# Rainfall Timing as a Key Driver of Cicada Peak Emergence in Urban Habitats

## TL;DR

Cicadas in urban areas emerge about two weeks after monsoon rains, showing that rainfall timing is a key trigger for their synchronized emergence.

## Contribution

The study identifies monsoon rainfall timing as the primary cue for cicada emergence in urban habitats.

## Key findings

- Peak cicada emergence consistently occurred about two weeks after monsoon rainfall.
- Poisson GEE analyses confirmed a significant 2–3-week lag between precipitation and emergence counts.
- Emergence synchrony varied by species and habitat, but the rainfall–emergence relationship remained robust.

## Abstract

Cicadas time their mass summer emergence with remarkable precision, but the environmental trigger for peak timing remains unclear. By monitoring three cicada species across two urban parks with contrasting habitat types in Seoul, South Korea, we found that emergence peaks consistently occurred about two weeks after monsoon rainfall. This pattern was consistent across species, although the degree of emergence synchrony varied between park types. Our results indicate that precipitation timing is the primary cue for peak emergence. As monsoon rainfall becomes increasingly unpredictable under climate change, these findings provide important insight into how urban cicada populations may respond to future shifts in precipitation regimes.

Synchronous emergence is a widespread adaptive strategy in cicadas, yet the proximate cues governing its timing in urban environments remain poorly understood. We examined the emergence phenology of three common urban cicada species (Cryptotympana atrata, Hyalessa maculaticollis, Graptopsaltria nigrofuscata) across two urban parks with contrasting habitat structure (a closed-canopy urban forest park vs. an open urban park) in Seoul, South Korea, over three summers (2015–2017). Despite interannual variation in rainfall amount and timing, peak emergence consistently occurred about two weeks after the monsoon rainfall peak. Poisson generalized estimating equation (GEE) analyses confirmed that antecedent precipitation at a 2–3-week lag significantly increased emergence counts across all three species, while precipitation one week prior had no significant effect. Emergence synchrony varied among species and habitat conditions, but the rainfall–emergence lag relationship was robust across years and sites. These findings demonstrate that precipitation timing is a key driver of peak cicada emergence in urban habitats. As East Asia experiences increasingly variable monsoon rainfall under climate change, understanding precipitation-based phenological cues will be essential for predicting the dynamics of urban insect populations.