As Spring Segues into Summer, the Great Eastern Brood Arrives.
Periodical cicada, Magicicada sp., Fort Harrison State park. (Photo by VR Schmalhofer.)
In a typical year, an encounter with a cicada takes place during the hot, sultry days of July and August. Green-and-black above and startlingly white below, the arrival of these large, stout-bodied, green-eyed insects is inextricably linked with the dog days1 of summer, and the occasional chirring call of a male cicada singing to potential mates is part of the background noise of the season.
When it comes to cicadas, though, 2021 is anything but typical!
If you live in the right area (or wrong area – depending on your point of view) and step outdoors in May and June, you will experience the sublime symphony (or raucous cacophony) of periodical cicadas.
Run-of-the-mill annual cicadas these are not!
Periodical cicadas, which belong to the genus Magicicada, are exceptionally long-lived insects and display one of two degrees of periodicity: 17 years and 13 years.2-4 Although cicadas are a diverse group of insects – there are over 3000 species of cicadas worldwide5 – Magicicada are found only in North America.3, 5
In most years, somewhere in North America a late spring emergence of Magicicada species occurs.6 Members of a given year's cohort are referred to as a brood, and broods are usually relatively small and have fairly restricted geographic distributions.3 This year, though, is different: 2021 is the emergence year for Brood X – also known as the Great Eastern Brood – the largest of all periodical cicada broods. Every 17 years, Brood X makes headlines as vast numbers of juvenile cicadas emerge from their underground tunnels, undergo their final molt to become adults, and begin the serious – and seriously noisy – business of ensuring the next generation of Brood X periodical cicadas.
Emergence holes of periodical cicadas, Fort Harrison State Park. (Photo by VR Schmalhofer.)
Life cycle of a 17-year cicada
Female (left) and male (right) Magicicada cassinii (Cassin's 17-year Cicada). (Photos by VR Schmalhofer.)
High up in the trees, a female periodical cicada uses her ovipositor (a long, pointed, egg-laying organ) to make slits in young twigs, in which she lays her eggs. After a period of 6-10 weeks, the eggs hatch, and young cicadas (called nymphs) drop to the ground where they dig into the soil and settle in among the plant roots.3 Cicadas belong to the insect order Homoptera and, like all Homopterans, are fluid-feeders. Consequently, they have mouthparts that are modified for piercing and sucking, and cicada nymphs use their straw-like mouthparts to tap into a plant's root system and sip plant juices. Initially, the very small (1/10 of an inch long!) nymphs feed on the roots of small plants, such as grasses.3 Later, nymphs burrow deeper (8-12 inches down) and tap into tree roots to feed.3
Cicadas, like all arthropods, have an external skeleton (exoskeleton) and spend their lives effectively encased in full body armor. As a cicada grows, its soft tissues increase in size, but its exoskeleton does not. At intervals, a cicada sheds its old exoskeleton and grows a new, larger exoskeleton to accommodate its increasing size. The exoskeleton-shedding process is a molt, and intervals between molts are instars. After 12-13 years have passed, a nymph reaches its fifth (and final) juvenile instar.3 In its 16th year, it tunnels upwards to within 4-6 inches of the soil surface and waits.3 In the spring of its 17th year, when soil temperatures become warm enough – soil temperature must reach 64 °F before a cicada will leave its underground home2 – the wingless cicada nymph emerges, climbs a nearby upright object (usually a tree trunk) and undergoes its final molt to become an adult.
Molted exoskeletons of periodical cicadas, Fort Harrison State Park. (Photo by VR Schmalhofer.)
A newly molted adult is soft and white. At this time, before the exoskeleton – and wings! – harden, a cicada cannot fly and is extremely vulnerable to predators. The striking colors of adulthood – black body, red eyes, orange wing veins – are achieved within a few hours of molting, but the new exoskeleton takes about five days to fully harden.3 Initially, more males than females emerge, so males take the brunt of predator impacts as birds, cats, foxes, lizards, fish, snakes, and many other animals gorge on the bonanza of insects.3 Later, the number of females emerging exceeds that of males.3 Five days after emergence and molting, once their new exoskeletons have hardened, males begin singing to attract females. Only males sing, using a pair of organs called tymbals to produce their distinctive love songs. Once mated, females find suitable twigs in which to oviposit (lay eggs). A brood's adult presence aboveground lasts about six weeks, though individual adults live for only 2-4 weeks.3
How do you tell the difference between cicada species?
