Over the past few weeks we have explored some of the mysteries surrounding the surprisingly large emergence of periodical cicadas in the Washington Metropolitan Region. We learned about accelerating Brood X cicadas, watched the emergence of cicada nymphs from their subterranean crypts, and learned how male cicadas make their ethereal sometimes earsplitting sounds.
One question that came up during an interview last week, “How to you tell the guys from the gals?” At first glance it is not apparent how guys are distinguished from babes in the world of cicadas. However, the task is pretty simple and requires very little skill. As discussed in last week’s episode, only male cicadas sing and the tymbal organs responsible for sound production are easily seen on either side of the cicada just beneath the wings. For most terrestrial creatures genitalia of males and females differ rather dramatically, and cicadas are no exceptions. Viewed from the underside, the terminal segment of the male cicada is dome-shaped.
By contrast, the female cicada has oval shaped genitalia with a distinct point at the terminus. Within the female’s external genitalia is a hollow saber-like structure called an ovipositor. The ovipositor of a cicada has evolved to cut a slit in woody tissue of a tree or shrub and provide a conduit for eggs to move from the reproductive tract of the female into the wound of the branch.
Gouges laid in the woody tissue are called egg nests, and into each nest the female deposits from 5 to more than 20 eggs. This process may be repeated on multiple branches and on different plants. The number of eggs laid by a single female can exceed 500 distributed over dozens of nests. With an astounding reproductive potential like this, it is easy to see how a single widespread cicada brood might contain trillions of cicadas.
With pulsing abdomen, a female cicada injects her brood into slits in a branch through a tubular appendage called the ovipositor.
More than 80 species of woody plants may be used as hosts by egg-laying females with angiosperms greatly preferred over gymnosperms. While the mere presence of adult cicadas poses no threat to humans, the egg-laying behavior of females can damage trees, especially young ones whose canopies are comprised of many tender young shoots. This damage may dramatically change the normal growth habit of the tree. Egg-laying may also weaken branches and cause them to hang down (flag) or snap from the tree.
The following information is excerpted from a publication by Robert Ahern, Steve Frank, and Michael Raupp: “Trees commonly damaged by cicadas are fruit trees, oak, maple, dogwood, and redbud, but over 200 species are susceptible. Cicadas will not damage most evergreen trees. Periodical cicadas cause damage to trees when they gouge slits in thin branches to lay eggs. Cicadas lay eggs in branches 1/4 to 3/8 inch in diameter. Twigs of this size that have slits gouged into them by female cicadas may die. This damage, called flagging, will not kill large, established trees. However, small trees with many small branches could suffer considerable damage or die from extensive cicada oviposition. At greatest risk of damage from cicadas are trees less than six feet tall. The most effective way to protect small trees from cicada damage is with mesh netting. The netting must have holes 3/8" in diameter or smaller to prevent entry by cicadas. To be effective, protective netting should be installed prior to cicada emergence in mid-May and remain on the trees until cicadas are gone at the end of June.” So, now is a fine time to purchase netting and protect your saplings if cicadas are chorusing in your neighborhood this spring.
To learn more about local emergence of periodical cicadas, please check out ‘PBS News Hour/The Rundown: Cicadas strike back four years early. But why?’
Thanks to everyone that has participated in the Cicada Citizen Science project. With the Big Boy Band singing their hearts out in the treetops, more than ever we need to know if you have seen or heard cicadas. Please report you sightings at the link below.
CALL TO CITIZEN SCIENTISTS: we need your help! If you see periodical cicada nymphs, shed skins, adults on vegetation or on trees, and hear cicadas chorusing in the treetops, please report your sightings to magicicada.org. Cicada experts are attempting to map the distribution of these magnificent and magical insects. Thank you for your help.
The wonderful articles “The ecology, behavior, and evolution of periodical cicadas” by K. S. Williams and C. Simon and “Viable Hybrid Young From Crossmated Periodical Cicadas” by Jo Ann White were used as resources for this episode.