Transfer of DDT and metabolites from fruit orchard soils to American robins (Turdus migratorius) twenty years after agricultural use of DDT in Canada

Wildlife contamination studies found high levels of DDT and associated meta bolites in bird eggs from Canadian orchard sites during the early 1990s. Th e present study investigated local dietary uptake of DDT and geographic var iability in tissue concentrations in the same orchards. A soil-earthworm-ro bin food chain was chosen for study, as early surveys showed that robins co ntained the highest levels of DDT of several avian species and because publ ished research indicated that earthworms were a probable dietary exposure r oute. Organochlorine pesticides and PCBs were measured in soil, earthworm, robin egg, and robin nestling samples collected from fruit orchards and ref erence sites. High average DDE (soil: 5.2 mg/kg; earthworm: 52 mg/kg; robin egg: 484 mg/kg dry weight) and DDT (soil: 9.2 mg/kg; earthworm: 21 mg/kg; robin egg: 73 mg/kg dry weight) concentrations in Okanagan (British Columbi a) samples confirmed that previously recorded contamination was common in t he region. Concentrations detected in Simcoe, Ontario, orchards were not as high but were still significantly elevated relative to levels in soils and robins from reference areas. Significant positive linear regressions betwe en soil and earthworm concentrations and consistent trends in food chain ac cumulation suggested that robins were acquiring DDT and metabolite (DDTr) b urdens locally. Low concentrations of DDT and DDTr in robin eggs collected from nests in nearby nonorchard and post-DDT orchard habitats suggested tha t the local sources were in orchards. Persistence of DDT in orchard food ch ains is likely due to a combination of retarded degradation rates for DDT i n soil and its extensive use historically. DDT concentrations in some robin eggs and earthworms were at levels comparable to those observed in field s tudies where mortality or reproductive effects occurred.