In a rare phenomenon, billions, possibly trillions, of periodical cicadas are set to emerge in more than a dozen US states this spring, including populous areas like Chicago, Nashville, and St. Louis. Naturalists have already spotted the first arrivals, marking the simultaneous appearance of two particular periodical cicada broods, which hasn't happened since 1803.

The Northern Illinois brood, known as Brood XIII, spends 17 years underground before emerging, while the Great Southern Brood, or Brood XIX, lives underground for 13 years. This year, the two broods will overlap, but they won't coincide again until 2245. This unusual convergence means that the insects will infiltrate a much larger geographical area than in most years.

Experts have offered guidance on how to prepare for the upcoming cicada season. The appearance of small holes resembling tiny chimneys near tree roots is a signal that the periodical cicadas are about to emerge from their underground lair. When the soil temperature reaches around 64 degrees Fahrenheit (17.8 degrees Celsius), the cicada nymphs begin to burst forth.

Upon surfacing, the cicadas make their way to vertical surfaces, usually tree trunks or fences, where they shed their outer skeletons and prepare to spend four to six weeks in a frenzy of eating, mating, and egg laying. The most noticeable sign of their arrival is the droning buzz created by the male cicadas' chorus, which can be as loud as a lawn mower.

Female cicadas use an egg-laying organ called an ovipositor to lay their eggs inside slits they cut into tree branches. While they may cause minor damage to very young saplings, cicadas generally do not harm plants, trees, or flower beds. Protective netting can be used to shield small saplings from potential damage.

Cicadas serve as an abundant food source for various animals, including pets. Although they may cause some digestive discomfort if consumed in large quantities, cicadas are not inherently harmful when eaten by humans or animals. John Lill, a professor of biology at the George Washington University, suggests replacing fear with fascination and embracing the opportunity to witness a natural phenomenon that has delighted people for centuries.

To further contribute to research and the understanding of these insects, individuals are encouraged to download community science apps like the Cicada Safari app and document their sightings through photographs. Cicadas play a valuable role in the natural environment by loosening and aerating the soil as they tunnel, and their decomposition adds nutrients to the soil after they die.

While more than 3,000 species of cicadas exist worldwide, only nine are periodical, with seven of them confined to the eastern United States. The reason behind why periodical cicadas evolved to emerge every 13 or 17 years remains unclear, given their unusually long lifespan for an insect.

For individuals who are not fans of these buzzing creatures, experts suggest staying indoors or planning a vacation during the peak of the emergence. "If you really are a cicadaphobe, I would plan a trip away," advises Paula Shrewsbury, a professor in the department of entomology at the University of Maryland.

In either case, whether one views them as a spectacular wonder of nature or an anxiety-inducing nuisance, the dual emergence of periodical cicadas promises to be a remarkable event, offering a unique chance to witness and study the glorious and mysterious cycles of the natural world.