The life history of Hexagenia limbata (Serville) (Ephemeroptera:Ephemeridae) in two Minnesota streams.

A.D. Anderson and M.G. Butler.Department of Biological Sciences, North Dakota State University, Fargo, North Dakota 58105

Much of our collective knowledge of the life history of Hexagenia limbata comes from Lake Erie, the Mississippi River, and a variety of lentic systems, but the population ecology of this species has been largely overlooked in small streams. Given the general phenotypic plasticity of mayflies, the unique environmental pressures of small lotic systems likely influence the life history of H. limbata. Better understanding of H. limbata’s natural history will permit biological monitoring to focus on sublethal effects of habitat degradation, in contrast to approaches based on mere presence/absence or relative density.

During 2004 and 2005, sampling was conducted in two Minnesota headwater streams, the Straight and Shell Rivers, to determine life history characteristics of H. limbata. Size-frequency analysis of nymphal head width indicated two cohorts in both streams. Densities in the Shell varied from 892/m² (16 September 2004) to 201/m² (11 July 2005) following late-June emergence, whereas densities in the Straight were much lower, ranging from 44/m² (7 June 2005) to 8/m² (11 October 2004). The sex ratio of all individuals collected from the Straight was 1.2 females/male (n=99) and 1.4 females/male (n=1713) for the Shell. Densities and voltinism are within previously reported ranges, however sex ratios differ.