Do 17-Year Periodical Cicadas Affect Soil Health and Composition?

Periodical cicadas, particularly those with a 17-year life cycle, are among the most fascinating and enigmatic insects in North America. Emerging en masse every 17 years, these cicadas erupt from underground nymph stages in colossal numbers, producing one of nature’s most impressive biological phenomena. While much attention has been given to their eerie mating calls and tree-biting behaviors, less discussed is their impact on soil health and composition. This article explores how these elusive insects influence the soil ecosystem during their subterranean nymphal phase and subsequent emergence.

Understanding 17-Year Periodical Cicadas

Periodical cicadas belong to the genus Magicicada and are famous for their synchronized mass emergences after spending 13 or 17 years underground. The 17-year cicadas primarily inhabit the eastern United States. Their lifecycle includes:

• Nymph Stage: Nymphs burrow underground immediately after hatching and feed on xylem fluids from tree roots.
• Emergence: After 17 years, nymphs tunnel to the surface simultaneously, molt into adult cicadas, mate, lay eggs, and die within a few weeks.
• Egg & Larval Stage: Eggs hatch into larvae that drop to the soil and burrow underground to start the next cycle.

This prolonged underground existence has significant implications for soil processes over nearly two decades.

Cicada Nymphs as Soil Inhabitants

During their long nymphal stage, periodical cicadas live entirely below the surface. Their interaction with soil can be broken down as follows:

Root Feeding Behavior

Nymphs use specialized mouthparts to tap into tree roots, extracting nutrient-poor xylem sap. While this sap feeding might seem minimal in terms of nutrient extraction, studies suggest it can cause subtle impacts on root physiology:

• Root Stress: Repeated feeding wounds may induce localized stress responses in roots.
• Exudate Changes: Damaged roots might alter exudate composition, which influences microbial communities in the rhizosphere (root zone).

Soil Aeration through Burrowing

To feed on roots at varying depths (typically 1-2 feet below ground), cicada nymphs create extensive tunnels:

• Increased Porosity: Movement aerates compacted soils.
• Improved Water Infiltration: Tunnel networks facilitate water movement through soil layers.
• Enhanced Gas Exchange: Aeration helps oxygenate root zones critical for plant health.

Such physical modifications can improve overall soil structure and promote beneficial microbial activity.

Organic Matter Contribution

Nymphs eventually emerge by digging upward tunnels to the surface; when they die, their bodies decompose underground or on soil surfaces. This contributes organic matter rich in nitrogen:

• Nutrient Input: Their biomass adds to the pool of organic nitrogen and other nutrients.
• Microbial Activity Boost: Decomposition stimulates heterotrophic microbes crucial for nutrient cycling.

Altogether, these factors suggest cicada presence can modestly enrich soil nutrient content during emergence years.

Emergence Effects on Soil Surface

The mass emergence of millions of adult cicadas creates several notable effects on soil’s surface environment:

Surface Disturbance from Emergence Holes

Each nymph creates a vertical tunnel approximately 2-3 cm wide when emerging. Millions of such holes per hectare can disturb the topsoil layer:

• Soil Mixing: Facilitates redistribution of organic material between horizons.
• Seedbed Preparation: Loosened soil may aid seed germination for some plant species.
• Potential Erosion Risk: In regions with heavy rain immediately after emergence, disturbed soils may be prone to erosion.

Nutrient Pulse from Decomposing Bodies

After mating and egg-laying, adults die in large numbers on vegetation and soil surfaces:

• Mass Decomposition Event: Provides a short-term nutrient flush as carcasses break down.
• Soil Fertility Spike: This pulse could boost nitrogen and phosphorus availability temporarily.
• Microbial Population Response: Enhanced resources stimulate decomposer microbes and fungi.

This seasonal input may affect plant growth cycles by providing nutrients when plants resume active growth in spring following cicada emergence.

Impact on Soil Microbial Communities

Soils are complex habitats where microbial diversity drives nutrient cycling and plant productivity. Periodical cicadas influence these communities through multiple pathways:

Changes in Rhizosphere Chemistry

Root damage from nymph feeding alters patterns of root exudates, carbohydrates, amino acids, and other organic compounds released into the soil:

• These changes shape microbial populations by favoring certain bacteria or fungi adapted to utilize altered exudates.

Increased Organic Inputs Supporting Microbes

Decomposing cicada bodies provide labile carbon sources that stimulate microbial growth:

• Studies have shown elevated populations of bacteria involved in nitrogen mineralization following emergence events.

Potential Shifts in Fungal-Bacterial Balance

Physical aeration from burrows combined with organic inputs may shift balance between aerobic fungi and bacteria:

• Improved oxygen levels favor aerobic decomposers.
• Nutrient pulses can encourage different functional groups within microbial communities.

Such fluctuations influence long-term soil fertility and ecosystem resilience.

Long-Term Ecological Implications for Soil Health

While individual emergence events cause short bursts of disturbance and nutrient input, repeated cycles over centuries may lead to more profound ecosystem effects:

Enhanced Nutrient Cycling

Regular pulses of nitrogen-rich organic matter promote sustained fertility in forest soils where periodical cicadas live.

Improved Soil Structure through Bioturbation

Repeated burrowing aerates compacted forest soils supporting root growth and microbial habitats.

Biodiversity Support Through Habitat Modification

Cicada tunnels provide microhabitats for other soil fauna like earthworms and nematodes that further enrich soil health.

Limitations and Areas for Further Research

Though current studies highlight positive contributions of periodical cicadas to soil ecosystems, limitations remain:

• Most research focuses on short-term impact around emergence periods; long-term monitoring is sparse.
• Effects likely vary by soil type, vegetation cover, climate zone, and cicada density.
• Interactions with other soil organisms such as root pathogens or mutualistic fungi have yet to be fully elucidated.

Future investigations using experimental plots comparing soils with/without cicada populations over multiple cycles would provide deeper insights.

Conclusion

The 17-year periodical cicada is more than just an acoustic signal heralding nature’s cycles, it plays an integral role beneath our feet by influencing soil health and composition. Through their prolonged underground nymph stage involving root feeding, extensive tunneling, and eventual mass emergence followed by decomposition, these insects contribute to improved soil aeration, enhanced nutrient cycling, and dynamic microbial ecosystems. While not causing dramatic shifts alone, their cyclical presence adds complexity and resilience to forest soils in North America. Understanding these subterranean engineers better can enrich ecological knowledge and inform forest management practices that value all components of healthy ecosystems, even those hidden deep underground.