EFFECTS OF TEMPERATURE, FOOD QUANTITY, AND NYMPHAL REARING DENSITY ON LIFE-HISTORY TRAITS OF A NORTHERN POPULATION OF HEXAGENIA (EPHEMEROPTERA, EPHEMERIDAE)

Hexagenia limbata (Serville) and H. rigida McDunnough from Southern Indian Lake, Manitoba, were reared at 10-degrees, 15-degrees, and 20-degrees-C to assess temperature effects on growth. Subsequently, H. limbata was reared at two food levels and two or three nymphal densities (depending on size class), while temperature again was varied, to determine whether these factors affected life-history features such as growth, development, fecundity, and mortality. Nymphs of both species grew at 8-degrees-C, 2-degrees below the published temperature threshold for development. At both limited and non-limited food levels, growth and development rates increased with increasing temperature, but food limitation slowed both growth and development within a temperature, treatment and resulted in increased degree day requirements. Nymphs reared at 20-degrees-C at a non-limiting food level grew into significantly larger and more fecund adults than those reared at 20-degrees-C at a limiting food level, or at 15-degrees-C under non-limiting food conditions. Growth and development rates were not affected by rearing density. Thermal requirements observed in the laboratory were near the lower end of those observed in nature, and are probably a reflection of the effect of temperature on development under near-optimal conditions. Growth responses to temperature were similar for the two species.