ECOLOGICAL RESTRUCTURING IN EXPERIMENTAL AQUATIC MESOCOSMS DUE TO THEAPPLICATION OF DIFLUBENZURON

Diflubenzuron is a chitin-inhibiting insecticide that is registered fo r use in controlling a multitude of invertebrate pests. The selective mode of action of diflubenzuron provided us with a unique tool for exa mining the indirect responses of an experimental aquatic ecosystem fol lowing direct impacts at the primary consumer (invertebrate) trophic l evel. Twelve outdoor aquatic mesocosms (0.1 ha each) were randomly ass igned to three treatments of diflubenzuron (four mesocosms/treatment): control, monthly (five total 10-mu g/L applications), or biweekly (tw ice monthly; nine total 10-mu g/L applications). Direct impacts on inv ertebrates (numbers and species composition of insects and zooplankton ) and indirect impacts on fish (mortality, growth, and reproduction of bluegill sunfish and largemouth bass), phytoplankton (biomass and pro ductivity), and water quality were measured over 16 weeks posttreatmen t. Both monthly and biweekly treatments of diflubenzuron decreased tot al numbers (fivefold reduction) and species richness (twofold reductio n) and increased dominance (twofold increase) of zooplankton within 4 weeks posttreatment; Cladocerans, Copepods, and Rotifers were all sens itive. Diflubenzuron reduced species richness of emergent insects, whi ch resulted in increased dominance by a few species; however, impacts on emergent insects were confounded by a treatment x time interaction. Direct reductions in invertebrate grazers caused indirect increases i n algal biomass. Indirect effects on biomass (50% reduction) and indiv idual weight (50% reduction) of juvenile bluegill occurred because of apparent decreases in invertebrate food resources. No statistically si gnificant impacts were observed on adult bluegill or largemouth bass f or the duration of the experiment. Results indicated that di8ubenzuron had both direct and indirect impacts on the experimental aquatic ecos ystems under the conditions tested.