Brood X: The Cicadas Are Back
Vol. 9 Issue 2, Spring 2004
By Cliff Fairweather
They have red eyes, red wing-veins, an astonishingly loud song, and emerge by the millions. They’ve been waiting underground for 17 years and they’ll be among us from approximately mid-May through mid-June. “They” are Brood Ten (usually written Brood X) of the periodical cicada. You have been warned.
Not that there’s anything to worry about. They don’t bite, sting, spread disease, cause famine, or try to share your picnic. Well, they might literally crash your picnic, but they probably won’t stay unless you choose to make them part of the menu.
Periodical cicadas are one of the great natural phenomena of the eastern deciduous forest. Twelve broods of 17-year cicada and three broods of the more southern 13-year cicada emerge on different schedules in different regions of the eastern half of the United States. They occur nowhere else in the world.
Brood X, one of the largest, most widespread periodical cicada broods and covering at least parts of 15 states, including the Washington, D.C. region, last emerged in 1987. I vividly recall driving down I-66 near Manassas, Virginia, with the car windows open, and hearing clearly the cicadas over the considerable din of traffic. I was awed by this auditory expression of cicada abundance.
Now, 17 years later, the progeny of that emergence are digging their way topside. They’ll stop just below the surface, waiting for some unknown trigger to complete their excavation. Visible signs of their imminent arrival above ground should begin appearing within the next few weeks.
Just Another Silly Love Song?
The story of this year’s periodical cicada emergence began at about the time I was listening to their fathers sing that June afternoon on I-66. Impressively loud, periodical cicada song measures at around 100 decibels, comparable to a lawnmower and only slightly below a chainsaw.
Only the male cicada–whose sound producing organ is a ridged membrane called a tymbal–sings, which it does primarily to attract females. A pair of tymbals flanks the first abdominal segment of the male, just behind and under the wings. Powerful muscles flex these membranes like drum heads to produce a sound that is then amplified by the male’s hollow, resonating abdomen.
Both male and female cicadas hear the males’ songs through their tympana, another membrane on the underside of their abdomens. Females, who ultimately make mate selections, need to hear for an obvious reason, but hearing is just as important for male periodical cicadas. Hearing the singing of the first males in a chorus recruits additional males until a large aggregation of singers acts as a sound beacon for females.
Males will change their tune, depending on where they are in the process of attracting a mate. They first give a calling song that attracts both females and other males. A responsive female then replies with wing-flicks that provide both a visual and an auditory signal to a male which, as he closes in on the wing-flicking female, switches to a series of three different courting calls. The male goes through his courting-call repertoire as he gets closer to the female, stopping altogether only after he mounts her and begins to copulate.
Male cicadas also produce an alarm call when disturbed. It’s a rougher version of the calling song, perhaps to startle potential predators. To hear and feel the alarm call, simply pick up or gently press down on a handy cicada.
What’s in a Name?
Periodical cicadas–often, though incorrectly, called 17-year locusts–are members of the insect order Homoptera, which includes familiar insects such as aphids, tree hoppers and their kin, and the annual or dog-day cicada. Locusts belong to a different insect order, the Orthoptera, which is made up mostly of crickets and grasshoppers. Some locust species do occur in huge swarms, which is probably how they became etymologically confused with cicadas. Just to add to the confusion a bit more, the root of the name of another group of orthopterans, Tettigoniidae, is Greek for cicada. This group includes katydids and other long-horned grasshoppers, but no cicadas.
Now that that’s straightened out, let’s deal with one other confusing aspect of cicada nomenclature: annual versus periodical cicadas.
Annual cicadas, also known as dog-day cicadas, or harvestflies, have one-, two-, or three-year life cycles. Unlike the late-spring emerging periodicals, annual cicadas emerge as adults in the “dog days” of July and August. Since annual cicada broods overlap, one brood or another emerges every summer in a smaller and less concentrated emergence than the periodicals.
Annual and periodical cicadas demonstrate some physical differences as well-our local annual cicadas are a bit larger and bulkier than the periodicals, and lack the distinctive red eyes and wing veins. Further, while individually quite loud, they don’t form the large calling choruses that make periodical cicadas so auditorily distinctive. More obscurely, the tymbals of annual cicadas are covered, while those of the periodicals are not.
