Factors influencing the Interaction of insecticidal Mists and flying Insects

Abstract

An attempt has been made to assemble the important factors concerned with the atomisation of a kerosene spray and to consider the behaviour of the spray alone, and in its relation with an insect flying through it.The degree of atomisation attained by the spray gun is dependent upon nozzle design, the spraying pressure, the relative rate of liquid and air flow through the nozzle and such physical properties of the spray fluid as viscosity and surface tension.All atomisers produce droplets of a variety of sizes which leave the gun with a considerable velocity and are carried along by the air current. At this stage they are at their maximum size and possess their maximum velocity so that the chance of impacting with an insect is great. Later on when the droplets have lost much of their velocity and volume (if they contained a non-volatile insecticide they will have become much more concentrated), they are much less likely to impact. It can thus be seen that a non-toxic carrier can increase apparent toxicity of a spray mist by assisting in the production of the momentum necessary for impaction (David and Bracey, 1944). On the other hand a reduction in the size of the droplet decreases the rate of loss from the air space by precipitation. In connection with evaporation the enormous change in concentration which occurs should be borne in mind. Within a few minutes the smallest droplets will have entirely evaporated, leaving only the non-volatile solute and before this stage is reached the droplet will have become super-saturated. Finally this non-volatile solute will exist as minute nuclei which, when they result from a fairly typical insecticidal formula containing up to say 5 per cent. nonvolatile material, may be so small as to settle only very slowly.Regarding the response of the insect to the spray mist, it may be said that this depends upon the resistance of the insect and the dose of insecticide picked up. Since the spray mist takes some time to distribute evenly and since it is more easily picked up during or immediately after spraying and by insects in flight, it can be seen that the dose received by individual insects will vary a good deal unless the insects behave identically so that under these conditions there is little correlation between the comparative effect recorded on individuals and their relative innate resistance.To the above remarks arising out of the theoretical discussion which formed the first part of this paper must be added the following conclusions from the experimental work.