Protection of radish from cabbage root fly damage by seed treatments with organophosphorus and carbamate insecticides

SUMMARY Ten seed treatment formulations were assessed in the laboratory and glasshouse for their phytotoxicity to radish and effectiveness in preventing cabbage root fly damage. Bendiocarb, dimethoate, mecarphon and chlorfenvinphos were phytotoxic; carbophenothion, diazinon, iodofenphos and pirimiphos‐ethyl gave negligible protection. Chlorpyrifos and isofenphos performed adequately and so were compared at 14 and 20 g a.i./kg of seed in four field experiments with chlorfenvinphos soil treatments. With less than one‐third of the a.i. needed for soil treatments, the seed treatments gave similar or, in one experiment, better protection than 2·2 kg chlorfenvinphos/ha incorporated in the soil before sowing. Insecticide residues in mature radish from soil treated at 2·2 kg a.i./ha with a chlorfenvinphos e.c. were 0·04–0·06 μg/g, within the current Codex tolerance of 0·1 μg/g; the same dose of a granular formulation of chlorfenvinphos produced residues of 0·06‐0·21 μg/g. Chlorpyrifos residues in mature radish grown from seed treated at 20 g a.i./kg were 0·12‐0·65 μg/g, mostly less than the most relevant Codex tolerance, for carrots, of 0·5 μg/g. Seed treatment with the same dose of isofenphos, for which there are no Codex tolerances, produced residues of 0·17‐0·27μg/g. No insecticide treatments are currently approved in the U.K. to protect radish from damage by cabbage root fly larvae. Soil treatment with a chlorfenvinphos e.c. seems to be suitable for protecting single radish crops. If suitable formulations of chlorpyrifos or isofenphos were made available commercially, seed treatment with these insecticides would lessen the risk of residues accumulating in soil and contaminating successive radish crops grown in the same soil. Seed treatment would also provide an alternative to applying smaller doses to the soil for second or subsequent crops.