Why Do Cicadas Stay Underground For 17 Years

Cicadas, those buzzing insects that emerge in droves during the summer, have fascinated scientists and nature enthusiasts for centuries, particularly due to their unique life cycle. One of the most intriguing aspects of cicadas is the 17-year periodical cicadas, which spend the vast majority of their lives underground. This extended subterranean existence raises a fundamental question: why do cicadas stay underground for 17 years? The answer involves a complex interplay of evolutionary strategies, predator avoidance, and the exploitation of environmental factors.

Understanding Cicadas and Their Life Cycle

To fully appreciate the significance of the 17-year dormancy, it's essential to understand the basics of cicada biology and their life cycle. Cicadas are insects belonging to the order Hemiptera, characterized by their piercing-sucking mouthparts, which they use to feed on plant sap. There are over 3,000 species of cicadas worldwide, divided into two main categories: annual cicadas and periodical cicadas.

Annual cicadas, also known as dog-day cicadas, emerge every year. Their life cycle typically spans two to five years, with different broods emerging annually, giving the impression that they are always present. Periodical cicadas, on the other hand, are a group of seven species found only in North America, notable for their synchronized mass emergences at specific intervals of 13 or 17 years.

The life cycle of periodical cicadas begins when adult females lay their eggs in the twigs of trees. After six to ten weeks, the eggs hatch, and tiny nymphs drop to the ground, burrowing into the soil. These nymphs then begin their long subterranean journey, feeding on xylem fluid from plant roots. Throughout their time underground, the nymphs undergo several molting stages, growing larger with each molt.

After 17 years, the mature nymphs emerge from the ground in massive numbers. This synchronized emergence is a spectacle of nature, with millions of cicadas appearing in a relatively short period. The adults mate, lay eggs, and die within a few weeks, completing the cycle. The specific triggers that prompt the nymphs to emerge after 17 years are still not fully understood, but environmental cues such as soil temperature and root nutritional changes are believed to play a role.

The Evolutionary Advantages of a Long Dormancy

The primary reason for the 17-year underground dormancy is believed to be an evolutionary strategy to avoid predators. This is a multifaceted approach that involves several key advantages:

• Predator Avoidance Through Prime-Numbered Cycles: The 17-year cycle is a prime number, meaning it is only divisible by 1 and itself. This is significant because it minimizes the chances of predators synchronizing their life cycles with that of the cicadas. If cicadas emerged every, say, 12 years, any predator with a 2, 3, 4, 6, or 12-year life cycle could easily adapt to prey on them. A prime-numbered cycle ensures that any potential predator would have a much harder time aligning its reproductive cycle with the cicadas' emergence.
• Swamping Effect: Emerging in massive numbers overwhelms predators. This strategy, known as predator satiation, ensures that even if predators consume a large number of cicadas, enough will survive to reproduce and continue the cycle. The sheer volume of cicadas appearing at once makes it impossible for any predator population to consume them all.
• Reduced Competition: By remaining underground for an extended period, the cicadas avoid direct competition with other insect species for resources. The nymphs feed on tree roots, a resource that is generally abundant and not heavily contested. This allows them to grow and develop without facing the intense competition that might occur above ground.
• Stable Environment: The underground environment offers a relatively stable temperature and humidity, protecting the cicadas from the harsh fluctuations of the surface world. This stability is crucial for their long-term survival, as it reduces the stress and energy expenditure associated with adapting to changing conditions.

Scientific Explanations and Theories

Scientists have proposed several theories to explain the evolutionary basis of the 17-year cycle, focusing on predator avoidance, resource exploitation, and genetic factors.

Predator Avoidance Hypothesis

The predator avoidance hypothesis is the most widely accepted explanation for the 17-year cycle. As mentioned earlier, the prime-numbered cycle and the swamping effect are key components of this strategy. Empirical evidence supports this hypothesis, as studies have shown that cicada populations with shorter life cycles tend to suffer higher predation rates.

Research has also indicated that the emergence of periodical cicadas can have a significant impact on predator populations. For example, bird populations may experience a temporary boom during cicada emergences, but this is often followed by a decline as the cicada food source disappears. This boom-and-bust cycle makes it difficult for predators to adapt specifically to cicada emergences.

Resource Exploitation Hypothesis

Another theory suggests that the 17-year cycle allows cicadas to exploit resources more effectively. By spending a long time underground, the nymphs can tap into deep root systems, accessing nutrients that may not be available to other insects. Additionally, the synchronized emergence of adults may facilitate more efficient mating and egg-laying, maximizing reproductive success.

However, this hypothesis is not as well-supported as the predator avoidance theory. While it is true that cicada nymphs feed on tree roots, there is no clear evidence that a 17-year cycle provides a significant advantage in resource acquisition compared to shorter life cycles.

Genetic and Evolutionary Factors

The genetic basis of the 17-year cycle is still not fully understood, but research suggests that it is controlled by a complex interaction of multiple genes. Studies have shown that different broods of periodical cicadas are genetically distinct, indicating that they have evolved independently over time.

One theory proposes that the 17-year cycle may have evolved gradually through natural selection. Initially, cicadas may have had shorter life cycles, but those with slightly longer cycles experienced lower predation rates and higher survival rates. Over time, this led to the evolution of the 17-year cycle as the optimal strategy for survival.

