Natural events often occur in predictable cycles. In temperate North America, we are accustomed to the annual production of the leaves, flowers and seeds of our deciduous oaks and maples. Agave americana, the giant agave native to Mexico and Texas, is commonly known as the century plant due to its enormous periodic bloom of a decade or two. Even celestial events occur with clockwork predictability, like the visit of Halley’s comet every 75 years. If you live in the eastern United States, get ready.
In the spring of 2021, trillions of periodical cicadas are expected to emerge in parts of the following states: Delaware, Georgia, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia. They will be a source of wonder and consternation as they emerge from the earth and lay eggs in treetops. (Photo 1)
What are periodical cicadas?
Taxonomically, periodical cicadas are members of a large clan of insects known as Hemiptera, with piercing-sucking mouthparts and gradual metamorphosis, meaning juveniles are called nymphs rather than larvae. During development, there is no pupal stage. Other familiar members of this clan and close relatives of cicadas include leafhoppers, spittle bugs and lanternflies. Periodical cicadas differ from their relatives, annual and dog-day cicadas, which appear yearly in summer and autumn throughout North America and much of the world.
By virtue of their visits in cycles of 13 or 17 years in distinct locations, periodical cicadas are unique in the animal realm. These regional, periodic visits are called broods, and there are three broods of 13-year cicadas and 12 broods of 17-year cicadas. Scientists classify the different broods using Roman numerals. Periodical cicadas emerging in 2021 are known as Brood X, with the X, of course, adding an element of consternation that will be pondered by many and exploited by the media. A common misconception regarding periodical cicadas is that they are but a single species. They are not. There are four species of 13-year cicadas, which go by the names of Magicicada neotredecim, M. tredecim, M. tredecassini and M. tredecula, and three species of 17-year cicadas, called M. septendecim, M. cassini and M. septendecula.
While Native Americans were fully aware of periodical cicadas by the time the first colonists landed on the New World’s shores, colonial Europeans had never experienced a massive appearance of large, boisterous insects emerging from the earth and flying to treetops. In 1633, William Bradford, the first governor of Massachusetts, wrote, “All the month of May, there was such a quantity of a great sort of flyes like for bigness to wasps or bumblebees, which came out of holes in the ground … and ate green things, and made such a constant yelling noise as made all the woods ring of them, and ready to deaf the hearers.”
For immigrants escaping persecution in parts of Europe, this vast, disturbing natural event awakened long-forgotten fears of the eighth biblical plague, the plague of locusts. Throughout the colonies, the name locust soon became attached to periodical cicadas. Of course, we know locusts are grasshoppers, chewing insects of the Orthoptera clan that sometimes appear in astounding numbers, consuming everything plantlike in their path. However, journalists and a misinformed public continue to refer to periodical cicadas as locusts.
Seventeen years underground, and then what?
As you read this article, there are literally trillions of cicadas in subterranean galleries a foot or more beneath the soil’s surface. Densities of periodical cicadas can be staggering, with some areas harboring as many 1.4 million nymphs per acre (3.5 million per hectare). (Photo 2) Brood X nymphs entered the soil in the summer of 2004 after hatching from eggs deposited by their mothers in small branches in the treetops. After burrowing into the soil, they feed on small roots of several different species of deciduous trees, but they also may feed on rootlets of gymnosperms and herbaceous plants, including grasses. Nymphs and adults pierce xylem elements with their sucking mouthparts and consume xylem fluid. Hatchling cicadas, known as first-instar nymphs, shed their exoskeletons four times, developing into fifth-instar nymphs by their 17th year, when they emerge from the soil.
