The physical state of deposits from various formulations of insecticides applied to different materials is considered in relation to the availability of the insecticide to insects making contact with the surface of the treated material.

Applications of oil solutions of insecticides to absorptive material result in absorption of the solution, and the insecticide is thereby rendered ineffective. On a nonabsorptive surface, oil solutions are the most efficient formulations, but only so long as the deposit remains wholly or partly liquid; if evaporation of the solvent gives a dry crystalline residue, this is much less efficient than the original solution.

On some fibrous absorptive surfaces, a solution of DDT in oil remains in a supersaturated condition in and around the fibres. If crystallisation is stimulated by mechanical means, the resulting crystals project from the treated surface and it is suggested that, in these circumstances, crystals are more readily available to insects than the oil solution from which they were derived.

DDT on compressed wall board shows the induced crystallisation to a greater degree than other insecticides. It is concluded, therefore, that wall board and similar surfaces should not be used in comparing the potencies of chlorinatedhydrocarbon insecticides applied as oil solutions, as DDT would be more available than others. Some factors that affect the degree of induced crystallisation are mentioned.