Cicada brood X to emerge in May by the billions: here’s what you need to know

By Alexandra Johnson

The spring of 2021 will hold a spectacle that has been forming for nearly two decades.  This is the year experts predict that Cicada Brood X will emerge from underground and molt. 

According to Penn State Extension’s website, these are known as Periodical Cicadas, meaning that the insects spend the first 17 years of their life as nymphs living underground.  

The life cycle of a periodical cicada

The extension’s experts state that once the nymphs emerge, they will climb up onto a nearby tree and molt into the Cicada adult form that we are all familiar with. Their flying form has large red eyes, a big black body, and a mouth capable of producing a very loud screeching sound. 

These bugs have the longest life cycle of any known insect, and experts still do not fully understand why their lifecycle is so long, or how they manage to all emerge at exactly the same time. 

Local residents may have questions too, as multiple Cicada prediction maps show that Southeastern Pennsylvania will be a hotspot for brood X. Multiple studies show that approximately 1.4 million cicadas can be found in a single acre once the mass emergence begins.  

So what exactly can we expect when millions of insects emerge in our backyards in late spring? Many residents of Delaware County do not know whether or not to protect their plants, children, or homes from this mysterious invasion.  

Here’s everything you need to know about this event. 

Will cicadas damage the trees and plants in my yard?

For the most part, no. Dr. Greg Krawczyk, an entomologist at Penn State Extension, explains that only very young trees that have been recently planted are at serious risk.  “The most obvious damage is done during the egg-laying process,” Krawczyk writes. “The slits made by the female in small branches severely weaken them; often the weakened branches snap off in the wind.”      

“You only have to be careful with young trees with thin branches,” agrees Holly Thorpe, the master gardener coordinator at the Penn State Extension. 

Do adults bite or sting?

“Cicadas don’t bite or sting,” Thorpe states.  “They don’t have any defensive weapons at all.” 

Scientists describe adult cicadas as having a very short life cycle compared to their nymph stage.  Because they are only alive for such a short time, all of their energy is geared towards mating, not defense. 

Why are cicadas so loud?

An adult cicada lands on a surprised boy’s nose. | Photo by Jim Mayo/ Chicago Tribune.

A recent paper published in the Annual Review of Entomology states that only male cicadas make sound, and they produce it by vibrating a body part known as a tymbal.  

It also discusses that males can vibrate these tymbal organs so fast, that they can produce a sound as loud as 90 decibels. A decibel measuring chart confirms that 90 decibels is approximately the same force of sound as an airplane taking off.  

These insects have made themselves heard for a long time, at least as far back as William Bradford, the first governor of Massachusetts.  He wrote about their high decibel calls stating that these bugs “made such a constant yelling noise as made all the woods ring of them, and ready to deaf the hearers.”

How long will the cicadas be around?

Krawczyk states that the females will begin laying eggs approximately seven to 10 days after emergence. The insects do not last long, only surviving in their adult forms for about two to four weeks according to the Cicada Crew Website at the University of Maryland. 

Thorpe recounts that the last brood of periodical cicadas were gone by July for the most part in Delaware County. 

Why does this only happen every 17 years?

A freshly emerged cicada sits on a tree. | Photo by Jason Wambsgans/Chicago Tribune.

While the specific number of years is genuinely still a mystery among scientists, there is an explanation for why so many insects emerge at the same time. This is a classic evolutionary strategy that is referred to as predator satiation. 

Biologist Richard Karban explains this in a recent study, stating that individual adult survival was higher in a larger group. Adult cicada survival was lower in locations where the number of individuals was smaller, and more were eaten. 

For example, if 1,000 cicadas were to emerge within 24 hours in a field, and the predators there could eat 600 before they were full, then 400 cicadas would have the opportunity to mate. Conversely, if only 500 emerge on the first day and 500 emerge several days later, the predators would eat every single cicada each day. 

Although this survival strategy has been studied extensively, no scientists have reached a solid conclusion as to why these insects only mate every 17 years, and how they coordinate a mass exodus from the earth. Several studies have been published suggesting that soil temperature plays a vital role, but that is only a piece of the puzzle, experts say. 

A Cicada emergence indicator formula developed by Gene Kritsky suggests that the insects will emerge around May 17 or 18 in Delaware County. 

Once the necessary precautions have been carried out, this will truly be a spectacle for everyone in the county to witness. 

Content by Alexandra Johnson at

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