Movement and residual toxicity of carbofuran in acid mineral soil

In greenhouse miniplots with soil moisture moving upward from the subsoil, toxicants of carbofuran moved upward and became detectable at the soil surface in ca. 3, 7, and 28 days after 10% granules of the insecticide were banded 1, 3, and 10 cm, respectively, below the surface. The test soil had a pH of 5.2 and contained 2.9% organic matter. Toxic residues reaching the soil surface from a depth of 5–10 cm increased over a period of 8–12 months and gradually decreased thereafter. Fifteen to 20 ppm in the surface soil was highly toxic to flies contacting the soil surface. Amounts in excess of 40 ppm in the surface layer caused knockdown in 10–15 and estimated death in less than 30 s. The LD₅₀ for cabbage maggot eggs on the surface of treated soil was 8.2 ppm and for larvae 0.86 ppm. Chemical analysis records correlated well with those of bio-assay tests. In field microplots, the addition of water during rainstorms (giving a total of 50 mm) demonstrated that toxic residues of carbofuran could be leached downward to at least 45 cm. However, under the normal weather conditions of 1980 and under abnormally dry conditions most of the detectable residues remained near or above the point of application.