Brood X is Emerging!

Most of us have kicked 2020 to the curb and are searching for new things to look forward to. Add this to your list for “Must Do Spring 2021” — you won’t be able to do it again for another 13-17 years. Brood X will emerge this spring in 15 states, including Tennessee.

What the heck is Brood X? 

Each labeled brood is a generation of cicadas. The 2021 Tennessee brood will be Brood X. Unlike the green, annual cicadas, periodical cicadas are known for their black bodies and five bold red eyes. They are unique to North America and at 17 years, are the longest-lived insects known. Periodical broods each have their own range and are concentrated in the central and eastern regions of the United States, and some areas are home to multiple broods. Their huge, simultaneous emergence every 17 years is one of nature’s great mysteries.  Where have they been for the last 17 years? Underground, since 2004!  They are always there, just underground in a nymph form that we don’t see.

What’s going on underground?

Cicada nymphs remain underground at half a foot to two feet deep, molting through five instars (a phase between two periods of molting in the development of an insect larva), and emerge from the ground in the fifth instar. In order to grow, they suck fluids from the roots of plants. Female periodical cicadas lay their eggs in tree branches so when the ant-like nymphs hatch, they can fall to the ground, suck on some tree roots and dig in for a really long growth period. 

What makes them emerge? 

On just the right spring day, when soil temperatures reach 64 degrees Fahrenheit, after sunset and often after rain, the nymphs climb their way to the surface and emerge together.

Scientists don’t fully comprehend what triggers the cicadas to emerge — some think the insects have a method of counting the number of times deciduous trees cycle their leaves and emerge after the right number. Some simply agree it takes the nymphs 17 years to mature, but are unsure of the prompt that elicits mass confirmed emergence. Dr. Chris Simon, a molecular systematist at the University of Connecticut’s Department of Ecology and Evolutionary Biology has a theory: “The year of cicada emergence is cued by what I and others believe to be an internal molecular clock. The clock is most likely calibrated by environmental cues that signify the passage of a year, such as the trees leafing out, changing the composition of the xylem fluid on which they feed. The molecular clock keeps track of the passage of years.”  

As proof of this theory, in 2007 in Cincinnati there was a warm January, a hard freeze in February and then a normal spring, which caused maple trees to produce two leaf sets that year. Hundreds of cicadas feeding on those tree roots emerged a year early after the trees produced 17 leaf sets in 16 years.

Dr. Gene Kritsky, an entomologist at Mount St. Joseph University in Cincinnati, Ohio believes soil temperature drives the mass materialization within the appearance year. “During the last emergence in our area, I planted temperature probes in the ground all over campus to track the emergence,” he stated. “The cicadas emerged over a period of two weeks after the soil temperature reached 65 degrees.”

There may be 1.5 million periodical cicadas per acre! Research suggests that their huge numbers allow them to overwhelm predators, so enough of them will live on to breed and perpetuate the brood. Safety in numbers!

Leaving behind visible exit holes, periodical cicadas quickly crawl to any nearby vertical structure, preferably a tree or shrub. They shed their skins as they molt into adults, leaving behind their empty shells. Shortly after molting, their wings unfurl and their yellow-white skin darkens as the exoskeleton completely hardens.

They’ve emerged... what now?

They sing! After 17 years underground, it’s time to find a mate and time is short. Males sing to attract females using their tymbal, which are two rigid, drum-like membranes on the underside of their abdomen.

Most cicadas love the sun, so rain and cloudy skies will dampen their song. Temperature also affects whether they will sing or not — too cold or too hot and the songs weaken. Some cicadas will sing depending on the number and proximity of other cicadas in their area. Periodical cicadas, when there are enough in a given area, will synchronize their songs and form a chorus, like a group effort to attract females. With millions of cicadas present, the sound can reach 100 decibels — the same sound as standing next to a motorcycle revving its engine!

After answering the male cicada by flicking her wings, the female will mate with a male and lay eggs, using her ovipositor like a saw to cut slits into living twigs and branches and then place her fertilized eggs inside. This may damage young branches, which makes the periodical cicada a potential temporary problem for nurseries and orchards. If you have particular trees you need to protect, you can put nets or bags over trees for the period of time the cicadas are active. 

“While they may cause cosmetic damage to trees when laying their eggs, cicadas actually provide a number of benefits to nature," comments Jim Fredericks, chief entomologist for the National Pest Management Association. These benefits include pruning mature trees, aerating the soil, providing a food source for predators, and once they die, their bodies serve as an important source of nitrogen for growing trees. A single female may mate multiple times and lay upwards of 600 eggs during the three- to four-week lifetime of adults. Eggs hatch in six to ten weeks. The tiny ant-like nymphs emerge from the egg and fall to the ground, burrowing in to start the 17-year cycle again.

Can cicadas harm people or pets?

In short, no. Cicadas are often mistakenly referred to as locusts. Locusts are members of the grasshopper family, which have chewing mouthparts and may damage crops. Cicadas have sucking mouthparts and do not chew, but feed on the xylem of plants. Periodical cicadas will not bite. They have been known to land on people, but don’t freak out — they won’t cause any harm. They aren’t drawn indoors, so no need to worry about them setting up shop inside your living room. 

An explosion of cicadas provides a bounty of food for wildlife and some humans. They are considered a delicacy in many countries and in parts of the United States. Some claim that cicadas are high in protein, but recent research conducted at the University of Cincinnati’s College of Engineering has determined that cicadas may contain high levels of mercury. National Geographic points out that cicadas are gluten-free, low-carb, and low fat, so why not throw a dinner party featuring cicadas during the emergence? There are even cookbooks to help you decide how to serve them. 

Cicadas in adult form are so massive in numbers that even if millions of them are eaten, there are still millions left to breed. However, the nymphs can be in trouble while underground. Sucking on plant roots and living underground makes them highly susceptible to chemicals like insecticides and poisons. This classifies them in a vulnerable class of insects and is another reason to use natural methods when gardening. 

I’ll be collecting cicadas to feed the opossums, red fox, American crow, American toad, tiger salamander, turtles, and some of our raptors at Reflection Riding. It’s a free, natural prey item full of nutrients and a great form of enrichment. 

Join me in celebrating this super cool natural phenomenon this spring. Happening only every 17 years, this is something to marvel at and celebrate. Maybe a local brewery will make a periodical cicada brew!