Experts recognize three species of 17-year cicadas and all three species occur in Brood X-Magicicada septendecim, M. cassini, and M. septendecula. Another four species of Magicicada are recognized among the 13-year forms. Local cicada researcher John Zyla says entomologist William Davis, whom he describes as the father of cicada research in the late 19th and early 20th centuries, gave periodical cicadas their genus name, Magicicada-choosing the word “magic” because of the way they seemed to magically appear every 13 or 17 years.
The three Magicicada species are easiest to tell apart by their calling songs, although physical and habitat differences can help in identification. M. septendecim’s song is often described as sounding like the word “pharaoh.” Found in a variety of habitats, it is the largest of the three-around 2 1/2 to 3 centimeters long-and displays broad orange stripes on the underside of its abdomen and orange markings ahead of the wings and behind the eyes.
The smaller M. cassini, a little over two centimeters, prefers floodplain habitats and has narrow orange stripes on the underside and no orange ahead of the wings or behind the eyes. Its call is a series of ticks followed by high-pitched buzzing. Meanwhile, M. septendecula is pretty similar to M. cassini in size and appearance and the two can be difficult to tell apart without hearing the songs. Occurring less commonly than the other two 17-year Magicicada species, its calling song is a series of 15- to 30-second phrases. Their habitat preference is upland areas.
A number of Magicicada broods occur in the mid-Atlantic region. In his research, John Zyla focuses on what appear to be 20- to 30-mile gaps between the boundaries of most broods. Interestingly, very few Magicicada occur in Charles County, Maryland, which lies between the range for Brood II in St. Mary’s County and Brood X, which extends south into Prince George’s and northern Calvert counties. A similar gap occurs between Brood X and Brood V in northwestern Garrett County. Zyla hopes his research, which depends on the help of volunteer cicada monitors, will eventually reveal the reason for these gaps.
From Branch to Root
After finding suitable males and mating 17 years ago, Brood X fertilized females began to go about the business of egg-laying. Female cicadas have a sharp, saw-edged ovipositor-or egg-laying appendage-that they use to cut longitudinal slits into smaller twigs and branches.
They will leave up to a total of 400 eggs, with 10 or so elongate, white eggs in each slit. In six to ten weeks, the nymphs hatch out, fall to the ground, and dig into the soil below their host tree or shrub.
Magicicada species choose broadleaf deciduous woody plants for egg-laying, avoiding conifers, sumacs, and other plants that produce large amounts of resins in response to injury. This sticky sap appears to destroy eggs, although annual cicada species can successfully oviposit in conifer species such as loblolly pine.
Cicada ovipositing usually kills the affected twigs, causing their leaves to quickly turn brown. Often, the entire twig or branch droops in a phenomenon called flagging. Dr. Don Messersmith, an entomologist at the University of Maryland, calls the egg-laying effects a natural pruning and says that it can actually be good for mature forest trees. Young trees might have more difficulty, however-since a large proportion of their structure consists of branches and twigs of the appropriate size for cicada egg-laying, they can experience a fatal pruning. Adult cicadas do feed, contrary to some information on the internet and elsewhere, but their feeding activity has little effect on trees and shrubs.
As for effects on larger ecosystems, a study by scientists at the University of Kansas, Spatial Variability in Oviposition Damage by Periodical Cicadas in a Fragmented Landscape, suggests that egg-laying by periodical cicadas might have long-term effects on forest succession. In particular, the authors suggest that cicada egg-laying could alter the dynamics of forest succession in disturbed areas, such as old fields with new woody growth. Magicicada appear to be especially attracted to younger trees. Possible effects include changes in the pace of succession and the mix of plant species, although the authors say more research is needed.
In any case, the lesson for gardeners this year should be obvious. This is not a good spring to plant new trees and shrubs. Waiting until the end of June, when the emergence has ended, will avoid any problems with cicadas on new plantings. You can go ahead and plant your annuals and perennials though-cicadas are not attracted to flowers and other non-woody plants for feeding or egg-laying.
Adult cicadas can live four to six weeks unless they meet with a predator, accident, or disease. All of the adults are gone by the end of June, although John Zyla notes that unseasonably low temperatures in early June can bring an abrupt and permanent halt to Magicicada mating activity.
