Not all cicadas increase thermal tolerance in response to a temperature gradient in metropolitan Seoul

Hoa Quynh Nguyen,Yoonhyuk Bae,Seongmin Ji,Hortense Serret,Soyeon Chae,Ye Inn Kim,Jeongjoo Ha,Yikweon Jang

doi:10.1038/s41598-020-58276-0

Abstract

Rapid anthropogenic alterations caused by urbanization are increasing temperatures in urban cores, a phenomenon known as the urban heat island (UHI) effect. Two cicada species, Cryptotympana atrata and Hyalessa fuscata (Hemiptera: Cicadidae), are abundant in metropolitan Seoul where their population densities correlate strongly with UHI intensities. Such a positive correlation between cicada density and UHI intensity may be possible if cicada abundance is linked to a certain thermal tolerance. We tested this hypothesis by investigating variation in morphology and thermal responses of two cicada species along a thermal gradient in Seoul and surrounding areas. The morphological responses were measured by metrics such as length, thorax width and depth, and mass. The thermal responses were measured in terms of minimum flight temperature, maximum voluntary temperature and heat torpor temperature. First, we observed a species-specific variation in thermal responses, in which C. atrata displayed a higher thermal threshold for maximum voluntary and heat torpor temperatures than H. fuscata. Second, a positive association between temperature conditions and body sizes were displayed in females H. fuscata, but not in either conspecific males or C. atrata individuals. Third, C. atrata exhibited similar thermal responses regardless of habitat temperature, while H. fuscata in warmer areas showed an increase in heat tolerance. In addition, H. fuscata individuals with bigger thorax sizes were more heat-tolerant than those with smaller thoraxes. Overall, our research is the first to detect a variation in thermal responses and body size of H. fuscata individuals at a local scale. More investigations would be needed to better understand the adaptation mechanisms of insects linked to UHI effects.

Highlights

Rapid anthropogenic alterations caused by urbanization are increasing temperatures in urban cores, a phenomenon known as the urban heat island (UHI) effect
Thermal responses measured at the pronotum (n = 158) showed that C. atrata became fully coordinated at an Minimum flight temperature (MFT) of 24.81 ± 1.72 °C, reaching the thermoregulation level at an maximum voluntary temperature (MVT) of 37.58 ± 2.01 °C, and becoming heat torpid at an heat torpor temperature (HTT) of 46.92 ± 2.71 °C
Body temperatures measured inside the mesonotum (n = 114) indicated that MFT was achieved at 23.88 ± 1.80 °C, MVT at 38.13 ± 1.99 °C, and HTT at 49.56 ± 1.87 °C

Summary

Introduction

Rapid anthropogenic alterations caused by urbanization are increasing temperatures in urban cores, a phenomenon known as the urban heat island (UHI) effect. Cryptotympana atrata and Hyalessa fuscata (Hemiptera: Cicadidae), are abundant in metropolitan Seoul where their population densities correlate strongly with UHI intensities. Growth rates of cicada nymphs demonstrate a positive association with climate: those inhabiting warmer regions are more www.nature.com/scientificreports likely to grow faster than counterparts in cooler regions[21] This may facilitate the development of larger body size in cicadas in the former group[22], which in turn directly promotes greater fecundity of females[23] and indirectly contributes to higher mating success in males[24]. Provided that the prevalence of C. atrata and H. fuscata populations in urban area is due to greater thermal tolerance to urban conditions, their thermal responses can be expected to be associated with the localized clines of habitat temperatures intensities

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