Periodical Wonders

Periodical cicada broods

If you live in parts of southern, central, or northeastern Pennsylvania, you may have already seen Brood X cicadas emerging. After developing underground for seventeen years, Pennsylvania’s largest group of periodical cicadas is ready to spend a few short weeks shedding their exoskeletons, mating, and laying their eggs. Also known as the “Great Eastern Brood,” Brood X cicadas emerge in 15 states across the eastern U.S. and Midwest.

A Vital Part of Our Ecosystem

Periodical cicadas are distinct from the typical, annual cicada in a few ways. They emerge much earlier than annual cicadas, usually in May or June when soil temperatures reach 64 degrees. Instead of the green and brown coloration of annual cicada species, mature periodical cicadas have a black body, red eyes, and orange wing veins. And they emerge seemingly overnight as a massive group, which helps them protect against predators.

These large insects can be a nuisance with their loud, droning calls and clumsy flying. But they pose no threat to humans or pets, and their presence is good for our ecosystems. Cicadas create tunnels when they dig their way out from underground, helping to aerate the soil. They also provide a feast for local wildlife. Birds, rodents, moles, snakes, and fish all gorge themselves on freshly emerged cicadas. And after they complete their life cycles, the millions of cicadas will decompose, helping to fertilize forest soil with a boost of nitrogen.

Periodical cicada. Photos by Dr. David Ingber.

Female cicadas do have the potential to damage trees when they lay their eggs. They cut small gashes in tree branches, where they will insert their eggs to mature into nymphs. This sometimes causes branches to break off and die. However, this process also provides natural pruning for the trees, and, assuming the tree is healthy, generally stimulates new growth. If you have young or vulnerable trees, you can cover them with netting to prevent cicadas from accessing their branches. It’s also important to lay off the pesticides for cicada control. They do much more harm than good to the local ecosystem and will hardly make a dent in the cicada population.


Periodical Cicadas & Warming Temperatures 

Like many other natural cycles, periodical cicadas are potentially susceptible to a changing climate.

“There is a concept in agriculture and pest management called ‘degree-days,'” explained Dr. David Ingber, Faculty Specialist in Biology at the University of Scranton. “It is possible to predict developmental events in a plant or insect species by the average temperatures of each day. If the temperature is above a certain threshold, which is unique for each species, a population will progress in its development. If it is below, then it will not.”

“This means that as the average daily temperatures increase over the years, degree-days will be accumulated faster, and those life events will subsequently happen sooner – as long as temperatures aren’t so high as to damage the insects. The take-home here is basically that as our average temperatures increase, the insects will develop faster! This could mean that it may take less and less time for the cicada to develop as time goes on.”

Like many other natural cycles, periodical cicadas are potentially susceptible to a changing climate.

It isn’t uncommon for small numbers of periodical cicadas to emerge off-cycle. According to Dr. Ingber, if enough breeding cicadas emerge early or late, new broods can form. But because each brood emerges so infrequently, it can be challenging to track patterns.

“Long story short, the jury is still out on the exact effects that climate change will have on our 13 and 17 year cicada, but it is a reasonable assumption that consistently high average temperatures may result in faster emergences,” said Dr. Ingber.

Interested in helping to map the emergence of Brood X? Document your cicada sightings with the Cicada Safari smartphone app.