Insecticidal activity of pyrethrum extract and its four insecticidal constituents against house flies. III. —Knock‐down and recovery of flies treated with pyrethrum extract with and without piperonyl butoxide

The influence of concentration and of time after treatment on knock‐down and recovery from the action of pyrethrum extract alone and with piperonyl butoxide at 2 : 1 and 1 : 5 w/w, was studied on house flies (Musca domestica L.) by a measured drop method. The unanaesthetised flies, held by suction during dosing, were inspected after treatment at frequent intervals for 5 h. and once on the following day. The tests were done at 20† with the materials tested dissolved in n‐dodecane. Most of the flies treated with a fixed dose of pyrethrum extract alone or with piperonyl butoxide remained normal for a very short time (the latent period). The number of flies knocked down increased very rapidly during the knock‐down period and reached a maximum at the knock‐down end‐point. Past this point a proportion of the flies knocked down recovered, the rest died. The relative and absolute durations of each of these periods were influenced by the concentration of pyrethrins and the presence of piperonyl butoxide. The interval of time between treatment and knock‐down end‐point was the same (10 min.) over a wide range of concentrations when the flies were treated with pyrethrum extract alone, and the flies treated with the extract alone recovered rapidly after knock‐down end‐point. The synergist inhibited recovery, so that more flies died, increased the toxicity of the extract during knock‐down and prolonged the duration of knockdown. The influence of piperonyl butoxide, small initially, increased with the passage of time after treatment. Attention is drawn to some of the shortcomings of methods commonly used to compare the knock‐down action of insecticides.