The Speed of Action of insecticidal Sprays and Deposits and its Use in assessing the biological Efficiency of BHC, DDT and Pyrethrum
Authors: Kettle D. S. 
Published: July 2009

This paper is concerned firstly with the standardisation of a technique for the estimation of the speed of action of insecticidal sprays and deposits on adult houseflies (Musca domestica), as indicated by their speed of knock down, and secondly with the application of this technique to the assessment of the biological efficiency of insecticidal solutions. The main conclusions are:—

The speeds of action of both sprays and deposits are greatly affected by the duration and intensity of illumination to which the flies have been exposed before treatment and by the ratio of the sexes in the treated population. The first factor was eliminated by conditioning the flies to an intensity of illumination of 20 foot-candles overnight, i.e. at least 17 hours, and the second was minimised by estimating the median knock down time for 50c per cent. instead of 50 per cent, where 50c=0·67x+0·y, and x and y represent the percentages of males and females respectively in the treated population. This relationship appeared to be independent of the technique as it was the same for 0·05 per cent. gammexane in petroleum distillate used as a fly spray and for deposits of pure p.p. DDT put down from petroleum distillate. The experimental evidence, on which this correction is founded, is set out in full.

It was found that there was a separate straight line relationship between the percentage knock down in terms of probits and the time after spraying (or the time of exposure) in minutes for male and female M. domestica. It follows therefore that such a relationship is not true for a population of mixed sexes. The method of calculating the corrected median knock down time (K.D.50c) is detailed.

The speed of knock down of solutions of pyrethrins, the gamma isomer of BHC and the para para isomer of DDT were proportional to the concentration of the insecticide in the solution. This conclusion has been applied to the estimation of gammexane in solutions of unknown strength and examples are given in which the calculated and actual values are compared and some conception is obtained of the accuracy of the method.

Experiments with DDT deposits on aluminium plates showed that:—

(a) The speed of action was independent of the deposit providing that this was above 2 mg. p.p. DDT/sq. ft. Although when the deposit was applied to paper-covered panels the speed of action varied with the magnitude of the deposit.

(b) A deposit of p.p. DDT from a solution of pure (100 per cent.) p.p. DDT acted more quickly than an equal p.p. DDT deposit from a solution of technical (80 per cent.) p.p. DDT.

(c) The action of a deposit was more rapid when the deposit was in a form which could act both as a stomach and contact poison and hence the ball milled technical DDT applied as a suspension acted more quickly than an equivalent deposit of technical DDT from petroleum distillate because the crystals of the latter were too tightly adherent to the panel for the flies to ingest them.

The significance of these results is discussed under the appropriate section.