Use of outdoor freshwater pond microcosms: II. Responses of biota to pyridaben

The effects of pyridaben, an insecticide-miticide on zooplankton, macroinve rtebrates, and fish were studied in outdoor freshwater microcosms using an analysis of variance design with three chemical concentrations (0.34, 3.4, 34.0 mu g/L) and one untreated control randomized among 24 tanks. Each trea tment was replicated six times. Monitoring was conducted during an 11-month baseline phase followed by a total of three months for treatment and postt reatment phases. Two applications of a wettable powder formulation were spr ayed directly below the water surface with a 30-d interval between treatmen ts. Copepoda adult abundance was significantly reduced at 34.0 mu g/L but r ecovery occurred within 6 weeks after application. Abundance of copepoda na uplii was significantly reduced at 3.4 and 34.0 mu g/L, after applications one and two; effects were more severe at 34.0 mu g/L and recovery was more rapid at 3.4 mu g/L. Abundance of Rotifera was reduced at 34.0 mu g/L, afte r applications one and two, and recovery occurred within 8 weeks for all gr oups except Polyarthra and Keratella. Of the most abundant Cladocera. abund ance of Alona was not significantly affected and abundance of Latonopsis wa s significantly reduced at 34.0 mu g/L, after applications one and two, but recovery occurred within 6 weeks. Abundance of Latonopsis also was signifi cantly reduced at 3.4 mu g/L, after applications one and two, but recovery occurred within 2 weeks. A significant decrease occurred in the abundances of Cnidaria, Insecta, and Hydracarina at 34.0 mu g/L, only after applicatio n one. Pyridaben was toxic to bluegill at 34.0 mu g/L, but was not acutely toxic at the laboratory 96-h LC50 concentration (similar to 3.4 mu g/L).