A comparison of the fate and effects of two pyrethroid insecticides (lambda-cyhalothrin and cypermethrin) in pond mesocosms.

: The fate and effects of two pyrethroid insecticides (lambda-cyhalothrin and cypermethrin) were investigated in replicated 25 m(3) pond mesocosms. Three pesticide treatments which simulated spray drift deposition were examined: 0.7 g a.i. ha(-1) cypermethrin and 0.17 and 1.7 g a.i. ha(-1) lambda-cyhalothrin. Based on the use rate and pesticidal activity of the chemicals, the cypermethrin and lower lambda-cyhalothrin rates were approximately equivalent. After applications, pyrethroid residues in the water column declined rapidly. Treatment-related effects were observed on some macroinvertebrate taxa, most notably the Asellidae and Gammaridae. Surfacedwelling insects also suffered initial knock-down, particularly in the 1.7 g a.i. ha(-1) lambda-cyhalothrin treatment, but there was recovery after the spray period. No adverse effects occurred on algae, macrophytes or zooplankton, but there were occasional enhancements (e.g. algal biomass and abundances of copepod nauplii and Rotifera) which may have been indirect effects. An overall comparison of the treatments indicated that the higher lambda-cyhalothrin rate had the greatest effects, whilst the cypermethrin application had a somewhat greater impact than the lower lambda-cyhalothrin treatment rate (due to effects on peracarid crustaceans). The study indicated that should spray drift occur at the levels expected for either pyrethroid's normal use patterns, potential impacts on natural aquatic ecosystems would be minor and transient.