Many individuals are infected by a parasitic fungus specific to Magicicada, called Massospora cicadina. Cicadas infected early in an emergence develop asexual fungal spores that render the animal sterile and cause the back of their abdomens to fall off, exposing a white mass of spores that infect other cicadas before the carrier dies. The secondarily infected cicadas develop a sexually-reproducing generation of spores that cause the ends of their abdomens to break off. Once released, the sexual spores lie dormant for 17 years until the next emergence. Some entomologists believe that the 13-year cycle of some periodical cicada species is an adaptive response to Massospora cicadina.
Our soon-to-emerge periodical cicadas began life 17 years ago as nymphs the size of small ants. Their shape immediately out of the egg, enclosed by a sheath-like membrane, was rather like a larval fish with rear projecting appendages that resembled flippers as much as legs. While such a form might seem ill-suited for digging underground, a cicada nymph’s first task is to struggle out of its egg shell and twig-slit nest. Their streamlined form and rear projecting appendages ease their forward progress out of the nest, but resist backsliding.
Once free of the egg slit, a cicada nymph loses its sheath, either before or after falling to the ground, which frees its appendages, including front-legs that are robust and well-adapted for digging. It promptly starts working its way two inches to two feet underground, exploiting cracks and fissures in the soil to find rootlets of a tree or shrub, which it will tap into to feed on sap with its piercing-sucking mouthparts.
The underground natural history of cicada nymphs is not well understood because it occurs out of sight, but we do know that they go through a series of instars, or growth stages. How many is not precisely known, although five or six are the numbers commonly given. An instar occurs each time an insect sheds its exoskeleton and, with a few exceptions, insects shed only during their immature stages. Once an insect reaches its adult stage-with fully developed wings and reproductive organs-it doesn’t shed again.
After they’ve wriggled into the ground and found an appropriate rootlet for feeding, the nymphs remain active for about five years, digging deeper into the ground. Then they become virtually dormant for a seven year period, feeding only enough to maintain their metabolism. In their last five years underground, the nymphs become active again and start working their way towards the surface. They get to within an inch or two of the surface by the autumn before they emerge, go through their last nymphal instar, and become dormant again until they are ready to emerge. During this time, a nymph can grow from about 1/12 of an inch to 1 1/4 inches, and develops red compound eyes and the wing buds that hold its folded-up wings ready for deployment.
Seventeen years underground might not seem like much of a life to us, but it has its advantages for a slow, vulnerable, long-lived insect larva. It offers safety from most predators and other dangers of a surface life, although moles and shrews are probably major predators, especially as they come closer to the surface prior to emergence.
Up and Out
Shortly before Magicicada nymphs emerge, they excavate chambers just below the surface. Digging with their powerful front legs, they use their claws to pick away at dirt and then use their tibiae and femurs to rake up the loose dirt and press it into the sides of their burrows.
You might find short mud turrets sticking above ground from these chambers, especially in damp soil-resembling miniature versions of burrowing-crayfish chimneys, they are formed when the nymphs push mud out of their burrows.
The exact cue for emergence is not fully understood, although soil temperature may be an important factor. Sometime between mid-May and early June, the nymphs will depart their subterranean nurseries at night and climb up a tree trunk, fence post, wall, or any other handy structure. Most members of a population of cicadas usually emerge within a couple of nights of each other. Nocturnal emergence helps reduce the risk of predation, especially by birds. Also, freshly molted adults are vulnerable to water loss, so the cooler, more humid night air offers them a less dessicating environment.
Once an emerged nymph has reached a suitable location, it grips the surface firmly with its front claws in preparation for their final molt when its back splits open and it wriggles free of its nymphal exoskeleton. The newly molted adult is soft and white, except for a couple of black eye-spots on its back, which John Zyla believes serve as a predator deterrent. A newly molted cicada extends its wings by pumping hemolymph (insect blood) into its wing veins. The animal then waits until its new adult exoskeleton and wings harden, usually by the following morning, during which time the animal’s body also darkens.
