Some Factors Influencing the Toxicity of Soil Applications of Chlorpyrifos and Diazinon to European Chafer Grubs1

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Abstract
Chlorpyrifos- and diazinon-treated soils of 8% moisture were more toxic to 3rd-instar grubs of Amphimallon majalis (Razoumowsky) than were treated soils containing 13 or 18% water. Differences in soil moisture did not affect degradation rates of chlorpyrifos, but formulation did. Halflives of ca. 7, 9, and 16 days were obtained with 22.4 EC, 0.95 G, and 0.88 G formulations, respectively. However, degradation of diazinon (5 G formulation) was more rapid as soil moisture increased, with half-lives of ca. 12 and 6 days for 8 and 13–18% moisture, respectively. Soil moisture did not affect diazinon uptake by grubs, but did affect chlorpyrifos uptake during a 22-h contact with treated soil. Total body residues were 1.0–1.5 ppm for grubs in diazinon-treated soil at all moisture levels, whereas higher chlorpyrifos residues (3.2 ppm) were found in grubs from soil with 8% moisture than from soil with 13–18% moisture (1.8 ppm).

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