Environmental cues linked to development and the exact timing of emergence are not fully understood, but the nutritional quality of plants consumed, soil temperatures and day length are all believed to play a role. When soil temperatures reach about 64 F, the massive synchronous emergence of cicada nymphs from their galleries will begin and last for only a matter of days. In the spring of 2020, at locations in Maryland, Virginia, the District of Columbia, Kentucky and Ohio, early-rising periodical cicadas of Brood X emerged between April 19 and June 14 (Raupp et al. 2020). Synchrony is critical for periodical cicadas. Their bizarre strategy for survival is to simply overwhelm hungry predators by filling all of their bellies and leaving yet enough cicadas to survive and perpetuate their species. This strange survival scheme is called predator satiation. It has proven successful for hundreds of thousands of years.
The bulk of cicadas emerge at dusk and move from the soil to vertical structures including trees, underlying vegetation and human-made structures such as buildings and lawn furniture. After shedding their last nymphal skin, adults expand their wings before their exoskeleton hardens. In the first 24 hours, the toll on emerging cicadas will be vast, but those that survive move to the treetops to mature. After several days, with wings and acoustic organs functional, males fly, aggregate in clusters of trees and begin an ear-splitting chorus designed to attract other members of their species. Membranes on both sides of their abdomen, called tymbal organs, vibrate to produce a variety of calls that can approach 100 decibels, an intensity slightly lower than that of a leaf blower or chain saw.
Once members of the same species are assembled, males use courtship calls to woo potential mates. If a female likes the male’s performance, she signifies her willingness to mate with an audible flick of her wings. Inseminated females eventually move to small branches on favored trees to lay eggs. Using a rigid appendage on the abdomen called an ovipositor, the cicada cuts slivers into twigs and deposits batches of 20 to 30 eggs into each of these egg nests. (Photo 3) Individual females may lay up to 600 eggs during the course of a lifetime. In the relative safety of the egg nest, eggs develop for six to 10 weeks, after which time tiny cicada nymphs drop from the canopy to the ground.
Within a few minutes, they burrow into the soil, where they locate roots of plants and begin feeding for the next 17 years.
What injury do cicadas cause?
Injury caused by xylem-feeding of adult cicadas is inconsequential. The real insult to woody plants comes from wounds caused when cicadas slice branches to insert eggs. This injury causes the tips of many branches to wither and die just distal to the sites of egg laying. These dead terminals droop, and the injury is called flagging. Eventually, dead terminals may break entirely and drop, littering the ground below with branches. Branches that do not break and drop may eventually enclose the ovipositional wound, but the wound site may be structurally deficient and may break at a later time. Concern also exists that egg-laying wounds may be entry points for pathogens to colonize plants. (Photos 4 & 5)
With respect to the types of plants used for oviposition, the bad news is that periodical cicadas are broad generalists. An important study conducted by Miller and Crowley (1998) at the Morton Arboretum of 140 genera of woody plants revealed that more than half sustained injury caused by ovipositing females of Brood VIII. Some of the most heavily attacked, and those experiencing the greatest twig breakage, included Acer, Amelanchier, Carpinus, Castanea, Cercidphyllum, Cercis, Chionanthus, Fagus Quercus, Myrica, Ostrya, Prunus, Quercus and Weigela. Several genera sustained no injury despite being surrounded by trees that were attacked. They included Rhus, Asimina, Berberis, Gymnocladis, Viburnum, Euonymus, Maclura, Abies, Larix, Picea, Pinus, Pseudotsuga and Phellodendron.
A more recent account of 42 common woody-plant species added several new genera to the list, and found that all but 10 were used as ovipositional hosts for Brood X cicadas in Delaware. This study found that native and non-native woody plants were equally likely to be used for oviposition by cicadas, but alien plants, those with no other known congener in the United States, were less likely to be used for oviposition (Brown and Zuefle 2009). (Photo 6)
Several other factors besides taxonomic identity of a plant affect the use of a plant as an egg-laying host for cicadas. Small, bushy plants tend to receive fewer egg nests than those having simpler structure with longer branches. Several studies revealed trees near forest edges and branches with sunny exposures sustain more cicada injury. This places nursery stock, orchards and recently transplanted saplings in commercial and residential landscapes at elevated risk, particularly if there are established trees nearby with a history of supporting cicadas.
