DDT (p,p'-dichlordiphenyl-trichlormethyl-methane) has been shown to be very toxic to the housefly, Musca domestica, L., when applied as a spray containing 0·1 per cent, w/v or more in kerosene. DDT at 1 per cent, gives an effective spray for practical use, but at lower concentrations the speed of knock-down is too slow. This disadvantage can be overcome by the admixture of a small amount of pyrethrum. A mixture of 0·03 per cent, w/v total pyrethrins and 0·1 per cent, w/v DDT in kerosene is recommended as a suitable fly spray for practical use. This mixture can be stored in glass bottles in the dark for at least 17 months at 27·5° C. without marked deterioration. Its toxicity may be increased by the incorporation of a suitable activator, e.g., sesame oil or isobutylundecylenamide.

The impurities in crude preparations of DDT appear to exert no toxic effect on houseflies.

DDT acts more slowly when dissolved in industrial methylated spirit than in kerosene.

Attention is drawn to the residual film effect of DDT on walls, etc., and an example is given to show how this effect was employed in the control of a fly population in a kitchen.

The symptoms of poisoning of flies by DDT are compared with those caused by pyrethrum. DDT is relatively slow in action; it is a severe irritant, causing the flies to buzz violently at one stage; it induces marked tremors during the coma preceding death; and, at death, it causes the legs to be tightly flexed at the femoro-tibial joint. When the pyrethrins and DDT are mixed in a spray, these constituents, so far as can be judged from the train of toxic symptoms, act independently, so that the rapid paralytic effect of the pyrethrins keeps the insects immobile until the slow lethal effect of the DDT supervenes.

This work has been carried out as part of the programme of research of the Pest Infestation Laboratory, and this account is published by permission of the Department of Scientific and Industrial Research.