RELATION BETWEEN PARTICLE SIZE AND SHAPE OF INSECTICIDAL SUSPENSIONS AND THEIR CONTACT TOXICITYL: I. D.D.T. SUSPENSIONS AGAINST TRIBOLIUM CASTANEUM Hb

Fairly coarse aqueous D.D.T. suspensions can be made by the exchange of solvent method. By varying the method of preparation the mean size of crystal in suspension can be altered. The principles involved are outlined, and methods and apparatus are described for the preparation of the following suspension types: colloidal D.D.T., a suspension containing elongated hexagonal plates, a suspension of somewhat elongated two‐dimensional aggregates of plate‐like crystals, and three suspensions of needle‐shaped crystals of different lengths. All the D.D.T. crystals are of the same fundamental crystal habit. These suspension types were selected to study the effect of both crystal size and shape on toxicity. The suspensions were tested against Tribolium castaneum Hb., using a recently developed dipping technique. It was shown that the results obtained were independent of any operation peculiar to the technique. Suspensions tested by a spraying method gave data similar to those obtained by dipping, except for certain anomalous results obtained where the crystals were considerably broken up in passing through the spray nozzle. Toxicity differences were assessed by the probit method. The lines obtained for the crystalline suspensions were generally parallel to one another, but the colloidal D.D.T. gave a line differing in slope from the others. The dipping tests showed that, within the range of crystal sizes up to about 400 μ, toxicity increased with increasing needle length; breadth was less important, but a suspension of needle‐shaped crystals was as toxic as one containing considerably larger plate‐shaped crystals at the same weight/volume concentration. Colloidal D.D.T. was less toxic than any crystalline suspension. Thus, taking the median lethal concentration for 400 μ needles as 100, the values for the other types were: for 120μ needles, 210; for plate aggregates (240 × 140 μ), 260; for 40μ needles, 390; for 60 × 15 μ plates, 510; and for colloidal D.D.T., 1700. These differences in toxicity were largely paralleled by retention of greater amounts of D.D.T. from the coarse suspensions than from finer ones; two extreme types were of almost identical toxicity, comparing the percentage kills produced by equal amounts of poison retained, and not by equal concentrations in suspension. The methods of carrying out these retention tests are described.