While flagging and limb breakage occur in the short term, there is little evidence that cicadas pose a long-term threat to the vitality of trees, especially older established ones (Miller and Croft 1998). A study of early successional trees found no clear effect of cicada ovipostion on growth rates or radial growth of trees attacked by cicadas (Clay et al. 2009).
Are there natural agents limiting cicada populations?
As mentioned previously, periodical cicadas evolved the strange predator-
satiation strategy as a means of survival in the face of intense pressure from so many creatures anxious to eat them. Starlings, grackles, robins, blue jays, blackbirds, sparrows, titmice, vireos, gulls, terns and several other feathered reptiles eat cicadas. Snakes, turtles and fish consume them. Skunks, squirrels, mice and other small mammals eat cicada adults and nymphs. Many predatory arthropods, including spiders, centipedes, opilionids, ants, stink bugs, assassin bugs and flies, have been observed feeding on various life stages of cicadas. A specialized fungus, Massospora cicadina, infects and kills large numbers of cicadas in each brood and, in a fascinating twist, becomes a sexually transmitted disease in cicada populations.
Cats and dogs will consume large numbers of cicadas in 2021. Cicadas in general, and periodical cicadas specifically, were and are important sources of protein for indigenous people, including Native Americans.
In addition to death due to biotic agents such as predators and disease, abiotic factors, including extreme weather conditions such as thunderstorms, doom many. Human activity such as deforestation, agriculture and urbanization with attendant proliferation of impervious surfaces is responsible for local extirpation of cicada populations. In recorded history, two broods of cicadas have disappeared, Broods XI and XXI.
Preventing cicada injury to trees
While a typical knee-jerk reaction might be to treat trees with insecticides, several scientific studies show this may not be the best way to go. Protecting trees from cicada injury is of the utmost importance to fruit growers, where injury to highly susceptible trees such as apples, peaches and cherries directly impacts yields and profits. Important data collected in a commercial orchard clearly demonstrated the efficacy of using netting rather than insecticides to protect trees from egg-laying cicadas. Trees netted with 1.0-cm mesh sustained virtually no damage, whereas trees treated several times with potent carbamate and synthetic pyrethroid insecticides received eight to 25 times more injury from cicadas. (Chart 1)
Mesh size does matter. While 1.0-cm mesh performed well, when mesh size increased to 2.5 cm, cicada damage was as severe as that of untreated trees (Hogmire et al. 1990). Another important finding of this study was that netting proved to be only slightly more expensive than insecticide applications. A second trial with active ingredients listed by the Organic Materials Review Institute (OMRI) for use in the production of organic crops found six applications of emulsions of kaolin clay, neem and karanja oils to be ineffective in reducing the number of egg nests, while fabric netting provided complete protection from cicadas (Frank 2020). Cicadas actively move about and lay eggs for a period of several weeks, necessitating repeated applications of contact insecticides as new cicadas arrive.
Do soil injections of neonicotinoids provide longer-lasting protection to small trees compared to exclusionary nets? Our research demonstrated that soil drenches of imidacloprid applied to sapling Tilia were only about half as effective at preventing egg laying compared to 1.0-cm mesh nets (Ahern 2005). The material cost to enclose a 3-meter- (10-foot-) tall tree was $2.82 in 2005. Time to enclose a sapling was a matter of minutes. (Photo 7)
Netting clearly provides superior protection and may be cost effective for small trees, but what about mature trees? Aforementioned studies indicate that the effects of cicada injury, while dramatic, likely have minimal negative effects on the long-term growth of trees. However, in addition to improving the short-term appearance of injured trees, careful sanitary pruning of damaged branches may enhance wound closure and reduce structural defects in branches as they mature.
How should we prepare for the impending arrival of Brood X?
