Particle size of insecticidal suspensions and their contact toxicity

With each of nine DDT analogues, two types of simple aqueous suspension were made, one containing crystals of uniform size, and the other containing colloidal particles. The toxicities of each pair of suspensions were compared by a dipping method on adult Oryzaephilus surinamensis L., kept at 11 degrees C. for 24 hr. after treatment. The colloid was always as toxic as the crystals, or more toxic. This difference in toxicity varied from one analogue to another. It was related to the physical properties of the analogue in the following way. Dipping applies poison to the insects; the amounts retained could usually be found by extraction and microanalysis. Retention of poison from colloidal suspension was about the same for each analogue; but crystals were retained either more or less efficiently than colloid, depending on their size and shape. Poorest retention was found with plate-like crystals of about 25 mu. The true ratio of toxicities for each analogue (colloid : crystals) was found by correcting the observed ratio for differences in retention; but even then there were still big differences amongst the ratios given by the different analogues. The true ratio was taken as a measure of the difference in speed of action between the two forms of poison. If the compound could kill O. surinamensis by fumigant action alone, the true ratio was very small. Otherwise, it seemed to depend on two physical properties related to the process of solution of poison in the wax of the insect cuticle. They were both measured in in vitro tests. The film made by evaporating a drop of colloidal suspension on glass is at first globular, but it may sooner or later crystallize. The film made by evaporating a drop of a suspension of crystals can be dissolved in olive oil. If the film of crystals brought about saturation of olive oil slowly and if, in addition, the deposit from colloid crystallized slowly, the true ratio was large. Unless the analogue had both of these properties, the true ratio was small. If the analogue had one or other of the properties, the true ratio could be increased either by increasing the crystal size or by decreasing the rate of crystallization. Physical properties of this sort may often affect the results of tests of insecticides, and should be taken into account when the toxicities of different compounds are compared. There seems to be no relation between lipoid-solubility and toxicity of compounds in the DDT group. Rate of solution is probably more important