In 2024, a 13-year brood emerged in the same year as a 17-year brood.
Cicada population surges, particularly the dramatic emergences we sometimes witness, are fascinating events influenced by several biological and environmental factors. These surges are primarily driven by the unique life cycles of cicadas, particularly periodical cicadas, as well as various ecological and climatic conditions.
The Life Cycles of Cicadas
Cicadas are divided into two main types based on their life cycles: annual cicadas and periodical cicadas.
1. Annual Cicadas
Life Cycle: Annual cicadas emerge every year and typically have a shorter life cycle ranging from two to five years, but their overlapping generations result in some emerging each year.
Population Fluctuations: These cicadas can experience fluctuations in population density due to environmental factors, but their annual appearance tends to be relatively consistent.
2. Periodical Cicadas
Life Cycle: Periodical cicadas are famous for their extended life cycles of 13 or 17 years. These cicadas spend most of their lives underground as nymphs, emerging in massive synchronized broods at the end of their developmental period.
Population Surges: The emergence of periodical cicadas is highly predictable and occurs on a strict cycle. When a brood emerges, millions of cicadas can appear simultaneously, leading to dramatic population surges in a specific region.
Factors Influencing Cicada Emergence
Several factors contribute to the timing and intensity of cicada population surges:
Environmental Triggers
Temperature: Soil temperature is a key trigger for the emergence of cicadas. For many periodical cicadas, the optimal soil temperature for emergence is around 64°F (18°C) at a depth of 8 inches. When this temperature is reached, it signals to the nymphs that it is time to emerge.
Weather Patterns: Mild winters followed by warm, wet springs can lead to early or more intense emergences. Conversely, harsh weather conditions can delay or reduce the emergence.
Population Synchronization
Genetic Synchronization: Periodical cicadas are genetically programmed to emerge simultaneously after 13 or 17 years. This synchronization helps overwhelm predators and increases the chances of successful reproduction, a survival strategy known as predator satiation.
Brood Overlaps: Occasionally, different broods of periodical cicadas may overlap, leading to exceptionally large emergences when more than one brood comes up in the same year.
Predator-Prey Dynamics
Predator Satiation: The sheer number of cicadas emerging at once is a strategy to overwhelm predators. While many cicadas are eaten, enough survive to ensure the continuation of the species. This strategy can result in years of low predator pressure followed by a massive surge in cicada numbers.
Predator Populations: Fluctuations in predator populations can also influence cicada numbers. Lower predator numbers in preceding years can lead to higher survival rates for cicada nymphs, contributing to larger emergences.
Habitat and Land Use Changes
Urban Development: Changes in land use, such as urbanization and deforestation, can impact cicada populations by reducing available habitats or altering soil conditions. These changes can lead to fluctuations in local cicada populations.
Agricultural Practices: Agricultural activities that disturb the soil can affect nymph development and emergence patterns, either delaying or reducing cicada populations in certain areas.
Natural Population Cycles
Boom-and-Bust Cycles: Cicada populations naturally fluctuate in boom-and-bust cycles. After a large emergence, cicada numbers may decline as resources are depleted, only to surge again when conditions become favorable.
Long-Term Cycles: For periodical cicadas, the 13 or 17-year life cycle ensures that significant population surges are a regular, albeit infrequent, occurrence.
The surges in cicada populations are a result of their unique life cycles, environmental conditions, and ecological interactions. Periodical cicadas, with their synchronized mass emergences every 13 or 17 years, create the most dramatic population surges. Understanding these factors helps us appreciate the complexity and predictability of cicada emergences, and prepares us to manage their impacts on our gardens and landscapes.
FAQs About Cicadas and Your Garden
Do cicadas have a purpose?
Cicadas are not dangerous and can provide some environmental benefits including: Cicadas are a valuable food source for birds and other predators. Cicadas can aerate lawns and improve water filtration into the ground. Source: epa.gov
How long do cicadas last?
Cicadas are above ground for only four to six more weeks—just long enough to mate, fertilize or lay eggs, and start the cycle all over again.
Why Do Some Cicadas Appear Only Every 17 Years?
Shortly after a 17-year cicada nymph hatches from its egg, it burrows into the ground, where it spends—as its name suggests—the first 17 years of its life. When it emerges from the ground, it lives only four to six more weeks—just long enough to mate, fertilize or lay eggs, and start the cycle all over again. Source: Brittanica.com
Are cicadas harmful?
Cicadas are largely harmless to humans and animals. They don't bite and they don't sting. They lack the physical structures to hurt people that other insects have: they don't have stingers so they can't sting and their mouth parts are a more like a straw than teeth so they can't bite. Source: Purdue.edu
Are cicadas bad for gardens?
Contrary to popular opinion, adult cicadas do not cause serious plant damage from their feeding activities, but do damage plants as the result of their behavior of cutting small slits in the plant they use for places to deposit their eggs. Source: Orkin.com
How Do Cicadas Impact Shrubs and Trees?
Most mature trees and shrubs can handle cicada eggs, but you may need to prune a few smaller branches if minor damage occurs. Pencil-thin branches of very young trees and shrubs are most at-risk for damage from cicada eggs. If you have planted a tree or shrub in the last three years, we recommend protective netting to prevent cicada damage.
Cicada Nymphs and Plant Root Damage
Cicada nymphs can eat plant roots, but usually don't cause noticeable damage. But plant growth may be reduced if there is an extremely large number of nymphs feeding on the plant's root system.
