Return of the Magi(cicada)
- Published: May 13, 2021
By Don Cipollini
This article first appeared in the newsletter of the YS Tree Committee.
By now, many of us are aware that southwestern Ohio will experience a mass emergence of periodical cicadas (Magicicada species) this Spring. A great deal of information exists on this phenomenon, as well as a fair amount of misinformation. So what do you need to know? Here are a couple of scientific tidbits and fun facts that may be just enough to impress your friends and maybe win a round of trivia or two.
The emergence that we will experience in the Yellow Springs area is part of a group of 17-year periodical cicadas known as Brood X. Broods are large groups of individuals that all emerge at the same time and have the potential to mate and are generally labeled with Roman numerals. Brood X is one of the larger broods, with large emergences expected in the Midwest and in several states along the east coast.
The cicadas that will emerge as adults this year began their journey in 2004, when they dropped from tree branches as nymphs and burrowed in the soil. There, they spent the last 17 years attached to tree roots feeding on xylem fluids with their piercing and sucking mouthparts. They will periodically move around underground and can burrow down to about eight feet. How they keep track of time is still somewhat of a mystery, but it likely involves sensing and “counting” seasonal pulses in plant resources as their host plants go through their annual growth cycles. Sensing seasonal temperature fluctuations may also play a part. When their internal clock strikes 17, they will move upward in the soil and build an exit tunnel at the soil surface in preparation for emergence when soil temperatures reach 64 degrees. This usually occurs in mid-May in this area, especially after a soil-softening rain, and then the next stage in their life cycle begins. It is said that mole activity will increase as cicadas slowly move upward in the soil in preparation for emergence. We have indeed noticed this very phenomenon in our own yard; whether a coincidence or not, I don’t know.
Upon emergence from the soil, cicadas will climb on the nearest upright object — even you, if you stand still long enough! — shed the skin of their final nymphal instar, and emerge as winged adults (imagoes). It can take a few days for their exoskeleton to fully harden and for them to be “functional” as adults in terms of flying, singing and mating. Three different species will be part of Brood X, including Magicicada septendecim, M. septendecula, and M. cassini. Mature adult periodical cicadas are generally black-bodied with red eyes, unlike the greenish, yellow-eyed annual cicadas we see every year, but a number of morphological features distinguish the different species, including variation in body size — cassini is smaller — and in the coloration of their abdomens. Grab a few this year and see if you can see any differences. Once aboveground, the whole raison d’etre of periodical cicadas is to reproduce as much as possible before they get eaten or die. In fact, a major hypothesis for the evolution of the mass emergence phenomenon is to satiate predators with so many adults that at least some will live long enough to reproduce. Just about anything that will accept meat in its diet will try to eat cicadas, including some humans. The most persistent rumor I have heard is that they taste like asparagus. I cannot attest to that just yet, but I may give it the old college try this Spring.
Adults can feed, much as they did belowground, but they generally do not expend much energy doing so and the damage they may do in this fashion is only minor. What they do is spend much of their time singing, making short flights between perches, and mating. Males do all the singing, using a special organ called a tymbal located behind the wings on the abdomen, and their different calls have a variety of meanings. Mating calls can be so loud that male cicadas will disable their own eardrums to protect themselves from the noise. In response to male mating calls, females respond with wing flicks to indicate their interest and the fun begins. Cicadas can be best described as polyamorous; they mate continuously and with many partners. The three species that will make up the emergence manage to avoid each other by having slightly different calls and activity patterns, despite sharing the same habitats.
Once mated, females will begin the egg laying process, which involves cutting slits in pencil-sized tree branches with a saw-like ovipositor and inserting 10–20 eggs at a time. A single female will lay up to 200 eggs during her lifetime and will die after she has deposited all her eggs. Eggs will hatch in 6–10 weeks and the first-instar nymphs that emerge will fall to the ground to start the process all over again. Cicadas will lay eggs on a wide variety of woody tree and shrub species, but they seem to have some preferences, like for oaks, and they will avoid most conifers. Oviposition damage by cicadas can injure small or weak trees but it will have little long-term impact on large trees. Some species appear to tolerate and heal oviposition wounds with little effect on their health or appearance. Some species, like oaks, tend to abscise small branches beyond the oviposition sites, which will initially drape and hang dead for a while before falling to the ground. This phenomenon is called flagging and it is a natural form of pruning. Newly planted, or otherwise small trees, can be protected from oviposition damage with 1-cm mesh netting placed over the trees for about a month between mid-May and mid-June. New tree plantings can also be delayed until Fall to avoid damage.
And then, almost as fast as it arose, the cacophony will cease and all that will be left will be the carcasses of billions of cicadas and lots of satiated predators. The mass emergence has numerous ripple effects on species like birds and small mammals that often experience temporary population increases for a few years after an emergence. The carcasses also serve as fertilizer for the very trees on which the cicada nymphs feed, temporarily compensating for any of their feeding damage in the rhizosphere, where they will remain, silent and in darkness, until 2038.
*The writer is a professor of biology and director of the Interdisciplinary Environmental Sciences Ph.D. program at Wright State University. He lives in Yellow Springs.