Laboratory and field experiments were conducted with six populations of Chironomus riparius to test for variation among the populations in life history parameters such as growth. mortality and/or reproductive rates. Egg masses of C. riparius from five field populations and one laboratory population were obtained and used to initiate laboratory cultures for experimentation. Environmental effects on life history variations could be ruled out because experiments were conducted with larvae that had been reared in the laboratory for several generations under identical conditions. Populations were compared under a variety of experimental conditions to test whether they would respond similarly, if genetically similar. or differently, if their genotypes were different.

The populations were subjected to a variety of experimental conditions in the laboratory in which temperature and food quality were manipulated. The population collected from Strouble''s Creek. Virginia demonstrated a pattern of high growth rate and low mortality rate. While the population collected from the final clarifier at the Madison, Wisconsin sewage treatment plant showed a pattern of low growth rate and high mortality rate. When several populations were transplanted into a high chlorine environment at the Blacksburg, Virginia sewage treatment plant, the population that had been previously exposed to the chlorine showed a significantly lower mortality rate than the other populations. A series of toxicity tests revealed different sensitivities among several populations to an experimental pesticide. Differences among populations were also found in numbers of egg masses produced, numbers of eggs per egg mass, length of time until the onset of adult emergence, and the overall length of the emergence period.

The results of this study show the importance of genetic differences among populations of C. riparius. Many commonly measured life history parameters that are usually assumed to vary primarily from differences in environment have been shown to vary as well from underlying genetic differences between populations. The role of genetics should be assessed, whenever possible. in studies where comparisons between populations of aquatic insects are made, in order to determine the degree to which genetics and environmental variables contribute to observed population differences.