The infection of nymphal Baetisbicaudatus by the mermithid nematode Gasteromermis sp.

1. This study reports the infection in nymphs of a bivoltine mayfly host ( Baetis bicaudatus ) in a high-elevation watershed by the mermithid nematode Gasteromermis sp. Infection by Gasteromermis causes mortality in two ways. Fifty per cent of the infections do not successfully develop beyond the initial stage of penetration and result in the early death of both host and parasite.
2. Infected hosts that survive this initial stage are rendered completely sterile by the infection (reproductively dead). In addition to complete sterility, the emergence size of parasitized nymphs is reduced and development time lengthened compared with unparasitized nymphs.
3. Parasite infection levels are stable from year to year at one site, but with a higher incidence of infection in the mayfly summer generation. Size differences between the generations at the time of infection may account for their different susceptibilities.
4. Within a year infection levels vary seasonally and spatially from 1 to 71%. Seasonally, there is a condensation of parasitized hosts towards the end of development as unparasitized nymphs emerge earlier. Spatially, infection levels show a downstream decline that may result from upstream dispersal by infected hosts or differential parasite survivorship at different elevations.