Cicadas, known for their distinctive buzzing songs, are insects that frequently spark curiosity regarding their periodic appearances and eventual disappearance. Understanding when these insects vanish depends on distinguishing between their different types and life cycles.
Understanding Annual and Periodical Cicadas
Cicadas fall into two main categories: annual and periodical. Annual cicadas, often called dog-day cicadas, are observed every summer, typically from late spring through late summer. Their individual life cycles range from two to five years, but different broods emerge each year, creating the perception of a yearly appearance. These cicadas are generally larger and greener than their periodical counterparts.
Periodical cicadas are known for highly synchronized, mass emergences every 13 or 17 years. These insects (Magicicada) spend most of their lives underground. When they emerge, it is often in enormous numbers, covering specific regions. For periodical cicadas, their departure is a pronounced and widespread event, marking the end of their brief adult phase.
The Cicada Life Cycle and Their Departure
The departure of cicadas is tied to the completion of their life cycle. A cicada’s life begins when eggs are laid in the branches of woody plants. After six to ten weeks, tiny nymphs hatch, drop to the ground, and burrow into the soil. Underground, nymphs spend years feeding on sap (xylem fluid) from tree roots, undergoing developmental stages.
When mature, typically after 2-5 years for annual species or 13-17 years for periodical species, nymphs tunnel to the surface and emerge when the soil temperature at a depth of 12-18 inches reaches approximately 64 degrees Fahrenheit. Upon emergence, they climb onto vertical surfaces, shed their nymphal skins, and transform into winged adults. The adult cicada’s primary purpose is reproduction; males sing to attract females, and after mating, females lay their eggs. The adult lifespan above ground is relatively short, usually lasting only two to six weeks. Once reproduction is complete, the adult cicadas die, rather than migrating or hibernating.
Signs of Their Disappearance and What Follows
Observable indicators signal that cicada activity is winding down. The most noticeable sign is a significant decrease in the buzzing or singing, which is the males’ mating call. Fewer adult cicadas will be seen flying, and dead cicada bodies will accumulate on the ground. While empty exoskeletons are present throughout their adult phase, their widespread accumulation on tree trunks and other surfaces also signals the nearing end of the adult population.
After the adult cicadas die, their bodies contribute to the ecosystem in several ways. They return nutrients like nitrogen and potassium to the soil as they decompose, acting as natural fertilizer. The emergence tunnels created by the nymphs also help aerate the soil, improving water infiltration. The landscape returns to its usual sounds, and visible signs of cicada presence diminish as the next generation of nymphs develops underground, preparing for a future emergence.