There are three species of 17-year cicadas, and a brood may contain all three: Magicicada septendecim, Magicicada cassinii, and Magicicada septendecula.2, 3 All three look similar, having black bodies (when viewed from above), red eyes, orange legs, and wings veined in orange. Species differ, though, in size, color of the ventral abdomen, and vocalization. Magicicada septendecim is the largest (~30-35 mm in body length – wing length not included in this measurement!), and the underside of its abdomen is predominantly orange-brown in color (there may be thin black bands); a reddish band between the eye and wing joint may also be evident. Magicicada cassinii is the smallest species (~20-25 mm in body length) and has a solid black abdomen. Magicicada septendecula is intermediate in size (~25-30 mm in body length) and color: the underside of its abdomen is black with thin orange-brown bands.
Interestingly, each species of 17-year cicada has a 13-year counterpart (M. septendecim has two) that look and behave similarly.3
Magicicada septendecim (Pharoah Cicada), Fort Harrison State Park. (Photo by VR Schmalhofer.)
Why is it called Brood X?
Written records of periodical cicadas date back to the pilgrim colony in Plymouth (1634), though cicadas were known to and used by indigenous tribes for centuries prior2 – side note: periodical cicadas are high in protein and quite edible, being variously described as tasting like shrimp or cold canned asparagus. The first written observations of Brood X date to 1715 and were made in the Philadelphia area2 – this was, of course, before Brood X was known as Brood X.
Charles Marlatt, a USDA entomologist, inititated the brood numbering system in 1893.2 He designated cicadas emerging in 1893 (and every 17 years thereafter) as Brood I, those emerging in 1894 as Brood II, and so on. Marlatt designated numbers I to XVII for broods of 17-year cicadas, and XVIII to XXXI for broods of 13-year cicadas. (Use of Roman numerals is part of the naming convention for periodical cicada broods. Why Marlatt chose to use Roman numerals is unknown, but he may have done so to avoid confusion, as Arabic numerals are part of the common names of the species.)
There are only twelve extant broods of 17-year cicadas: I-X, XIII, and XIV.3 In some cases, an emergence of periodical cicadas simply doesn't occur in a particular year, and there has never been a recognized brood for that particular 17-year cycle. In other cases, broods formerly existed but have gone extinct. Habitat loss is the primary environmental stressor causing extinction among periodical cicada broods.3 Brood X is the largest and most widespread, with cicadas emerging in 15 states2 and in some places achieving densities of 1.4 million cicadas per acre!3
Periodical versus annual cicadas
Annual cicada, Neotibicen canicularis. With their green-and-black bodies and startlingly white bellies, so-called annual cicadas are easily distinguished from the red-eyed, black-bodied periodical cicadas. It is worth noting that annual cicadas don't actually have a one-year life cycle: their typical life span is 2-5 years, depending on the species – they are “annual” only in the sense that they are present every summer.7 In Indiana, a typical encounter with an annual cicada involves a member of the genus Neotibicen8 – most likely N. canicularis.9 Species within this group are commonly known as dog-day cicadas because adults are active during the hottest part of summer (i.e. the “dog days” of July and August). (Photos by VR Schmalhofer.)
In addition to size, color, life span, and timing of emergence, periodical and annual cicadas differ in two other key features of their life history: variability in size among adults and timing of emergence. Annual cicadas have a variable life span (typically 2-5 years) and molt to adulthood once they attain a certain size.3 Consequently, adults of a given species are all very similar in their physical proportions. Periodical cicadas, conversely, molt to adulthood after a set period of time has passed (no matter how large or small they are) – so there can be a great deal of variation in body size among individuals of a given species within a brood. The synchronized, mass-emergence events of periodical cicadas, in combination with their extremely long life spans, are thought to serve as defenses against predators. Although a given brood of periodical cicadas reappears in an area at regular intervals, the cicadas' life span is so long – much longer than that of many of their predators – it effectively makes periodical cicadas an unpredictable resource (from the predator's point of view). In combination with their “unpredictable” emergence, immense population size also protects Magicicada species. In years when a brood emerges in a particular area, the sheer number of cicadas overwhelms predators – predators cannot eat them all! – ensuring that some cicadas survive to reproduce. A similar phenomenon is seen in certain plants, such as bamboos (some species live for decades or a century before flowering and producing seeds in vast quantity) and oaks (which generally produce some acorns each year, but have synchronized mast events at irregular intervals in which the oaks in a region produce a much, much greater abundance of acorns that usual).
