Parts of the Northeast will hear the distinct buzzing sound of 17-year cicadas this spring.
Periodical cicadas are classified into different broods and can arrive from underground in 13 or 17-year cycles, according to the National Pest Management Association (NPMA). The broods are identified by roman numerals and are named based upon the geographic regions as well as the timing of their emergence.
This year's grouping is classified as Brood V and will emerge in southwestern and southeastern Pennsylvania, eastern Ohio, extreme western tip of Maryland, a small portion of northwestern Virginia and most of West Virginia, said Greg Hoover, an ornamental entomologist at Penn State University. This brood was last seen in 1999.
Entomologists say the cue for the insects to begin to move above ground occurs when soil temperatures reach around 64 degrees Fahrenheit. Recent rainfall can also be a factor.
"The emergence is usually proceeded by a rainfall event," Hoover said.
Hoover said this year's warmer winter conditions across the Northeast would not likely impact the arrival of the cicadas, and he added that there have not been any reports of early sightings. However, he said some Brood V cicada nymphs were discovered by gardeners in southwestern Pennsylvania last year.
"It's really beyond ambient or the air temperature, it's the soil temperature that will influence when they emerge," he said.
After the cicada nymphs complete their synchronized emergence above ground, they will develop into adults and begin the mating process. They only stay there for about a month to a month and a half where they will reproduce and eventually die. After the new cicada nymphs hatch from their eggs, they will burrow back underground to begin another 17-year cycle. Once underground, they will spend most of their time consuming juices from tree roots.
Typically, the cicadas will first be heard in late April or early May, said Dr. Jim Fredericks, chief entomologist for NPMA. The noise is produced by the males calling out to the females, he added.
There are a number of different theories for why the cicadas spend 17 years underground, Fredericks said. One of which is an ecological principal called predator satiation. In this case, the mass emergence of hundreds of thousands of cicadas at the exact same time is said to overwhelm the number of predators, like fish and birds, who will try to feed on the insects, he explained.
Mature cicadas are least active at night when they are up in the trees and in the morning when temperatures are lower, the NPMA said.
Cicadas are considered more nuisance pests than detrimental. However, they can present a concern to fruit orchards, Hoover said. Females lay eggs, usually hundreds of them, by slicing into the bark of twigs. This can structurally weaken the twigs and cause them to break, which is harmful to fruit-bearing trees.
"Orchards usually do treat their trees to keep the damage at a minimum to the twigs," Hoover said.
Periodical cicadas are the longest-lived insects in North America and their "sudden, springtime emergence" is a key reason for why the insect generates so much curiosity, according to information from Penn State's entomology department.
"Periodical cicadas are unique to eastern North America and they are found nowhere else in the world," Hoover said.
The number of cicadas that might be found in a certain area can vary. Some places could see thousands of cicadas per acre, while others could see hundreds of thousands per acre. Yet in locations like southeastern Pennsylvania, tree removal for residential development has diminished the cicada population, according to Hoover.
Fredericks said the numbers can be "surprisingly large" in some areas and described an occurrence when he observed trees "crawling with cicadas."
"The sound that they produce is extremely loud and it really is a wonder, a sight to see," he said.