Mathematical Models and Simulations

Mathematical models and computer simulations have been used to test the validity of these hypotheses. These models have shown that prime-numbered cycles and synchronized emergences can indeed provide a significant advantage in terms of predator avoidance. They have also helped to identify the conditions under which these strategies are most effective.

For example, simulations have shown that the effectiveness of the prime-numbered cycle depends on the diversity of predator life cycles in the environment. In environments with a wide range of predator life cycles, the prime-numbered cycle is more likely to be advantageous.

The Impact of Environmental Changes

While the 17-year cycle has proven to be a successful survival strategy for cicadas, it is not without its challenges. Environmental changes, such as habitat destruction, climate change, and the introduction of invasive species, can all have a significant impact on cicada populations.

• Habitat Destruction: The destruction of forests and other natural habitats reduces the availability of suitable breeding grounds for cicadas. This can lead to smaller population sizes and increased vulnerability to predators.
• Climate Change: Changes in temperature and precipitation patterns can disrupt the timing of cicada emergences. Warmer temperatures may cause cicadas to emerge earlier than usual, while changes in rainfall can affect the availability of food resources.
• Invasive Species: The introduction of invasive species, such as the Asian long-horned beetle, can pose a threat to cicadas by damaging their host trees. This can reduce the availability of food and shelter for cicada nymphs.
• Pesticide Use: The use of pesticides in agriculture and landscaping can directly harm cicadas, reducing their survival rates. Pesticides can also indirectly affect cicadas by reducing the availability of their food sources.

Brood Emergence and Regional Variations

Periodical cicadas are divided into different broods, each emerging in a different year and region. There are 12 broods of 17-year cicadas and 3 broods of 13-year cicadas. The broods are designated by Roman numerals, with Brood I emerging in 1893 and each subsequent brood following in numerical order.

The geographic distribution of the broods varies widely. Some broods are concentrated in specific regions, while others are more widespread. For example, Brood X, one of the largest and most well-known broods, emerges in the eastern United States, covering states such as Ohio, Pennsylvania, and Maryland.

The synchronized emergence of different broods can lead to fascinating regional variations in cicada activity. In some years, multiple broods may emerge in the same region, resulting in an even greater abundance of cicadas.

The Cultural Significance of Cicadas

Cicadas have been a part of human culture for centuries, appearing in literature, art, and folklore. In many cultures, cicadas are seen as symbols of rebirth, transformation, and immortality. Their long underground dormancy and dramatic emergence have captured the imagination of people around the world.

In ancient Greece, cicadas were associated with music and poetry. The philosopher Aristotle wrote about cicadas in his "History of Animals," noting their unique life cycle and their ability to produce sound. In Japan, cicadas are a common motif in art and literature, often symbolizing the fleeting nature of life.

In modern times, cicadas continue to fascinate and inspire. Their synchronized emergences attract tourists and researchers from around the world, providing opportunities for scientific study and public education.

The Future of Cicada Research

Despite the extensive research that has been conducted on cicadas, many questions remain unanswered. Scientists are still working to understand the genetic basis of the 17-year cycle, the specific triggers that prompt cicadas to emerge, and the long-term impacts of environmental changes on cicada populations.

Future research will likely focus on:

• Genomics: Advanced genomic techniques will be used to identify the genes that control the 17-year cycle and to understand how these genes have evolved over time.
• Environmental Monitoring: Continuous monitoring of cicada populations and their habitats will help to track the impacts of climate change, habitat destruction, and invasive species.
• Behavioral Studies: Detailed behavioral studies will be conducted to understand how cicadas interact with their environment and how they respond to different stimuli.
• Mathematical Modeling: Sophisticated mathematical models will be developed to simulate the dynamics of cicada populations and to predict how they will respond to future environmental changes.

Practical Advice for Dealing with Cicada Emergences

While cicadas are generally harmless to humans, their massive emergences can be disruptive. Here are some practical tips for dealing with cicada emergences:

• Protect Young Trees: Cicadas lay their eggs in the twigs of trees, which can damage young trees. Protect young trees by covering them with netting during cicada emergences.
• Avoid Pesticides: Pesticides can harm cicadas and other beneficial insects. Avoid using pesticides during cicada emergences.
• Enjoy the Spectacle: Cicada emergences are a unique and fascinating natural phenomenon. Take the time to observe and appreciate these amazing creatures.
• Clean Up Debris: After cicada emergences, there may be a lot of dead cicadas and shed skins. Clean up this debris to prevent unpleasant odors and to reduce the risk of attracting pests.

Conclusion

The 17-year dormancy of periodical cicadas is a remarkable evolutionary adaptation that has allowed these insects to thrive for millions of years. The prime-numbered cycle, the swamping effect, and the stable underground environment all contribute to their survival. While environmental changes pose a threat to cicada populations, ongoing research and conservation efforts can help to ensure that these fascinating creatures continue to grace our summers for generations to come. The mystery surrounding their extended subterranean life continues to captivate scientists and nature enthusiasts alike, highlighting the intricate and complex strategies that life on Earth employs to endure. Understanding the "why" behind the cicadas' 17-year underground stay provides valuable insights into evolutionary biology, ecological interactions, and the delicate balance of nature.