Periodical cicada, Magicicada sp., Fort Harrison State Park. (Photo by VR Schmalhofer.)
Environmental benefits of periodical cicadas
Periodical cicadas provide a number of environmental benefits. As previously mentioned, many animals eat cicadas, and this food surplus not only enables cicada predators to raise more young, but also ensures that the predators are in a better nutritional state (and therefore have a higher probability of survival) when faced with seasonal stressors, such as fall migration and winter food scarcity. Cicadas also benefit trees. Egg-laying by cicadas often causes the twigs in which the eggs are laid to flag (droop) and eventually drop off. Although this would appear to damage the trees, the “pruning” of new growth actually benefits trees and results in increased flowering and fruit production in the following year. Finally, cicadas improve the soil. Cicada tunnels help aerate the soil, and the decomposition of their bodies (when they die en masse as they emerged en masse) returns nutrients to the soil (as well as proving a food resource for scavengers and decomposers).
Love them or hate them, Magicicada will be gone after a few short weeks, and by July they'll be only a memory, not to return for another 17 years.
The USDA Forest Service has an excellent map that shows which broods will emerge where and in what years.
Cicada Safari is an app that allows users to help track cicada emergence and contribute to scientific research. The site also provides interesting information about periodical cicadas and links to other information sources.
Kate Wong's article in Scientific American provides excellent information, including a synopsis of the evolutionary history of periodical cicadas.
Infection by fungal parasites affects cicada behavior.10, 11 You may have heard that periodical cicadas are attacked by a fungus that alters their behavior (in ways that help spread the fungus) and eventually results is the cicada's “butt” (abdomen) falling off. An article in Smithsonian magazine describes the phenomenon. For those interested in the hard-core science concerning the influence of fungal parasite on host behavior, check out the article in PLoS Pathogen.
1. “Dog days” refers to the hottest, most uncomfortable part of the summer in the northern hemisphere – a time frame associated with the rising of the star Sirius (the Dog Star). “Dog days.” Wikipedia, 24 May 2021, wtbblue.com/wiki/Dog_days Accessed 18 June 2021.
2. Cicada Safari. Mount Saint Joseph University, wtbblue.com Accessed 18 June 2021.
3. Wong, K. “Brood X cicadas are emerging at last.” Scientific American, 10 May 2021, wtbblue.com/article/brood-x-cicadas-are-emerging-… Accessed 18 June 2021.
4. “Periodical cicadas.” Wikipedia, 13 June 2021, wtbblue.com/wiki/Periodical_cicadas Accessed 18 June 2021.
5. “Cicadas.” Wikipedia, 15 June 2021, wtbblue.com/wiki/Cicada Accessed 18 June 2021.
6. Liebold AM, Bohne MJ, Lilja RL. “Active periodical cicada broods of the United States.” USDA Forest Service Research Station, Northeastern Area State and Private Forestry, May 2013, wtbblue.com/foresthealth/docs/wtbblue.com Accessed 18 June 2021.
7. “Annual cicada.” Wikipedia, 5 May 2021, wtbblue.com/wiki/Annual_cicada Accessed 18 June 2021.
8. “Neotibicen.” Wikipedia, 27 February 2021, wtbblue.com/wiki/Neotibicen Accessed 18 June 2021.
9. “Dog-day cicada.” Wikipedia, 11 June 2021, wtbblue.com/wiki/Dog-day_cicada Accessed 18 June 2021.
10. Machemer T. “Cicadas fall prey to a psychedelic-producing fungus that makes their butts fall off.” Smithsonian Magazine, 20 May 2021, wtbblue.com/smart-news/cicadas-fall-prey-drug-produci… Accessed 21 June 2021.
11. Lovett B, Macias A, Stajich JE, Cooley J, Eilenberg J, de Fine Licht HH, et al. “Behavioral betrayal: How select fungal parasites enlist living insects to do their bidding.” PLoS Pathogen 16(6), 18 June 2020: e1008598. wtbblue.com/10.1371/journal.ppat.1008598 Accessed 21 June 2021.
Magicicada septendecim (Pharoah Cicada), Fort Harrison State Park. (Photo by VR Schmalhofer.)