Comparison of cold tolerance in eggs of two cicadas, Cryptotympana facialis and Graptopsaltria nigrofuscata, in relation to climate warming

Abstract

The population of the cicada Cryptotympana facialis began to increase in Osaka, Japan, during the late 20th century. Climate warming is considered a major cause, although the relationship between temperature and the cicada population increase remains unclear. By examining cold tolerance in overwintering eggs of C. facialis in relation to another cicada, Graptopsaltria nigrofuscata, whose population has recently decreased in Osaka, we tested the hypothesis that warming has caused the population increase of C. facialis by decreasing egg mortality due to winter temperatures. A short-term (24 h) cold exposure experiment demonstrated that the half-lethal temperatures (LT50) of C. facialis and G. nigrofuscata were -23.3°C and -28.9°C, respectively, although these extreme low temperatures never occurred in Osaka during the 20th century. Prolonged exposure to -5°C for up to 30 days had no harmful effects on the hatching rate in either species. Overwintering mortality was also assessed under naturally fluctuating conditions by transferring eggs to cooler elevated sites that mimicked the environment prior to the current warming. Eggs of C. facialis that overwintered at the cooler site exhibited similar hatching rates to those maintained at the original site. The results of these experiments consistently indicated that overwintering eggs of C. facialis possess adequate tolerance to the low temperatures of the 20th century. Therefore, we rejected our initial hypothesis that recent increases in the C. facialis population have been caused by warming-related reductions in overwintering egg mortality.

A Cicada that Ensures Its Fitness during Climate Warming by Synchronizing Its Hatching Time with the Rainy Season

“…During this period, air temperature in Osaka increased by 0.29°C per decade, probably due to the global warming trend and the urban heat island phenomenon. The relationship of decreasing winter severity to the population increase of C. facialis was ruled out in our previous study (Moriyama and Numata, 2009), and no causal relationship of warming to the compositional change has been proposed.…”

Desiccation tolerance in fully developed embryos of two cicadas, Cryptotympana facialis and Graptopsaltria nigrofuscata

“…Graptopsaltria nigrofuscata dwells in a wide range of habitats, whereas C. facialis aggregates in the warmer and more open habitats (Kato 1956; Hayashi 1984). In Osaka, although the two species frequently coexist, C. facialis occurs predominantly in locations characterized by extensive urban development, whereas G. nigrofuscata is prevalent in suburban and forest areas (Shiyake 2004; Takakura & Yamazaki 2007; Moriyama & Numata 2009). The difference in the humidity responses of fully developed embryos suggests a causal relationship between environmental humidity and habitat segregation in these two species, although temperature may also be implicated (Moriyama & Numata 2008).…”

“…Adult females of C. facialis were collected from the campus of Osaka City University (34.6°N, 135.5°E) and those of G. nigrofuscata from Utsubo Park (34.7°N, 135.5°E), Osaka, Japan, between late July and early September 2004 and 2005. These females were individually wrapped in moistened paper towel (Comfort Service Towel; Crecia, Tokyo, Japan) to obtain free eggs (modified from Kato 1956, see Moriyama & Numata 2009), and maintained at 25 ± 1°C under 16 h light and 8 h darkness (LD 16:8). Eggs laid on the paper were collected daily and placed in plastic Petri dishes (50 mm diameter, 10 mm depth) within sealed plastic pots (50 mm diameter, 60 mm depth).…”

The urban heat island effect is closely related to cicada density in metropolitan Seoul

“…Important life history events of these two cicadas, such as mating and oviposition, occur in summer. Moreover, although cicadas overwinter in diapause (Moriyama & Numata, 2008) and UHI is most intense in winter (Kim & Baik, 2005), the severity of winter weather conditions does not influence on hatching success of cicadas (Moriyama & Numata, 2009).…”

“…Daily minimum and maximum temperatures of 38 automatic weather stations within and surrounding metropolitan Seoul were accumulated from June to August 2014. This period was chosen because cicada nymphs are more likely to be influenced by hot weather conditions in summer than by coldness of winter (Moriyama & Numata, 2009;Sato & Sato, 2015). Important life history events of these two cicadas, such as mating and oviposition, occur in summer.…”