The cast off nymphal skin often remains attached to the place an adult cicada left it, and during the upcoming emergence, you are likely to see dozens or more of these exuvia hanging from tree trunks in areas with large periodical cicada populations. Take a careful look at the exuvia, especially at the split, or ecdysial suture, where the adult broke out. Often you’ll see what look like white threads sticking out of the split-these are part of the animal’s respiratory system. Insect don’t have any organs similar to our lungs. Instead, they have holes in the abdomen and thorax that lead to tubes, or trachea, to carry oxygen into the body. The trachea are part of an insect’s exoskeleton, even though they are on the inside, so when it sheds, the trachea linings shed as well.
Mass emergence is an important element to the evolutionary success of periodical cicadas. By coming out into the open by the millions, these vulnerable, clumsily flying insects overwhelm their predators. It’s not that lots of cicadas don’t get eaten-just about any animal that can handle an insect that big will gorge itself on cicadas-but the predators can eat only so many and there will still be millions left. Nate Erwin, curator of the Smithsonian’s insect zoo, notes that cicada predators, such as many bird species, realize greater reproductive success in an emergence year because of the additional food. But in subsequent years, that abundance of prey won’t be around to sustain the enlarged population of predators and their numbers will fall back to pre-emergence levels well before the end of the next periodical cicada cycle.
The odds that any one cicada will survive long enough to mate and reproduce are pretty good. With such a long time between emergences, there is little likelihood of a predator evolving a similar life cycle to specialize on periodical cicadas. Provisioning their young for such a long larval stage would not be feasible-they would have to evolve some other means of surviving the 17-year stretches between cicada emergences, an unlikely scenario given that specialization on annual cicadas works quite well. Cicada killers wasps, for example, specialize on annual cicadas. These wasps reach maturity in mid- to late-summer to match the emergence of their prey and stock their nest burrows with paralyzed cicadas as food for their larvae.
Not all periodical cicadas emerge according to schedule. Zyla says that these off-schedule individuals, which can appear one to four years before or after the main emergence, don’t successfully reproduce. He believes that aberrant emergences might be related to high nymph population densities.
Cicadas and People
I have friends who look upon the emergence of millions of large insects in a short period of time with some dread–especially in a season when most of us want to be outdoors. I don’t know how to counsel them other than to tell them get a grip, relax, and enjoy the spectacle.
The most harm a cicada is likely to do is to bump into you. Other than that, they’re pretty benign–wild rumors of cicadas flying en masse into people’s mouths notwithstanding. Their only objective is to reproduce, not to make life miserable for us. That’s not to say that there might not be a few annoyances.
Egg-laying periodical cicadas can cause a problem for ornamental and fruit trees, especially young specimens, but pesticides might not be a very effective response. At least one trial has shown no effect of pesticide use on cicada egg-laying damage. Some experts recommend covering vulnerable trees and shrubs with cheesecloth or netting, but others warn that such netting could be a hazard for birds. A more benign approach to protecting young trees is to simply knock the cicadas off with water sprayed from a hose. Perhaps the best advice comes from Loudoun County Master Gardener Karin McDowell, who says, “One should just let the cicadas do their thing.”
Actually, it’s the cicadas who have something to complain about. We have a much bigger and far more adverse effect on them than they could ever have on us. Assuming that as a cicada, you weren’t killed outright by the construction, imagine waiting 17 years underground for your short moment in the sun, only to find your path blocked by a parking lot.
Native Americans made good use of periodical cicada emergences in the same way they took advantage of shad runs, waterfowl migration, and other, more seasonal abundances-they ate them. Zyla says the Iroquois prepared cicada pie. Personally, I’m not too sure about turning something that appears only once every 17 years into a meal, if I don’t need to. I like my Magicicada singing in mass choruses and I’ll leave their consumption to their non-human predators.
I agree with Erwin, who suggests using the upcoming emergence to learn a little bit about habitat change and cicada distribution–perhaps by conducting a survey in your neighborhood, seeing which yards have cicadas, which don’t, and trying to figure out the reasons why. Periodical cicadas present us with no danger and relatively little nuisance. They’ll be gone after only a few weeks and their progeny won’t emerge for another 17 years. I hope you’ll join me in marveling at this display of natural abundance.
Test your knowledge of Cicadas–try our Cicadas Crossword Puzzle.