For arborists, now is an excellent time to plan and discuss the upcoming appearance of Brood X with clients. Step one involves determining if cicadas will be present on your clients’ properties. Even though Brood X will appear in more than a dozen states, their distribution will be patchy, meaning in some areas vast numbers will emerge, but miles away few or none will be seen. If you are familiar with the cicada history of your clients, you may already know that cicadas emerged at their properties in 2004. If your business is new to an area or if you have expanded your client base to new locations, talk to your clients and see if they have knowledge of what happened in their landscape 17 years ago.
If cicadas are likely, inventory properties proactively to evaluate which trees are at greatest risk and discuss plans for protecting those trees. Several commercial vendors sell plastic netting suitable for protecting trees, but remember, mesh sizes larger than 1.0 cm may not prevent ovipositional injury. Suppliers can be found on the internet, and netting can be purchased in bulk. Some naturalists have expressed concerns about nesting birds becoming trapped in netted trees, so inspect trees prior to netting to avoid harming wildlife.
Urban foresters and planners should consider delaying planting woody plants in the spring of 2021 in areas known to support populations of periodical cicadas. If trees were planted in the fall of 2020 or in recent years past, consider protecting them.
Excellent information on all things related to cicadas can be found at the Cicada Mania website https://www.cicadamania.com/.
Get ready. Just as cherry trees bloom each spring and Halley’s comet stops by every 75 years, Brood X cicadas will return in 2021.
Michael J. Raupp, Ph.D., is professor emeritus in the Department of Entomology at the University of Maryland, College Park, Maryland. He has published many articles, made numerous presentations and frequently appears on television and radio. His most recent book, 26 Things that Bug Me, published by ISA, introduces youngsters to the wonders of insects and natural history, while Managing Insects and Mites on Woody Landscape Plants, published by the Tree Care Industry Association, is a standard for the arboricultural industry. His websites include www.bugoftheweek.com and www.youtube.com/user/BugOfTheWeek.
Ahern, R.G., S.D. Frank and M.J. Raupp. 2005. Comparison of exclusion and imidacloprid for reduction of oviposition damage to young trees by periodical cicadas (Hemiptera: Cicadidae). Journal of Economic Entomology. 98: 2133-2136.
Clay, K., A. L. Shelton and C. Winkle. 2009. Effects of oviposition by periodical cicadas on tree growth. Canadian Journal of Forest Research. 39: 1688-1697.
Cook, W. M., and R. D. Holt. 2002. Periodical Cicada (Magicicada cassini) Oviposition Damage: Visually Impressive yet Dynamically Irrelevant. The American Midland Naturalist. 147(2): 214-224.
Frank, D. L. 2020. Evaluation of Organically Acceptable Methods to Control Periodical Cicada (Hemiptera: Cicadidae) Oviposition Injury on Nonbearing Apple Trees. Journal of Entomological Science. 55(2): 210-218.
Hogmire, H.W., T.A. Baugher, V.L. Crim and S.I. Walter. 1990. Effects and control of periodical cicada (Homoptera: Cicadidae) oviposition injury on nonbearing apple trees. Journal of Economic Entomology. 83: 2401-2404.
Kritsky, G. 2004. Periodical Cicadas: The Plague and the Puzzle. Indiana Academy of Science Press, Indianapolis, Indiana. 147 pp.
Miller, F., and W. Crowley. 1998. Effects of periodical cicada ovipositional injury on woody plants. Journal of Arboriculture. 24(5):248-253.
Raupp, M. J., C. Sargent, N. Harding and G. Kritsky. 2020. Combining Data from Citizen Scientists and Weather Stations to Define Emergence of Periodical Cicadas, Magicicada Davis spp. (Hemiptera: Cicadidae. The Maryland Entomologist. 7(4):31-42.
Williams, K. S., and C. Simon. 1995. The ecology, behavior and evolution of periodical cicadas. Annual Review of Entomology. 40:269-295.