HIGH COUNTRY— Cicadas are appearing along the East Coast as Brood X emerges for the first time in the past 17 years. While the cicadas that sing in the trees each year during the summer come annually, there are a variety of cicadas that emerge cyclically, skipping years and some, like Brood X, only returning after nearly two decades.
Dr. Clare Scott-Chialvo, professor of entomology and evolutionary biology, and Dr. Pablo Chialvo, lecturer of biology, both at Appalachian State University, said that Brood X, also known as Brood 10 or the Great Eastern Brood, is just one of 15 periodic cicadas in the United States. They note that in the mountains of western North Carolina, the region has also seen Brood XIV which last emerged in 2008 and will emerge again in 2025, and Brood IX which last emerged in 2020 and will return in 2037.
Chialvo-Scott and Chialvo said that one theory of why these cicadas emerge each 17 years is that it “minimizes the probability of encountering predators upon emergence.” For example, they noted, “If a predator has a life cycle of one, two, six, nine, or 18 years, the adults will overlap at the right time to each of the cicadas,” however, with a 17-year cycle cicadas will encounter fewer predators whose reproductive cycle syncs up with that of the cicadas.
According to Chialvo-Scott and Chialvo, emerging all at once in overwhelming numbers is a strategy known as “predator station,” in which the cicadas maximize how many individuals will survive and reproduce by having a large population. Mayflies, they said, engage in similar behavior.
Dr. Ray Williams, professor of biology and entomologist at Appalachian State University, said that while it is unlikely western North Carolina will see a large emergence of cicadas this year, the Great Southern Brood (Brood XIX) is expected to return to the High Country in 2024. He said the closest emergence to Boone of Brood X cicadas this year is in Wilkes County.
The number of cicadas can be tracked by documenting the “choruses” of the cicadas, according to Williams. A chorus, he explained, is when the males cicadas “sing” by “pulling a kind of flexible membrane on the side of their body back and forth like a drum. It makes a high pitched sound.”
Williams explained that the male cicadas do this to “produce a sound to attract the females. There can be lots and lots of them and it can be quite loud, and that’s when they’re really noticeable.”
It is easy for the public to notice cicadas, Williams said, because they are large for insects and can grow to “maybe a couple inches long.” Once they emerge, cicadas must replace their old exoskeleton with a new, larger exoskeleton to grow into. Williams said this is why “you often see this kind of weird light brown, almost crispy shell, usually attached to a tree or a plant.” While cicadas may look like locusts to many, they are actually distantly related locusts; rather, froghoppers such as spittlebugs are much more closely related to cicadas according to Scott-Chialvo and Chialvo.
Many insects besides cicadas have long periods of their lives in which they are flightless larvae, known as nymphs, and cicadas are no exception. The periodic 17-year cicadas emerging now, according to Scott-Chialvo and Chialvo, spend about 17 years underground as nymphs and feeding on plant fluids they obtain from roots. They said that cicadas have what are known as “piercing-sucking” mouthparts, which “allow them to (as the name suggests) pierce through roots or stems and suck up plant fluids.” Luckily, these mouths are incapable of biting humans and pets, according to Scott-Chialvo and Chialvo. If a large number of individuals feed on a single young tree, the experts noted that the cicadas could do damage. However, more mature trees will not be affected as much and are unlikely to harm gardens.
Scott-Chialvo and Chialvo recommend not using insecticides, but explain, “If you are worried about young trees, you can wrap them in mesh before emergence to prevent feeding damage.”
Another manner in which cicadas interact with young trees, Scott-Chialvo and Chialvo contend, is through laying eggs. Looking at the cicada’s egg-laying structure, its ovipositor, with the naked eye would not reveal anything special. However, by looking through a high-powered microscope, one can find “that the appendage is not only serrated to allow the insect to cut through woody material, but also fortified with a variety of metals, including iron, zinc, copper, and manganese,” Scott-Chialvo and Chialvo said. Using the ovipositor, female cicadas can open up small slits in wood to lay eggs within which, should the small wounds accumulate, can divert energy and resources within younger trees and leave openings for pathogens.
Cicadas are not emerging in the High Country yet, which Williams said is due to chance. “It’s just not their time yet,” Williams added.
According to Scott-Chialvo and Chialvo, however, the simple, “but important” question that arises for many people is how does a cicada track time underground and know when to emerge? They stated that the current theory is that cicadas have a molecular clock that keeps track not of calendar time, but the number of seasonal changes their host plant experiences. Williams said the cicadas grow slowly underground until they get to the size and stage where they can come out of the ground and form an adult.
“They live for a few weeks, which is not typical for insects, so they live for a fairly long time as adults above ground,” Williams said.
Cicadas, the doctors say, are sensitive to the changes plants experience each year, such as leaves falling off and growing back, that signal seasonal change. “That said,” Scott-Chialvo and Chialvo added, “this pattern can be thrown off if the weather is erratic and causes plants to, for instance, produce two sets of leaves in a year.”
Changes in the climate have altered cicadas’ schedules. “If you were to look at emergence dates over the past 50 years or so, you’ll find that cicadas are ‘ahead of schedule,” Chialvo said, adding that this is due to warmer average temperatures earlier in the season, and consequently, higher soil temperatures. Additionally, he said that scientists have also documented more dramatic shifts in emergence patterns. For example, a portion of Brood X, the 17-year cicada that is currently emerging, emerged in the Northeast in 2017 a full four years ahead of schedule.
The warming climate is also going to affect where cicadas can live, according to Chialvo. For Brood X, 64 degrees Fahrenheit at a depth of eight inches is the ideal soil temperature for emergence and reproduction. Areas which normally do not have the appropriate soil temperatures, such as colder areas farther north, will “over time become more amenable to cicada development and growth.” He added the caveat, however, that this assumes these areas “have enough trees to support new populations.”
Chialvo said that because cicadas initially develop in tree branches and spend their juvenile years absorbing nutrients from roots, deforestation has greatly reduced the geographic range in which cicadas can live.
In the regions cicadas emerge, they can provide plentiful, nearing unlimited, food for predators that eat cicadas such as other insects, spiders, snakes, mice and birds among others, according to Scott-Chialvo and Chialvo. Some people, Chialvo included, have eaten cicadas as well, noted as a rich source of protein. Chialvo shared that he has stir-fried the cicadas with vegetables and rice, noting they have an “almost shrimp-like flavor with a great crunch.”
The cicadas that are not eaten will die after a few weeks and release substantial amounts of nutrients back into the environment including carbon, nitrogen, and phosphorus. Impacting both terrestrial and aquatic plant productivity, Scott-Chialvo and Chialvo said that cicadas are “a bit like fertilizer in that regard.”
Residents of western North Carolina can look forward to 2024 for the emergence of the Great Southern Brood in the High Country. Until then, annual cicadas will continue to populate the High Country summers.