Why cicadas spend years underground

Introduction

Cicadas are famous for their long lives spent below ground: certain species emerge only every 13 or 17 years. But why do these insects spend so much of their life cycle underground? This article explains the biology, ecology, and evolutionary reasons behind cicadas’ prolonged subterranean stages in clear, non-technical language.

What does “spending years underground” mean?

Cicadas are a group of insects in the order Hemiptera. Most cicadas have a life cycle that includes an egg, nymph (juvenile), and adult stage. When people say cicadas “spend years underground,” they are referring to the nymph stage. After hatching from eggs laid in tree twigs or branches, the tiny nymphs drop to the ground, burrow into the soil, and remain there feeding and growing for many years before emerging as adults.

There are two broad life-patterns among cicadas. Annual cicadas (which can still take several years to develop individually) have overlapping generations and some emerge every year in a region. Periodical cicadas, found mainly in eastern North America, synchronize their development so that enormous broods emerge at fixed intervals — typically every 13 or 17 years.

How do cicada nymphs live underground?

Under the soil, cicada nymphs live in tiny vertical burrows. They do not feed on soil or plant roots in a destructive way. Instead, they use specialized mouthparts called stylets to pierce the roots of trees and shrubs and sip xylem sap — the water-and-minerals stream that moves from roots to leaves.

Xylem sap is low in nutrients compared with other plant fluids, so cicadas feed continuously and slowly. Their bodies and digestive systems are adapted to extract sufficient water and trace nutrients to support slow growth over many years.

While underground, nymphs periodically molt, shedding their exoskeletons as they grow. Each molt transforms the nymph closer to adult form until, at the final molt, the nymph digs to the surface, climbs a vertical surface such as a tree or stem, and emerges as a winged adult in a process called ecdysis.

Main reasons cicadas spend years underground

Several interacting ecological and evolutionary reasons explain why cicadas remain subterranean for many years. The key explanations are resource specialization, predator avoidance through timing and numbers, and life-history trade-offs that favor long juvenile development.

1. Resource-driven slow growth

As xylem feeders, cicada nymphs consume a food source that is abundant but nutrient-poor. Extracting enough nutrients from dilute xylem sap requires continuous feeding over extended periods. This physiological constraint leads to slow, steady growth rather than rapid development.

Because tree roots provide a relatively stable food source year-round (unlike seasonal above-ground food supplies), living underground allows cicadas to exploit that resource reliably across many seasons and climatic variations.

2. Predator avoidance through long juvenile stages and mass emergence

Spending many years underground reduces the chance that any one individual will be eaten during its vulnerable juvenile phase by predators that hunt above ground. For periodical cicadas, long and synchronized life cycles serve a second, crucial function: predator satiation.

Predator satiation is an ecological strategy where a species produces an extremely large number of individuals in a short time so predators cannot consume them all. When millions of cicadas emerge simultaneously after 13 or 17 years, local birds, mammals, and insects may eat large numbers, but most cicadas survive simply because predators become overwhelmed. Between mass emergences, predator populations that might specialize on cicadas decline because their periodic food supply is absent.

3. Avoiding synchronization with predator cycles

Periodical cicadas’ prime-numbered cycles (13 and 17 years) are thought to help avoid synchronization with the population cycles of predators and parasites. A predator species with a population cycle of, say, 2, 3, 4, or 6 years is less likely to consistently coincide with a 13- or 17-year emergence. Using prime numbers reduces the chance of regular overlap with common periodic predators’ peaks, lowering the risk that predators evolve to rely on cicada emergences.

4. Environmental stability and buffering

Underground life buffers cicadas from surface weather extremes, such as storms, droughts, and sudden temperature swings. The soil environment changes more gradually, which may be advantageous for slow-growing nymphs and reduces the likelihood that a single bad season will wipe out an entire cohort.

5. Evolutionary trade-offs

Long juvenile stages represent a trade-off. Slow growth delays reproduction, which could be disadvantageous if mortality rates were high. But the overall evolutionary success of cicadas shows that these trade-offs have paid off: the strategy minimizes per-year exposure to surface mortality while concentrating reproduction into events with high offspring survival because of predator satiation.

Why are some cicadas annual and others periodical?

The distinction between annual and periodical cicadas arises from differences in species biology and evolutionary history rather than a simple “choice.”

