Life-Cycle and Production of the Burrowing Mayfly, Ephemera danica: A New Method for Estimating Degree-Days Required for Growth

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Abstract
Growth, life-cycle and production of the burrowing mayfly Ephemera dnaica were investigated by taking monthly samples from April 1983 to June 1984 in the River Tud in Norfolk, U.K. A maximum likelihood method was used to estimate the number of degree-days required to complete nymphal growth. Minimum threshold temperatures for growth were estimated to be 2.6.degree. C for males and 3.1.degree. C for females. From the estimates of degree-days required to reach maturity and the temperature regime of the study site, it was concluded that some larger males should emerge as adults in 1 year, whereas the rest, and all females, should take 2 years to mature. This agrees with the pattern observed in size-frequency distribution of E. danica nymphs throughout the year. Predictions of time to emergence based on temperature data from other sites in Europe agree well with published accounts of the life-cycle of E. danica. Annual population production of the 2 year-classes of E. danica combined was 5.58 g dry weight m-2 year-1, with the older year-class contributing about 85% of the total. Females accounted for 60-70% of production. Monthly production in summer was as much as ten-times that in winter. Production-biomass ratios showed relatively low values (< 2.1), a pattern that may be general in semi-voltine species. The maximum likelihood method for estimating minimum threshold temperature and degree-days to complete growth can be used for aquatic and terrestrial insects in the field or laboratory.