Life histories of burrowing mayflies (Hexagenia limbata and H. rigida, Ephemeroptera: Ephemeridae) in a northern Canadian reservoir

1. The life histories and population dynamics of the burrowing mayflies Hexagenia Umbata and H. rigida were examined from 1986 to 1988 in four regions of an impounded lake in northern Manitoba, Canada. 2. Bottom temperatures reached 15–20°C, but ice-free periods of 4–6 months resulted in annual degree day (dd > 10°C) accumulations of only 210–650 in various regions and years. 3. The life cycles (egg to adult) of both Hexagenia species required 3yr in the three warmest regions studied and 4yr in the coolest area; total dd accumulations to complete the life cycle ranged from 1222 to 1468 dd > 10°C. 4. Fecundity (no. of eggs/) was strongly correlated to body size of subimagos, and subimago size was strongly correlated to length of nymphal exuviae, so field collection of nymphal exuviae could be used to determine both the size and fecundity of the emerging females. 5. Degree day accumulation in the final year before emergence was a better predictor of emergence timing than overall dd accumulations for the life cycle.