PARTICLE SIZE OF INSECTICIDAL SUSPENSIONS AND THEIR CONTACT TOXICITY IV. MECHANISMS OF ACTION OF DIFFERENT‐SIZED PARTICLES
Authors: McINTOSH A. H. 
Published: February 2008

Two earlier papers showed that in tests of suspensions by dipping, large crystals of DDT could kill grain beetles just as efficiently as colloidal DDT. But with rotenone, colloidal particles were far more toxic than large crystals; this difference was partly in their speeds of action. A third paper showed that in contact action the relative toxicity of small particles is increased by cooling the beetles after treatment. In tests by injection into milkweed bugs, particle size seemed to have no effect on toxicity of rotenone and DDT suspensions if the bugs were kept warm after treatment. In cool bugs, colloid acted more quickly than crystals, but the kills from the two types finally became the same. The time for this to come about was less for DDT than for rotenone and less still for DFDT, an analogue of DDT.

An explanation of these results is now given. In the action of contact poisons, attention is given to the waxy layer on the outside of the cuticle. Contact poisons must first of all dissolve in this wax layer, and it is suggested that the difference in action between rotenone and DDT is due to a difference in their solubilities in wax.

Small particles will always have the advantages, over large, of greater surface area and greater ability to enter openings in the body. With Oryzaephilus surinamensis, a main route of entry by rotenone into the body is possibly through the spiracles, and this may be why colloidal rotenone is so much more toxic than rotenone crystals. The solubility of rotenone in wax is thought to be small and if this is so, it will be easy to saturate the wax and the higher solubility of very small particles of rotenone will be of importance. The behaviour of rotenone particles of different sizes is therefore understandable.

Entry through the body openings is evidently unimportant for DDT, because large crystals kill as quickly as small ones. Penetration must be through the general cuticle. It is suggested that the solubility of DDT in wax is very high, compared with rotenone; the wax will not be easily saturated by DDT and small particles will not have the advantage of higher solubility, which is only helpful if saturation is reached.

It is shown theoretically that if solubility (in wax) does decide the relative behaviour of different‐sized poison particles, then colloidal poison should be more toxic, relative to crystalline poison, if the insects are kept cool after treatment than if they are kept warm.

The following explanation is offered for the injection results. The blood of milkweed bugs contains free droplets of oil, and these are mainly responsible for carrying poison from injected crystals to the site of action. Colloidal poison probably diffuses there directly, and more quickly, in the aqueous phase of the blood. The difference in speeds of action of colloidal and crystalline poison will depend on the ratio of dose to solubility in oil. If this is large, saturation of the blood is easy, and small particles, with their special properties, act more quickly than large. If the ratio is small, the difference in speeds of action may not be detectable.

This seemed to hold for the three poisons tested, but not by any means exactly. The ratios for rotenone, DDT and DFDT were in the expected order.

Finally, it is suggested that in contact action, the relative speeds of kill of different‐sized poison particles may be explained in a somewhat similar way, the solubility in cuticle wax, relative to the necessary dosage, being a controlling factor.