COMPARISON OF A FIELD AND LABORATORY-DERIVED POPULATION OF CHIRONOMUS-RIPARIUS (DIPTERA, CHIRONOMIDAE) - BIOCHEMICAL AND FITNESS EVIDENCE FOR POPULATION DIVERGENCE

A field (Jackson Pike) population of Chironomus riparius (Meigen) and a laboratory-derived culture were compared in terms of components of biological fitness (female fecundity, male and female pupal weight, and larval developmental period), response to selected insecticide exposure (malathion, parathion, propoxur, and dichlorodiphenyltrichloroethane [DDT]), and several biochemical parameters (mixed function oxidase, general esterase, acetylcholinesterase and glutathione-S-transferase activity) to determine the extent of population divergence (differential population characteristics) as well as to assess the accuracy of results obtained from tests performed on laboratory-cultured insect populations. Results from all comparisons suggest that these two populations have significantly diverged. For example, the laboratory larvae were characterized by enhanced susceptibility to malathion, parathion, DDT, and propoxur as determined at LC50s. Jackson Pike larvae, however, displayed resistance to these compounds, with LC50s values ranging from 13-250 times greater than that estimated for laboratory individuals. Furthermore, Jackson Pike individuals were characterized by significantly increased enzymatic activity (glutathione-S-transferase), differential oxidative metabolism (mixed function oxidase), reduced target site sensitivity to inhibition (acetylcholinesterase), and reduced general esterase activity compared with laboratory larvae. These results suggest that the Jackson Pike population has developed an effective resistance strategy against toxicants, presumably as a result of site specific selection pressures. No such compensatory mechanisms were observed in laboratory larvae. Finally, the Jackson Pike population was characterized by heavier, more fecund individuals as compared to the laboratory population. Difference in life history characteristics between populations also indicates divergence. In summary, the results of our study indicate that long-term laboratory culture conditions do not necessarily provide selection pressures characteristic of those experienced by field individuals. Therefore, information obtained from laboratory individuals should be used with caution in representing field responses of field populations.