Annual cicadas (often called “dog-day cicadas”) have staggered, overlapping generations: while an individual may spend several years underground, different cohorts emerge each year so adults can be seen annually. These species typically have shorter, less synchronized developmental schedules and do not form massive synchronized broods.

Periodical cicadas evolved highly synchronized, long cycles in specific regions. These synchronized cycles likely emerged because of local environmental conditions, predator pressures, and historical population dynamics that made synchronized emergence and long juvenile periods advantageous. Periodical cicadas are mostly restricted to parts of North America, where their unique life cycles have been well studied.

Mechanisms that time emergence

How do nymphs “know” when to emerge after many years? Research indicates cicadas use environmental cues and internal developmental clocks.

Temperature is a major cue. For periodical cicadas, a common trigger is ground temperature reaching a threshold in late spring — often around 64°F (18°C) at about 8 inches (20 cm) deep. Once soil warms enough for several consecutive days, mature nymphs begin to tunnel upward.

In addition, cicadas appear to use an internal, genetically regulated developmental program to count years. That program interacts with environmental signals so that nymphs develop slowly and complete the final maturation only when both internal age and favorable external conditions align. The details of how the internal “count” works are a subject of ongoing scientific research, and likely involve hormonal changes linked to molt cycles and environmental temperature patterns.

Examples and notable cases

Periodical cicadas (genus Magicicada) of eastern North America are the most famous example. They form large broods that emerge synchronously every 13 or 17 years. Each brood occupies a geographic area and appears like clockwork; different broods have different years of emergence.

Outside North America, many cicada species have multi-year underground stages but do not synchronize into massive broods. For example, some Australian and Asian cicadas develop over several years but have overlapping generations so adults can be heard regularly in summer months.

Ecological consequences of long underground stages

When cicadas emerge en masse, they influence ecosystems in several ways. Their bodies provide a sudden, large nutrient input to the soil as many die shortly after mating, which can boost local decomposition and plant growth. Eggs laid in twigs can cause some twig dieback, called flagging, which may prune trees lightly and open canopy gaps beneficial to some plants.

Predators benefit from the temporary increase in food supply. Bird and mammal populations may experience short-term feeding booms, although these rarely produce long-term population increases because the cicada pulse is infrequent. Periodic emergences may also affect insect communities through increased parasitism or competition for resources.

Common misconceptions

Misconception: Cicadas are harmful to humans or crops. In general, cicadas are not harmful to people and rarely cause lasting damage to mature trees. Young trees or shrubs can suffer from egg insertions or heavy feeding, but widespread economic damage is uncommon.

Misconception: All cicadas have 13- or 17-year cycles. Only certain North American species are truly periodical with these prime-number intervals. Many other cicada species have shorter or variable underground durations.

Misconception: Cicadas spend their lives entirely underground. Adults spend only a few weeks above ground to mate and lay eggs. The long subterranean nymphal period is only one phase of their life cycle.

What we still don’t fully understand

Scientists have made great progress, but questions remain. The precise genetic and hormonal mechanisms that set and maintain 13- or 17-year developmental cycles are still not fully resolved. Similarly, the full set of selective pressures that produced prime-numbered intervals, while strongly supported by ecological reasoning, continues to be explored through modeling and field studies.

Researchers also study how climate change and habitat alteration might affect cicada timing. Shifts in temperature patterns could alter emergence cues or the survival of different life stages, with unpredictable effects on synchronization and ecosystem impacts.

Practical implications for people

For most people, cicadas are a seasonal curiosity — loud but harmless. Gardeners and land managers might protect very young trees during an expected emergence year by using physical barriers or netting for a short period, but major interventions are rarely necessary or recommended.

From an educational perspective, cicadas provide a vivid example of life-history strategies: how organisms balance growth, survival, and reproduction over time to maximize evolutionary success in a given environment.

Conclusion

Cicadas spend years underground primarily because their immature stages feed on nutrient-poor xylem sap, which requires slow growth, and because long juvenile periods combined with synchronized mass emergence reduce the risks posed by predators. Environmental stability underground, evolutionary trade-offs, and timing cues like soil temperature all contribute. The result is a fascinating life-history strategy that shapes both cicada biology and the ecosystems they briefly but spectacularly enliven when they emerge.