The persistence and dissipation behaviour of tebufenozide, an ecdysone agonist, were investigated : (1) under laboratory conditions in aquatic models set up in glass aquaria, and (2) under Ðeld conditions in in-situ aquatic enclosures deployed in a mixed-wood boreal forest lake.
Two models were set up in the laboratory study (Study I), which was conducted at constant conditions of temperature, water pH and photoperiod. In Model I, partitioning of tebufenozide from sediment, treated at a concentration of 1400 kg kg~1, into untreated water was examined. The results showed that the chemical moved very little from the treated sediment into water. The concentration in sediment and water decreased gradually during the 90-day incubation period. Tebufenozide disappeared faster from the top layer of sediment than from the middle and bottom layers. The half-lives of disappearance were 64 days for the top layer but [90 days for the middle and bottom layers respectively. In Model II, partitioning from water, treated at a concentration of 350 kg litre~1, into untreated sediment was investigated. The results showed that the chemical moved from treated water into sediment due to adsorption. Little vertical downward movement of the adsorbed residues from the top layer of sediment occurred into layers beneath. The adsorbed residues were also not released readily back into water. The concentration in water and sediment decreased gradually during the 90-day incubation period. The half-life of dissipation from water was 67 days.
The Ðeld microcosm study (Study II), conducted under Ñuctuating conditions of temperature, water pH and photoperiod, involved application of tebufenozide onto aquatic enclosures at four concentrations of 0É05, 0É10, 0É26 and 0É5 mg litre~1. This study also showed that the chemical moved downwards from the applied location and was adsorbed onto sediment. The chemical persisted longer in Study II than in Study I. Tebufenozide, being photo-labile, probably degraded faster after constant exposure to light in Study I than after exposure to Ñuctuating light in Study II. At 90 days after treatment in Study I, only about 55% of the applied material persisted in the sediment, and there was little accumulation. In Study II, the material not only persisted but also was accumulated in the sediment, since at 92 days post-treatment the residues were about 25 times higher than the applied concentration level. Residues in water also decreased more rapidly in Study I than in Study II, because the concentration at 90 days posttreatment was about 41% of the applied value. In Study II, however, about 65% of the applied chemical persisted in water at 92 days post-treatment. While the long persistence of tebufenozide in both the laboratory and Ðeld studies was attributable to its low vapour pressure, low water solubility, high octanol/water partition coefficient etc., the di †erences in the persistence characteristics observed in the two studies were due to the Ñuctuating environmental conditions and water pH encountered in the Ðeld study, compared with the constant environmental conditions and water pH utilized in the laboratory study.