Two weeks ago, the cicadas emerged from years underground to become adults called imago. Maybe you noticed the holes poked in the dry ground. Maybe you noticed the notorious dog-day buzzing begin. Maybe you noticed their alien-like exoskeletons hanging haphazardly outside. Or, maybe, like me, you found one just beginning metamorphosis and watched. The whole metamorphosis took a little over 30 minutes in the dark on my porch, illuminated by my neighbor’s cell phone; we were mesmerized. I felt a little bit like I was watching something very private – I mean, this was the cicada’s extremely accelerated puberty, sotospeak.
Metamorphosis, an insect’s change from larva to adult, is otherworldly. The cicada pulses as the tender, green imago emerges from the hard, brown exoskeleton of the last instar nymph. Once the thorax emerges, it does this amazing backward bend and begins to unfold its wings. I couldn’t possibly relate with that cicada: What in the world might it be thinking, pondering, or feeling during this strange, inexplicable, and, for it, completely life-changing new experience. Since metamorphosis is a gradual process, I had a lot of time to wonder about that cicada in particular and also cicadas in general.
Why do cicadas spend a large portion of their lives underground? What do they do down there? How long are their lives aboveground? What do they eat, and how do they affect the forest?
When cicada nymphs emerge from eggs originally deposited in the bark of a twig, they burrow underground. I had always heard that cicadas stayed underground for 13 or 17 years (both prime numbers) before emerging to metamorphose. Apparently, this length of stay is only true for one North American genus, Magicicada, while most other cicada taxa stay underground for only 1-2 years. This explains why in the Carolinas I always heard cicadas every year, even though I’d heard that cicadas emerge every 13 or 17 years. The next Magicicada emergence is not slated until summer 2013 in NC, so it’s likely the cicada I saw belonged to some other genus.
Why 13 or 17 years? When cicada nymphs emerge, they spend a week aboveground before metamorphosing. During that time, they are big, juicy, and cannot fly. Predators love them, and pretty much everybody loves to eat them, including cicada killer wasps, praying mantises, lizards, birds (including wild turkeys), rodents, larger mammals, and even humans. There are two hypotheses about these prime-numbered cycles: (1) predators have a hard time responding to sudden large numbers of cicadas and synchronizing their own population numbers accordingly, and (2) if everybody of the same species emerges at once, it’s likely they’ll find someone of their species to mate with rather than mistakenly mating with another related species, which often produces infertile offspring. In a species with specific emergence years, hybridization would be especially bad, because the hybrid cicada would be genetically programmed to emerge when no one else was emerging and thus would not mate. These issues may have been especially advantageous to the survival of the species during the Pleistocene, because it was cold, and cicadas like to mate when it is dog-day hot. A lot of cicada populations probably did not make it through the Pleistocene, and those that did evolved strategies to deal with their smaller numbers, so that survival and mating were maximized. Let’s just say that ecologists got really excited about these hypotheses for a couple of decades and made a bunch of models of different scenarios of cicada population growth and emergence cycles. And, who can blame them? Ecologists are obsessed with explaining the weird and the anomalous. What they decided was this: While the first hypothesis certainly explains the mass numbers of cicadas, it does not explain the prime numbers. Since many other species of cicada emerge every two years (or a multiple of 2), then if these cicadas emerge on a prime-numbered cycle, they will miss some of these other species. Further, 17-year cycles will never mate in the same year as 13-year cycles (or, well, only every 200-some years). This is the most accepted hypothesis explaining why prime-numbered cycles would have developed, supported by mathematical models.
Cicada nymphs feed on sap in the roots of deciduous trees, and can dig down to depths more than 8 feet. Adult cicadas also eat sap, using a long proboscis that pierces a plant stem. Mostly, cicada feeding causes nominal damage. The adults are only above ground for a few weeks, chorusing (loudly) and mating. Females pierce twigs to make holes where they lay eggs, which also wounds twigs, but the damage is minimal. However, when cicadas are concentrated (which we know happens every 13 or 17 years), the multiplicative effects of this small-scale wounding can cause damage or death, especially in young trees or shrubs. If a cicada emergence is predicted (i.e., it’s been about 12 or 16 years since the last one), then you might want to hold off planting young deciduous shrub and tree saplings for a few years. You can check whether an emergence is predicted in your state here.
During prime-numbered emergence years, cicada numbers can be huge (as much as 1.5 million per acre). In the year before a periodic emergence, large cicada nymphs can cause reduced growth in trees and higher numbers of moles, which feed on the nymphs. However, the moles have a bad year the next year, because they suddenly don’t have any big nymphs to feed on but they have more mouths to feed because of the previous good year. Conversely, the trees have a good year the next year, because cicada carcasses decompose and add nutrients that fertilize the trees. So, cicadas have a great story for the power of numbers and strategy, so that they are a hidden driver of forest processes in North America.