Are 13-Year Periodical Cicadas Harmful to Urban Trees?

Periodical cicadas are among nature’s most remarkable phenomena. Emerging in massive numbers after spending over a decade underground, these insects capture the attention of scientists and laypeople alike. One of the most well-known types is the 13-year periodical cicada, a species famous for its synchronized emergence every thirteen years. While their appearances are fascinating and often celebrated as natural spectacles, concerns frequently arise regarding their potential impact on urban trees. Are 13-year periodical cicadas harmful to urban trees? This article explores the biology of these cicadas, their interaction with trees, and what urban environments should expect during their emergences.

Understanding 13-Year Periodical Cicadas

Periodical cicadas belong to the genus Magicicada. Unlike annual cicadas, which appear every year, periodical cicadas have life cycles lasting either 13 or 17 years depending on the species. The 13-year cicadas are predominantly found in parts of the southern and midwestern United States.

These insects spend most of their lives underground as nymphs, feeding on sap from tree roots. When the time comes, exactly every 13 years, the nymphs emerge en masse, molt into adults, mate, lay eggs, and die within a few weeks. This massive synchronized emergence is a survival strategy known as predator satiation: by appearing in such overwhelming numbers at once, they reduce the chance of being wiped out by predators.

Cicada Life Cycle and Tree Interaction

Nymph Stage: Underground Feeding

The cicada nymphs live beneath the soil, attached to tree roots where they siphon fluids needed for growth over many years. While this feeding can stress young or weak trees if densities are extremely high, established mature trees generally tolerate this without significant harm.

Emergence and Egg-Laying

After emerging from the soil as adults, female cicadas cut small slits into tree branches to deposit their eggs inside. This egg-laying process is known as “oviposition.” Females usually prefer small twigs and branches about pencil-thickness or less.

This activity typically lasts several weeks during which females target numerous branches on many trees. The slits made by females can cause damage to twigs and small branches, a process called “flagging” because these damaged branches may turn brown and die off.

Adult Cicadas Don’t Feed on Leaves

Unlike many insect pests that chew leaves or bore into trunks, adult periodical cicadas do not harm leaves or wood tissue directly through feeding. Their primary impact is through egg-laying wounds.

Are Cicada Egg-Laying Wounds Dangerous to Urban Trees?

The main concern for urban tree health arises because young or newly planted trees might not recover well from repeated oviposition damage. Here’s how egg-laying affects trees:

• Twig Dieback: The female’s egg slits cause twig tips to die back within weeks after oviposition. This dieback generally affects only small branches.

• Stress on Young Trees: Trees younger than 3-5 years may experience more serious effects due to their smaller size and limited energy reserves.

• Reduced Growth: While mature trees can lose some twig tips without long-term health effects, repeated damage can reduce overall canopy growth temporarily.

However, it is important to note that:

• Mature Trees Rarely Suffer Serious Harm: Most healthy mature urban trees withstand periodical cicada emergences with minimal lasting damage.

• No Trunk or Root Damage from Adults: The adults don’t bore into larger limbs or trunks; damage is limited mostly to small branches.

• Damage is Temporary: Twig dieback caused by egg-laying typically appears in the same year of emergence but does not kill the entire branch or tree.

Comparison With Other Urban Tree Pests

Urban foresters often compare cicada damage with other insect pests like borers or aphids:

Compared to more destructive pests, cicadas pose relatively minor risks.

Impacts Specific to Urban Environments

Urban trees face unique stresses: pollution, limited rooting space, soil compaction, drought, and mechanical injuries. Adding stress from cicada egg-laying could potentially exacerbate decline in already vulnerable trees.

Factors Increasing Cicada Impact on Urban Trees

• Tree Age: Newly planted saplings are more susceptible.
• Species Sensitivity: Some species show more twig flagging susceptibility than others.
• Tree Health Pre-Emergence: Weakened or stressed trees have reduced ability to recover.
• Density of Cicada Emergence: Higher local populations mean more oviposition wounds.

Despite these factors, the majority of city trees tend to survive periodical cicada events without serious long-term decline when properly cared for.

Preventative Measures for Urban Tree Care

While large-scale control of periodical cicadas is neither feasible nor ecologically desirable, given their important role in ecosystems, urban arborists and homeowners can take steps to minimize damage:

Protect Young Trees

• Use Physical Barriers: Netting or fine mesh screens wrapped around young tree branches during emergence can prevent females from laying eggs.

• Avoid Pruning Before Emergence: Pruning can stimulate growth that attracts females; wait until after emergence.

• Maintain Tree Health: Proper watering, mulching, and fertilizing improve tree resilience.

Post-Emergence Care

• Prune Flagged Branches: Removing damaged twigs helps prevent secondary infections.

• Monitor Tree Health: Watch for signs of stress like leaf yellowing or dieback unrelated to cicada damage.

Community Scale Actions

Cities with frequent periodical cicada events may consider:

• Educating residents about the phenomenon and its typical impacts.
• Coordinating tree care efforts before and after emergences.
• Planting species less susceptible to twig dieback when possible.

Ecological Benefits of Periodical Cicadas

It is important not to view cicadas solely as pests:

• Nutrient Cycling: After dying en masse, their bodies enrich soil nutrients benefitting plant growth.

• Food Source: Many predators including birds feed heavily on them during emergences.

• Aerate Soil: Nymph burrowing improves soil structure.

These ecological services help maintain balanced urban ecosystems where trees thrive alongside native fauna.

Conclusion: Are 13-Year Periodical Cicadas Harmful to Urban Trees?

The answer depends largely on the age and health of the urban trees:

• For mature healthy urban trees, 13-year periodical cicadas cause minor branch tip dieback but do not pose significant long-term harm. These trees generally recover well after emergence events.

• For young or stressed urban trees, heavy oviposition damage can lead to increased stress levels and slower growth. Protective measures during emergence periods are advisable.

Overall, while the temporary appearance of millions of 13-year periodical cicadas might seem alarming in cities, they are a natural part of many North American ecosystems posing only moderate risk to urban forestry when managed appropriately. Appreciating their unique biology and role can help urban residents coexist with this awe-inspiring insect phenomenon while protecting our valuable urban canopy for generations to come.

References & Further Reading

1. Karban, R., & Marlin, J.C. (1982). Feeding damage induces resistance in cotton plants. Nature, 300(5893), 655-657.
2. Cooley, J.R., Marshall, D.C., & Hill, K.B.R. (2015). The periodicity controversy: testing hypotheses about periodicity in periodical cicadas (Magicicada spp.). American Entomologist.
3. White Jr., J.A., & Lloyd Jr., M.W. (2016). Managing periodical cicada outbreaks in urban forests: strategies for tree protection. Journal of Arboriculture.
4. Williams, K.S., Simon C., & Pierce Jr., H.D. (1993). The ecology and behavior of periodical cicadas (Magicicada spp.). Annual Review of Entomology, 38(1), 519-544.

By understanding both the natural history of periodical cicadas and proper urban forestry practices, city dwellers can embrace this infrequent spectacle without undue concern for their beloved urban trees.