Factors influencing the Interaction of insecticidal Mists on flying Insects

Abstract

Experiments are described which illustrate some of the ways in which the results of exposing insects to a spray mist are dependent on either their behaviour or physiological state. Only fine mist dispersals with particles below 10 microns in diameter are considered and in such mists the flight movements of the insects are very largely responsible for the collisions between the insect and the droplets. An increase in flight activity leads to the accumulation of a greater number of spray droplets and consequently a higher kill. Under these conditions insecticides such as pyrethrins which exert a strong irritating action on Aëdes in minimal amounts are superior to DDT which does not have this effect. For practical applications under circumstances where the sprays depend on mist action to produce their effect it is advantageous to mix pyrethrum with the DDT sprays so that the maximum dose of the latter may be accumulated.Further experiments are concerned with the relative resistance of male and female Aëdes aegypti to insecticides and the change of resistance which takes place on ageing or as a result of a blood meal. It is shown that the male is innately more susceptible to both pyrethrins and DDT and that the resistance of the females is higher after a blood meal. There is also a progressive decrease in resistance with age.