Digging out intersexual and meteorological effects on cicada emergence using 10-year citizen monitoring

Understanding the mechanisms determining the emergence timing of herbivorous insects is ecologically important. However, little is known about the effect of climatic factors and the presence of conspecific individuals on their emergence in the field. In particular, it is challenging to investigate the seasonal emergence of cicadas because these insects have a long life-cycle and subterranean larval stages. We assembled a time-series dataset that consists of daily counts of emerging cicadas together with meteorological factors, using long-term collection of cicada exuviae by elementary and junior-high-school students. We then performed a non-linear time-series analysis, empirical dynamic modelling, to identify factors behind the timing of cicada emergence. Our findings are three-fold: (1) emergence of individuals of the opposite sex constitute a major driver of the number of individuals emerging per day, and this effect is stronger in males than in females, (2) as in other insect species, air temperature consistently affects cicada emergence, but its effects are relatively weak and (3) precipitation and humidity were causally related to emergence. These results are consistent with the theory of sexual selection as well as the fact that it is hard for the subterranean cicada larvae to use information about air temperature. Significantly, the findings are based on long-term field data collected by non-expert citizens.