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Cicadas Are Coming!
by Blake Layton - March 2011

This spring many Southeastern gardeners will witness one of the rarest and most spectacular events of the insect world, an emergence of periodical cicadas. Periodical cicadas only occur in the eastern United States, and they only emerge as adults every 13 or 17 years. This year the Great Southern Brood of 13-year periodical cicadas, also known as Brood XIX, will emerge through much of the Southeast.

It's the huge number of large, noisy, black and orange insects that makes an emergence of periodical cicadas so spectacular. Numbers range from tens of thousands to more than one million per acre. That's a lot of insects, and half of these, the males, will be singing as loud as they can in an effort to attract a mate. Individual male cicadas sing quite loudly; the combined voices of millions of cicadas create a roaring chorus that drowns out all other woodland noises. If you live in an area where an emergence occurs, you will know when it happens.

Like schools of sardines, periodical cicadas find safety in numbers. Some insects protect themselves from predators by hiding or using camouflage; others sting, and many use defensive chemicals that smell or taste bad. Periodical cicadas overcome predators through sheer numbers. Adults are slow-moving and easy to spot, but there are so many that birds, raccoons and other predators simply cannot eat them all before they reproduce.

Although this brood of periodical cicadas occurs over a 15 state area, distribution is somewhat spotty, and you may or may not see periodical cicadas in your backyard this spring. Parts of all Southeastern states except Florida will experience this event, but there is no state where the emergence will occur statewide. The accompanying map shows the approximate distribution of Brood XIX. If you live in a shaded area and have large numbers of hardwood trees in and around your neighborhood, you will probably see, and hear, periodical cicadas this year.

How Do These Differ From the Cicadas We See Every Year?


Periodical cicadas are easily identified by their black bodies and red eyes, but they only appear every 13 years (1.2 inches).

 

The large, green cicadas that sing from the trees in late summer and early fall are known as annual cicadas. Although annual cicadas take several years to complete a generation, the generations overlap so that annual cicadas emerge every year. Actually, there are several different species of annual cicadas, all belonging to the genus Tibicen. Depending on the species, annual cicadas may feed on roots of either deciduous or coniferous trees.

Periodical cicadas, genus Magicicada, take either 13 or 17 years to complete a generation, but there are no overlapping generations -- all members of a given population of periodical cicadas emerge as adults at the same time. In the southern portions of their range, periodical cicadas begin emerging in April, but adults may emerge into June farther north. Adult cicadas only live a few weeks. Today there are 15 recognized broods of periodical cicadas, 12 broods of 17-year cicadas and only three broods of 13-year cicadas. In general, 17-year cicadas occur in the more northern portions of the country, while 13-year cicadas occur in the South, but there is some overlap. Geographically, Brood XIX is the largest 13-year brood, occurring in portions of 15 different states. Periodical cicadas only feed on deciduous trees, and males begin singing soon after sunrise.

Just as there are different species of annual cicadas, there are also different species of periodical cicadas. In fact, the Great Southern Brood consists of four different species of 13-year cicadas that emerge at the same time, each with its own unique mating songs.
 
Cicada Life Cycle


Annual cicadas are seen, and heard, every year even though they take several years to complete a generation (1.5 inches).


The transition  from childhood to adulthood is always difficult, but cicadas complete it in a  single night. This adult is shedding  its nymphal skin.


Cicada nymphs leave dime-sized holes when they emerge from the ground. Scientist can estimate numbers of periodical cicadas per acre by counting the number of emergence holes per square yard.


Cicada nymphs spend years underground feeding on sap from tree roots. This small nymph was unearthed while preparing a flower bed (3/16 inches).

Except for the time it takes to complete a generation, the life cycles of annual and periodical cicadas are similar. To review the cicada life cycle, let's start with a male singing from a tree limb to attract a female.

