Food preference and relative influence of temperature and food quality on life history characteristics of a grazing mayfly, Ephemerella ignita (Poda)

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Abstract
In choice experiments, starved larvae of Ephemerella ignita (Poda) moved preferentially to compartments with a diatom-rich diet rather than to those with a detritus diet. Growth rate, mortality rate, larval developmental time, adult size, and fecundity of E. ignita were assessed in response to rearing on two diets (diatoms and detritus) at four constant temperatures (4.5 – 18 °C). On each diet, growth rate (range 0.30 – 6.2% length/d) increased with increasing temperature according to a power law (Y = axb). At any temperature, growth rate was significantly higher on the diatom diet than on the detritus diet. Mortality rate (range 0.2 – 6.5%/d) increased with increasing temperature and was higher on the detritus diet than on the diatom diet. Developmental time varied significantly with temperature for animals fed diatoms according to a hyperbolic relationship. The thermal requirement for completing larval growth from hatching to emergence on a diatom diet was estimated at about 950 degree-days above a threshold of 3.5 °C (range 9.5 – 18 °C). Very few larvae reared on detritus achieved the adult stage and their developmental time seemed longer. Subimago weight did not vary significantly with temperature, but fecundity (range 435 – 1320 eggs per female) was significantly lower at 9.5 °C than at 14.5 and 18 °C in larvae fed diatoms. Fecundity of females from larvae fed detritus was very low (range 5 – 150 eggs). The reproductive effort (number of eggs per milligram of subimago weight; range 66 – 125) followed a clear linear increasing relationship with temperature (9.5 to 18 °C) for animals reared on diatoms. Poor food quality could probably restrict distribution of E. ignita by reducing fecundity. In conclusion, these results show that the thermal equilibrium hypothesis must be extended to other environmental factors. They also support the hypothesis of a bivoltine cycle of E. ignita under favorable thermic and food conditions.