The bean fly, Melanagromyza. phaseoli (Coq.), the larvae of which tunnel in the stems of Phaseolus vulgaris, is a pest of this crop in Tanganyika. Field trials there in 1956 on haricot-type beans showed that dressing the seed with chlorinated-hydrocarbon insecticides could prevent attack for at least 30 days after germination; organophosphorus insecticides were less effective. In four further field trials (A-D), in 1957, the insecticides and types of formulation used were aldrin (wettable powder and emusifiable concentrate (EC)), endrin (EC), γ BHC (dry dressing and EC), dieldrin (dry dressing and EC) and a proprietary organophosphorus systemic (dry dressing). Wet dressings were applied by treating the seeds with the liquid and drying immediately, and in one treatment aldrin as a wettable powder was applied dry and the seeds then damped and dried at once.

Damage was assessed on an arbitrary scale on trials A and B, and yields measured on trials A, C and D. Results varied between insecticides and under different growing conditions, but wet dressing with endrin at 213 mg. per 100 g. seed, of which about 50 per cent, was retained on the seeds, resulted in the lowest damage, amounting to 7·7 compared with 52·6 for the control (no dressing), and the highest yield, amounting to 13·5 units compared with 8·6 for the control. All treatments, except that with the organophosphorus dust, reduced the damage significantly below that in the control. Wet dressings were usually more effective than dry dressings with the same insecticide, as regards both yield and reduction in damage. This may be associated with the better retention of wet dressings, which was shown by the results of chemical analysis of the residues on the seeds left in the drill hopper after sowing.