Only male cicadas sing, and the songs of each species are unique and only attract females of the same species. After mating, female cicadas use their strong, stinger-like ovipositor to cut a slit into the bark and sapwood of pencil-sized twigs and insert their eggs. Each female lays several hundred eggs in this manner, depositing a few dozen eggs in each twig. Eggs hatch in around six to eight weeks. Newly hatched nymphs fall to the ground and use their strong forelegs, which are well-adapted for digging, to burrow into the ground. Nymphs feed by sucking sap from tree roots and grow by occasionally shedding their old skin. Depending on the species, this development period can last as little as two years for some annual cicadas, to as long as 13 or 17 years for periodical cicadas.

Once nymphs are fully grown, they emerge from the soil at night, leaving dime-sized holes in the surface, crawl a few feet up a tree trunk, and take a firm grasp. Then they molt one last time, emerging as winged adults and leaving the shed skins attached to the tree trunk. Most nymphs emerge over a period of just a few nights. This synchronized nymphal emergence is probably triggered by soil temperatures. Newly emerged adults are white-colored and soft-bodied with wrinkled wings. It only takes a few hours for the wings to expand and the skin to begin to harden and gain color, but it takes several days for the skin to fully harden.

Are Cicadas Harmful?

Despite their black and orange coloration, colors that often signal danger in nature, periodical cicadas are not harmful to people, wildlife or pets. Neither are annual cicadas. Cicadas do not bite, and even though females can splinter tree twigs with their ovipositors, they do not sting. Cicadas are not poisonous. Many birds, mammals and reptiles prey on annual cicadas every year, and such predators literally gorge themselves when periodical cicadas emerge. I even know a few gastronomically adventurous entomologists who enjoy periodical cicadas fried like soft-shelled crabs or prepared in other ways.

One might think having huge numbers of immature cicadas sucking sap from the roots would adversely affect host trees, but forest trees seem to grow well despite this heavy load of root-feeding parasites. Perhaps the tillage and aeration the nymphs provide as they tunnel through the soil helps compensate for the sap they consume. Adults also suck sap from leaves and twigs, but the effects seem to be negligible.

Cicadas do damage trees, but in a unique way. The egg-laying scars created when female cicadas insert their eggs into twigs often cause the twigs to break. This usually occurs about 6 to 12 inches from the end of the twig, resulting in a hanging brown "flagged twig." Annual cicadas occasionally cause flagged twigs but numbers are so low this goes largely unnoticed. Periodical cicadas are a different matter. If you live in an area where periodical cicadas emerge this spring, you will see trees that are brown with flagged twigs after the emergence is over. Although only a portion of twigs with egg-laying scars break, host trees are often heavily pruned. This seems to cause little long-term harm to woodland trees, but can cause short-term declines in acorn production.

Fruit trees and newly planted ornamental trees can be severely injured by this egg-laying injury, and gardeners and nurserymen who live in areas where an emergence is expected may want to take steps to protect small, high-value plants. The best way for home gardeners to protect such plants is to cover them with netting that has mesh small enough to exclude cicadas until the emergence is over, but allows adequate light and ventilation. This is one of those difficult gardening decisions that must be made ahead of time. You may not be able to find appropriate netting at the local nursery once the emergence begins.

Obviously, you won't want to go to this trouble unless you are sure you are going to have cicadas. Search your memory and consult your gardening friends. If you had an emergence in your area in 1998, there will be another one this year. Then you have to decide if the plant is really worth protecting. Most plants will suffer little long-term effect from this cicada-inflicted tip pruning, but fruit trees and newly planted hardwood trees can be severely damaged, especially if the central leader is broken.

An emergence of periodical cicadas is one of the more amazing wonders of the natural world, an event that gardening naturalists will want to experience. If cicadas will not be emerging in your backyard this spring you may want to plan a family excursion to an area where they are. Standing in a woodland setting that is pulsing with the songs of millions of cicadas is an experience you will long remember.

  

(Photos by Blake Layton)  

 


Dr. Blake Layton is Extension Entomology Specialist at Mississippi State University.

 

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