The considerations affecting the design of a spray testing chamber are discussed, and a description is given of equipment specially intended for use with mosquitos as the test insect. In essentials the apparatus consisted of a metal chamber surrounded by a “Masonite” cabinet with an air space in between. In this space air, at a constant temperature, was circulated around the outer face of the inner metal chamber and so controlled the temperature of the spray chamber. The cabinet was fitted with lights; wet and dry bulb thermometers to check the humidity, which was controlled by blowing in steam; a fan to circulate the air; a spray gun; four ports through which the insect exposure cages were introduced into the chamber; a shelf to carry these cages; and an exhaust and an intake port to allow the mist to be removed at the conclusion of the test. Finally, there was a door on one side allowing access to the interior. It will be noted that the insects were introduced into the spray chamber in cages and not allowed to fly freely. This greatly facilitated the collection of insects at the conclusion of the test. Under the routine testing conditions adopted the insects were not introduced until four minutes had elapsed after spraying. They were then exposed for ten minutes and afterwards transferred to clean recovery cages.

The practice of caging the insects and placing them on a shelf opposite a window within the spray chamber permitted the insects to be closely observed and the rate of paralysis (knockdown) to be followed. In general, however, the rate of knockdown, when it occurs, was not so sensitive a method of comparing insecticidal efficiency as the twenty-four hour percentage kill.

In practice the apparatus was found to give satisfactory results. It permitted accurate comparisons to be made between different strengths of a given insecticidal solution, but care had to be taken to ensure that the non-volatile content of the sprays being compared for toxicity was essentially similar. When this was not so, other things being equal, the spray which produced the less volatile mist (i.e., the larger particle size) gave the higher kill. When working on activators, which are usually non-volatile materials, it was therefore necessary to add an equivalent quantity of, say, high vacuum pump oil to the control spray before making the comparison. Examples of the application of the method of pyrethrum, DDT, and activated sprays are given.

The set of standard conditions chosen for the routine testing procedure were sometimes rather arbitrarily fixed but usually with due reference to the requirements of the insects or to practical testing conditions. Since two of the conditions chosen, i.e., the use of caged insects and exposure after an interval of four minutes after spraying, were rather artificial from the point of view of practice and might conceivably have led to conclusions inapplicable to practical conditions, the effect of modifying certain of the standard procedures was investigated. The results of these experiments are fully described in the text. It was concluded that temperature influenced the level of kill recorded considerably, the kill being lower when the temperature of the spray chamber was dropped 10°C. Humidity seemed to have little or no effect within the range 40–80 per cent. R.H. Increasing the interval at which the insects were inserted after spraying from 0 to 10 minutes affected the level of kill without influencing the comparative efficiency of the insecticides examined. On the other hand caging the insects had a pronounced effect on the comparative efficiency of pyrethrin sprays with and without added non-volatile material. This was thought to be due to the delay in access of the spray droplets to the caged insects which permitted a sufficient interval for the differences in particle size to become established. With caged insects inserted before spraying, therefore, the biological efficiency of a pyrethrin spray without, added non-volatile material is much lower than when some such substance as high vacuum pump oil is added, whereas this difference is much smaller when the insects are exposed free in the cabinet (Table X).