Aquatic Fate

A challenge in interpreting aquatic fate of spinosyns is the numerous degradates that can be generated in low quantities. A further challenge is that the pattern of degradates shifts depending on the dominant degradation process associated with the test system or conditions. For spinosad, the traditional approach of using radiolabeled test substance in dark laboratory studies to identify metabolites for development of methods for nonradiolabeled field studies was generally followed. This approach was augmented by: (1) development of an immunoassay kit used in the aquatic microcosm study; (2) insecticidal screening of numerous spinosyns and several degradates; (3) eco-toxicology testing of degradates; and (4) the use of radiolabeled test material in an outdoor study. This approach provided a comprehensive characterization of the degradate profile and potential environmental impact, but required integration of multiple studies. For spinetoram, an aquatic field study at two sites in the USA resulted in reported half-lives of less than one day. The respective N-demethylated metabolites were formed rapidly following application, and these dissipated with a first order half-life of one to two days. Although spin-etoram has high sorption potential for soil and sediment, no partitioning to sediment was observed in these shallow water bodies because of the rapid degradation in the water column.

Collectively, the studies indicate that photolysis, rather than adsorption, is the major route of dissipation of spinosyn residues in water. Dissipation of the active ingredient occurs very rapidly and the N-demethylated products of

Figure 5.8

General environmental degradation pathways for spinsosyn A, the major component of spinosad. See Figures 5.2 and 5.7 for structures.

Figure 5.8

General environmental degradation pathways for spinsosyn A, the major component of spinosad. See Figures 5.2 and 5.7 for structures.

Degradation Pathway Soil
Figure 5.9 General environmental degradation pathways for XDE-175-J, the major component of spinetoram. See Figures 5.2 and 5.7 for structures.

spinosad and spinetoram are not persistent. For spinosad, it has been demonstrated that secondary pseudoaglycone metabolites are not persistent and have little biological activity. Generalized environmental degradation pathways based on studies in several environmental compartments (e.g. soil metabolism or aqueous photolysis, etc.) are shown in Figures 5.8 and 5.9. Pathways include both major (> 10%) and minor compounds as well as both tentative and confirmed structures.

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