What are periodical cicadas?
There are two groups of periodical cicadas – the 13-year and the 17-year – and both groups are members of the genus Magicicada. These ‘bugs’ are considered to be ‘true bugs’ in that they belong to the order Hemiptera and that they possess the characteristic "piercing-sucking mouthparts” (note: all their piercing and sucking is done in plants, not people – don’t worry). Periodical cicadas spend the majority of their life living underground and feeding on fluids from tree roots, but in the 13th or 17th year, the nymphs will begin to emerge synchronously (at the same time) and in huge numbers and start their search for love. You can actually see some signs of their emergence already – as many create cicada “chimneys” as they get closer to E-day. The chimneys (image right – all items circled in red are chimneys) are found only in wet soil and they are created by the cicadas as a means of ensuring their tunnels don’t fill with water. If you break off the top of the chimney (see video below) you can actually see right down into the tunnels. You will also find places were the cicada tunnels just open up to the outside, so if you peek down in you just might see a cicada getting ready to emerge!
When the cicadas do emerge, you will notice that they look different than the ones we see most other years. The cicadas that come out every year are generally green in color but the periodical cicadas are mostly black in color and have distinctive red eyes. The periodical cicadas are also a bit smaller than the annual cohorts (or group). And, as is the case with most insects, the females are slightly larger than the males.
What happens when they emerge?
Once the cicadas emerge, they will climb up a tree or other vertical structure and affix their legs in place and then they will begin their eclosion. Eclosion is the name we give to the process a nymph goes through when emerging from inside of their nymph cuticle (old skin) to become an adult cicada. They do this by splitting the back open of their nymphal skin and pulling their body out through the opening. When they first emerge, their wings are all squished inside and must be inflated, so they will hang upside down to allow their wings to fully unfold. Once their wings and their new cuticle (skin) hardens—which takes up to six days—then they are ready to get on with the business of finding a mate. Individual cicadas can live a couple of weeks above ground. In total, the adult cicadas will only be active for about four to six weeks and then all will be gone within two months - the next generation won't appear for 13 or 17 years.
Why do they only come out every 17 years?
Entomologists are not entirely sure why they only emerge every 13 or 17 years, but it is thought that it may be in response to predation pressure. If all of the nymphs emerge at once instead of slowly and over the course of many years, then there will be so many that it will not be possible for predators to eat them all. This ensures that the species will survive.
There is also a hypothesis that suggest that the number of years they take to mature - 13 or 17 - may be important, as both 13 and 1 7 are prime numbers (only divisible by itself and the number 1). Having a life cycle like this would make it difficult for predator species to have life cycles that respond or sync with the cicada life cycle. Since the cicadas emerge every 13 or 17 years, predators with two year or five year life cycles (or really any number of years besides one, 13, or 17) will only occasionally sync with the cicada emergence. For example - if a predator has a five year life cycle and it is emerging this year (2021), then they would emerge again in 2026, 2031, 2036, 2041 and so on.... but the 17 year cicadas we have her in Pennsylvania will next emerge in 2038 - so they would miss each other and would not emerge at the same time. The predator species in this example would not emerge at the same time as the periodical cicadas again until 2106 - which is five generations of cicadas later but 17 generations of the predator species!
What is that sound they make and why do they make it?!
The males sing for love! Male cicadas will use a special organ called a tymbal (circled in red below) to make their call. The calls are all species-specific – so only females of the same species will respond. It’s similar to a human male yelling to a human lady in French – if the lady only understands German, she probably won’t respond. Many males from the same species will gather together into cicada choruses and sing for the females together. When a female finds herself taken with a particular male she will respond with little wing-flicks which are apparently very attractive if you are a male cicada. The male and female will then mate and the female will oviposit (insert / lay) about 20 eggs eggs into woody plant stems. The female will repeat this until she lays 600 or more eggs! When the eggs hatch (6-10 weeks later) the wee babies will drop down to the ground and dig down into the dirt where they will live for the next 13 or 17 years – until the next emergence.
Click here for an audio clip I found on Wikipedia that lets you hear what the cicadas will sound like when they sing.
What do they mean when they say "Brood X"?
All periodical cicadas are organized into groups called brood based on the calendar year when they will emerge. All of the different broods have been assigned a Roman numeral and, in this case, it is Brood X (as in 10) that will be emerging. The map below (a USDA Forest Service map) shows the locations of the different broods of periodical cicadas. You can see that brood X, in yellow, actually emerges from several places in central and the north eastern US.
Are cicada's dangerous to me or harmful to my garden?
Cicadas are essentially harmless – they do not have a stinger, they don’t have mandibles (chewing / biting mouth parts), they are not poisonous or venomous, they won’t even eat your plants. In fact, the only harm that they will do would be to the roots of some young trees if they were planted in the years around the emergence. If you pick up a cicada there is still the chance that it could try to defend itself by sticking its piercing mouth parts into you instead of into a tree root, but that’s only if you mishandle them. So long as you just let them be or only handle them gently, then no harm will come to you or the cicada. The huge population is actually a benefit to many other species, as they become a significant food source during emergence years, and to the soil, as the cast away nymph skins (called exuvia) and deceased cicadas will be broken down and increase soil nutrients through decomposition.
Periodical cicadas are really neat and an emergence is always an exciting event, so if you get the chance to get out there and have a look at them this moth you should do it.
LIFE CYCLE OF PERIODICAL CICADAS:
I’m going to try to keep a somewhat updated map that shows the locations where we have found cicada holes or chimneys in our general area here in East Central Pennsylvania.
Finding even MORE chimneys at some of the areas around my house - look at the number of chimneys in this image: