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S U M M A R YSamples of plants and soil from two experimental sites, one clay loam and one peaty loam, were analysed chemically to try to explain differing results with y B H C and dieldrin seed-dressings applied to control wheat bulb fly on different soil types, and to suggest reasons for patchy plant stands and occasional failure to control the pest. Seed dressed with dieldrin yielded more than untreated seed on both sites, whereas seed treated with y-BHC yielded as much as seed treated with dieldrin on the clay site, but it yielded less than untreated seed on peat. The chemical analyses showed that the poor performance of y-BHC on the peat site could not be explained by its failure to persist, because the soil still contained considerable quantities of y-BHC at the time of insect attack. Possible reasons may be sorption of the poison by organic matter making it less available, and deeper sowing, permitting larvae to reach the plants without prolonged contact with the insecticide.Bayer 381 56 (0-ethyl S-p-tolyl ethyl phosphonodithioate) and trichloronate (0-ethyl O-z,4,5-trichlorophenyl ethyl phosphonothionate) persisted in soil long enough and were sufficiently toxic to wheat bulb fly to suggest that organophosphorus compounds might provide alternatives to chlorinated hydrocarbon insecticides for control of the fly.Analyses made on seeds dressed at the laboratory for the experiment showed that the amounts of insecticide on seeds were smaller than expected and that the amounts on individual seeds differed greatly. Of ten samples of seeds dressed commercially with y-BHC, three carried very little insecticide, and the variations in the other seven samples were greater than with experimentally applied dressings.

A method for determining insecticide on seeds treated with liquid formulations by extraction and gas chromatography of the extract was devised and used to estimate the amount of the insecticide on single seeds.Eight solvents were tested for their ability to extract aldrin from wheat seeds treated with a liquid formulation. Acetone-hexane was chosen for further investigation because in 24 hours at room temperature it extracted at least 95 % of the aldrin from treated seeds. This solvent almost quantitatively extracted chlorfenvinphos and carbophenothion at room temperature and y-BHC at 45" in 24 hours, without extracting substances which interfere with gas chromatography of the insecticides.The amount of insecticide found on single seeds varied widely, in seeds dressed both in the laboratory with commercial preparations (range 4-to 7-fold) and in 2 commercial plants (range 40-to 200-fold).