Influence of block and cell size, growing medium and insecticide dose on the behaviour and uptake of phorate by lettuce and uptake of carbofuran and chlorfenvinphos by calabrese

Abstract

Phorate residues in peat blocks and lettuce were determined following incorporation of the insecticide into different block and ‘Speedling’ cell sizes. Between‐block variability was influenced little by block size. Phorate oxidation was most extensive in the largest blocks containing the smallest dose. Total residue concentrations in the lettuce declined from the time of planting to harvest, although accumulation of insecticide continued and was related more to dose than to block or cell size. Residues in the lettuce at harvest exceeded the proposed maximum limit of 0.2 mg kg−1 in some treatments. Residues at planting comprised mainly the parent sulphoxide and sulphone, but by harvest, the oxygen analogue sulphoxide and sulphone predominated. Lettuce weight was not influenced by dose but was related directly to block size. Carbofuran and chlorfenvinphos residues were determined in calabrese sown into two sizes of blocks. At planting time, carbofuran residue concentrations were 100 times greater than those of chlorfenvinphos but residues of both insecticides in the mature heads were < 0.01 mg kg−1. Seedling weights in both sowings declined with increasing concentrations of the insecticides. It was concluded that manipulations of block size and the dose of insecticide need to be evaluated for individual insecticide/crop combinations to exploit the technique fully.