Effect of water temperature on the mass emergence of the mayfly, Ephoron shigae, in a Japanese river (Ephemeroptera: Polymitarcyidae)

Abstract

Summary1. Winged stages of Ephoron shigae were collected every day during their emergence period during a 6‐year period from 1989 to 1994, by net sweeps or a light trap along the Asahi‐gawa River in western Japan. Emergence occurred mainly in September.2. Coefficients of determination (r2) were calculated for the regression of the mean date of emergence against cumulative degree‐days during various periods from late March to early September.3. In both sexes the highest values of r2 were obtained for regression with degree‐days from late June to late August. This indicates that thermal conditions during the late instars affected the emergence timing most strongly. The timing of emergence can be well predicted from the cumulative degree‐days during summer.