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.
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