The dynamics of aquatic fungi parasitic in a stream population of the midge,Chironomus attenuatus

The dynamics of a midge community in which egg masses of Chironomous attenuatus were dominant and hosts to various fungal parasites were studied during three seasons at Mechumps Creek in Hanover County, Virginia. Catenaria spinosa, Couchia circumplexa, Atkinsiella entomophaga, a Plasmodiophora-like organism, and a number of water molds produced overall rates of infection in the egg masses ranging from 44.6 to 76.9%, and mortality in the eggs ranging from 28.8 to 41.4% during the 3-year period. C. spinosa and C. circumplexa were responsible for the largest amount of infection during each year of the study, and appeared and disappeared in response to changing water temperatures. Combined rainfall data over the 3-year period showed significant negative correlations with population densities and prevalence of disease. Studies also revealed significant negative correlations between infection and mortality levels and numbers of C. attenautus larvae collected 3–6 weeks later. The fungi were highly specific in their parasitism of C. attenuatus egg masses, even though egg masses of at least 10 other midge species were present at the study area. It is speculated that the host-parasite relationships and dynamics at Mechumps Creek are of frequent occurrence in many Virginia streams, where they constitute a natural control mechanism for C. attenuatus.