Factors influencing the Action of Dust Insecticides

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Abstract
The methods used to investigate the properties of dusts are described and, in a theoretical section, the relevance of the various physical properties to insecticidal action are considered.In order to investigate the effect of toxic and non-toxic dusts on insects the experimental procedure was simplified to eliminate all difficulties associated with the formation of uniform dust clouds and deposits. In problems relating to the adherence of the dusts to insects the actual quantity of dust was measured either by weighing the insects before and after dusting or by dyeing the dust with Sudan III and determining the quantity colorimetrically. The experiments were all conducted under known conditions of temperature and humidity.Non-toxic dusts killed insects by causing them to lose water. Not all non-toxic powders were equally effective when conditioned to the same relative humidity. All were without effect at saturated humidity and became progressively more rapid in action as the humidity at which the test was carried out was decreased (p. 32).The non-toxic dusts caused the insects to lose water by abrading certain areas of the cuticle ; the more extensive the abrasion the more quickly the insects died (p. 27).To be effective as an abrasive the dust must be hard and finely ground and, perhaps also, sharply angular. Thus materials which ranked high in Moh's scale of hardness were in general more effective than soft materials and hard materials became quite ineffective unless they contained material below about 10 μ, diameter. Presumably the coarser materials could not gain access to the articulations, etc., where abrasion usually occurred. This effect can be seen clearly with carborundum powders. Sharply angular glass was more effective than the same powder converted into rounded spheres (pp. 25–31).