution. Some are apparently extinct. The Broods are numbered with Roman numerals, I through XVII for the 17 year species and, for the three known broods of 13 year cicadas, XIX, XXII XXIII.
A map for Brood X can be found at this website.
A recently published, high resolution map of multiple cicada broods can be found at this website.
Why are cicadas sometimes called locusts? A locust is a type of grasshopper. (All locusts are grasshoppers, but not all grasshoppers are locusts.) Unlike cicadas, grasshoppers have chewing mouthparts and are members of the Order Orthoptera (crickets, katydids, grasshoppers). Cicadas belong to the Order Hemiptera (true bugs, cicadas, aphids, leaf hoppers, plant hoppers) and have piercing, sucking mouthparts. Locusts periodically emerge in terrific numbers and lay waste to the vegetation in their area. It is likely that an emergence of enormous numbers of cicadas caused the early English colonists to associate the cicadas with one of the eight plagues that the Bible says God sent to Egypt. Neither locusts nor periodical cicadas occur in England, so the colonists had never experienced such an eruption of insects. They seized upon the biblical plague passage to incorrectly call the cicadas locusts.
More Information:
This website has a lot of periodical cicada information, great photos and information about preventing possible damage to your trees. (link)
A great site for exploring information about cicadas of all kinds. (link)
