Why Do Cicadas Stay Underground for 17 Years?

Bill Swank
First Published: | Updated: February 27, 2024

Cicadas stay underground for 17 years (in the case of some periodical cicadas) as part of their unique evolutionary strategy to avoid predators and synchronize their emergence. This long developmental phase allows them to grow slowly while avoiding detection, and the synchronized mass emergence overwhelms predators, ensuring enough survivors to reproduce. Explore the fascinating evolutionary advantages of this prolonged lifecycle.

  • Cicadas spend most of their life underground as nymphs, with some species staying buried for up to 17 years, to avoid predators and competition for resources.
  • The emergence of cicadas is triggered by soil temperatures reaching approximately 64 degrees Fahrenheit, signaling the mature nymphs to surface for their brief adult stage, which is dedicated to mating and laying eggs.
  • Periodical cicadas, such as the 17-year and 13-year varieties, emerge in massive numbers in synchronized broods, overwhelming predators and ensuring species survival, while annual cicadas emerge every year.
  • Cicadas play a vital ecological role by serving as a food source for predators, aerating the soil, and contributing to nutrient cycling through the decomposition of their bodies, which enriches the soil with nitrogen.
  • Cicada emergences can be predicted using brood maps, which are important for environmental and agricultural planning, with the next major 17-year cicada emergence (Brood X) expected in 2038, and other broods, such as Brood XIII and Brood XIX, set to emerge in 2024.

Why Do Cicadas Stay Underground for 17 Years?

Cicadas are fascinating insects, not least because of their unique and lengthy developmental cycle. Understanding the lifecycle of cicadas is key to appreciating the evolutionary advantages and survival strategies they have developed over millions of years. So, why do cicadas stay underground for 17 years?

The Cicada Lifecycle: Developmental Stages and Evolutionary Advantages

Cicadas undergo a process called incomplete metamorphosis, which includes three stages: egg, nymph, and adult. After hatching from eggs laid in tree branches, the tiny nymphs fall to the ground and burrow into the soil. Here begins their extended stay underground, lasting up to 17 years for some species.

This prolonged nymph stage serves several evolutionary advantages. Firstly, it helps cicadas avoid predators, as their long absence from the surface world makes it difficult for predators to synchronize with their lifecycle. Secondly, the extended period allows them to develop and grow without competing with their own kind for limited resources above ground.

Survival Strategies While Living Underground

While underground, cicadas rely on xylem, the sap from tree roots, for nourishment. This diet is low in nutrients, which partly explains their slow growth rate. The nymphs live in a state of relative stasis, molting several times as they grow larger. Their survival is a testament to their ability to efficiently utilize the sparse resources available to them.

What Triggers Their Emergence?

The trigger for cicada emergence is primarily temperature. When the soil about eight inches deep reaches approximately 64 degrees Fahrenheit, the mature nymphs know it’s time to emerge. This temperature typically coincides with the late spring and early summer months. The long developmental phase ensures that when they do emerge, they do so en masse, which overwhelms predators and increases the chances of successful mating.

How Long Do Cicadas Live Underground vs. Above Ground?

The contrast between the life of a cicada underground and above ground is stark. While the nymph stage can last for an astonishing 17 years, the adult stage is fleeting.

Underground: The Nymph Stage

Underground, cicadas spend most of their life in the nymph stage. During this time, they go through five instar stages, molting and growing larger with each phase. Their behavior is characterized by tunneling and feeding on root sap, activities that support their growth and development.

Above Ground: The Adult Stage

Once they emerge, cicadas transition to the adult stage, which lasts a mere 4 to 6 weeks. During this time, their primary tasks are to mate and lay eggs for the next generation. Adult cicadas are known for their loud mating calls, which can be heard in chorus during the peak of their emergence.

Are There Cicadas Every Year?

While it may seem that cicadas come out only every several years, some areas do experience cicadas annually.

Annual vs. Periodical Cicadas

Annual cicadas, or “dog-day cicadas,” emerge every year in late summer. They have a shorter lifecycle than periodical cicadas and are not synchronized across large populations. In contrast, periodical cicadas emerge after 13 or 17 years in synchronized broods.

Regional Experiences with Cicadas

Some regions may see cicadas each year if they have overlapping broods of annual and periodical cicadas. However, areas with only periodical cicadas will experience the phenomenon less frequently.

Ecological Roles and Adaptation Strategies

Both annual and periodical cicadas play vital roles in their ecosystems. They provide a food source for various predators and contribute to the aeration of the soil during their nymph stage. Their different adaptation strategies, whether emerging annually or periodically, showcase the diversity of survival tactics in the natural world.

What Impact Do Emergent Cicadas Have on the Environment?

The mass emergence of cicadas is not just a noisy curiosity; it has significant ecological implications. These insects play a crucial role in their ecosystems, and their periodic arrival above ground influences various natural processes.

Ecological Significance of Mass Emergence

When cicadas emerge in large numbers, they become a substantial food source for a variety of predators, including birds, mammals, and other insects. This abundance of prey can lead to a temporary increase in predator populations. Additionally, the emergence of cicadas aerates the soil as they tunnel to the surface, which can benefit plant growth.

Effects on Local Ecosystems and Benefits to Other Species

The sudden influx of cicadas also provides nutrients to the soil when they die. Their decomposing bodies release nitrogen, an essential nutrient for plant growth, thereby enhancing the fertility of the environment. This nutrient cycling is a pivotal contribution to the health of forests and other habitats where cicadas are found.

Shaping the Environment: Natural Processes Influenced by Cicada Emergence

Cicadas also influence the environment through their selective feeding on certain tree species. By laying eggs in the branches of these trees, they can affect the composition of the forest. The small twigs damaged by egg-laying may die off, a process known as “flagging,” which can alter the canopy structure and potentially influence the forest dynamics.

When Will Cicadas Come Back?

Predicting the next emergence of cicadas is important for environmental and community preparedness. Various methods are used to forecast when and where cicadas will appear.

Prediction Methods: Cicada Brood Maps

Cicada brood maps are a crucial tool for scientists and the public alike. These maps chart the geographic distribution of different cicada broods and their expected years of emergence. By monitoring cicada populations and their developmental stages, entomologists can predict when a particular brood is due to surface.

Importance of Tracking Cicada Patterns

Understanding cicada patterns helps in several ways. It can inform agricultural practices, as farmers may need to protect young trees from egg-laying females. It also aids in biodiversity monitoring, as cicada emergences can impact various species in the food chain.

Insights into Next Expected Emergence Times

As of the most recent data, the next major 17-year cicada emergence, Brood X, actually occurred in the spring of 2021. The next expected emergence of this brood will be in 2038. However, there are several other broods that will surface before then. In 2024, for instance, we can expect to see:

  • Brood X (17-year cicadas): Having last emerged in 2021, Brood X will next appear in 2038. This brood is one of the most densely populated and widely distributed of the 17-year cicadas.
  • Brood XIII (17-year cicadas): This brood is anticipated to emerge in 2024, and it will be found in parts of the Midwest, including Illinois, Indiana, and Iowa.
  • Brood XIX (13-year cicadas): Also set to emerge in 2024, Brood XIX will appear in the southeastern United States, affecting states like Alabama, Arkansas, Georgia, and Missouri.

It’s important to note that while the 17-year and 13-year periodical cicadas have synchronized emergences in specific years, annual cicadas, as their name suggests, emerge every year. These annual cicadas are more commonly seen and heard during the late summer and are not part of the synchronized broods.

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