Although damage by wheat bulb fly, Leptohylemyia coarctata (Fall.), can be prevented or reduced by various cultural measures these are not always practicable (Gough, 1957) and chemical methods were therefore investigated. Preliminary experiments suggested that seed dressings would be the simplest and most promising approach as Gough & Cohen (1954) found that spraying to kill adults was unsatisfactory.Seed dressings containing 20 per cent, -y BHC to prevent wireworm damage were introduced commercially about 1949-50, but observations on fields where this treatment was used indicated that it had little effect on attack by wheat bulb fly, and higher rates were therefore tested at the outset. About the same time, aldrin and dieldrin became available for experimental purposes, and seed dressings containing these materials were used in the first trial; other materials were introduced for comparison or as they became available and, in the later stages of the work, chemicals found promising by Bardner (1958Bardner ( , 1959 were included. Methods.Fields on commercial farms were usually selected for experiments on the basis of a fairly high egg count of wheat bulb fly, preferably over one and a half million per acre; these egg counts were based on 20 four-inch cores per field, the samples being examined by a flotation process (Cockbill & others, 1945). All cultivations, including sowing, were carried out by the farmer or his staff with his own equipment. Smaller trials were drilled with a hand machine.Dry seed dressings only were used and all seed was treated in a small barrel-type dressing machine, usually within 24 hours and invariably within a few days of sowing. Except where otherwise stated, all insecticidal seed dressings also contained a suitable quantity of organo-mercurial fungicide and the seed on all control plots in experiments where the insecticide was combine-drilled was treated with an organo-mereurial fungicide (O.M.). The rate of dressing was either 2 or 3 oz./bushel (equivalent to 2 or 3 g./kg.). Where possible, seed-drills were calibrated beforehand so that the sowing rate of the seed or any material combine-drilled would fall within the chosen range. The rates were checked after the drilling of each treatment by noting the difference between the weight of seed or material put in the drill and the residue.Counts of attacked and healthy plants and shoots were made on sample areas of two adjacent rows each 1 ft. long on one or more occasions and the number and condition of the larvae in the attacked shoots (or a sample thereof) were recorded. The number of samples taken depended on circumstances but was usually three to five per plot. Usually these samples were brought back to the laboratory for examination and dissection under a low-power binocular microscope though sometimes it was possible to make total-plant and damaged-shoot counts in the field.

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Water traps examined weekly were used to assess the activity of adult wheat bulb flies during the oviposition period in late July, August and eariy September. Traps were placed at various distances up to 1 mile (1-6 km) from known sources of infestation at Whittlesey, Cambridgeshire and at Rothamsted. Most flies were caught down wind from possible emergence sites and few were caught more than ^| mile (^0-4-0-8 km) away. The distribution of flies between traps was similar in both the first and second halves of the trapping period. At Whittlesey there was a significant positive correlation between the number of flies caught in a trap and the number of eggs laid in the field containing the trap. SUMMARYMethyl-bromide (1 lb/100 sq. ft), applied as in commercial practice, gave an increase in yield of both grafted and ungrafted tomato plants in glasshouse soils naturally-infested with root diseases. Grafted plants greatly outyielded ungrafted plants on treated and untreated soil where phytophthora stem and root rot was present. The efl"ect of the gas on corky root rot varied in the experiments and, on one nursery, control of the disease was not reflected by increased yield.

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.