The mode of action of inert dust insecticides on the grain weevil (Calandm p a -L.) has been investigated by determination of mortality-time curves and by a detailed study of the influence of dusts on the water relations of the insects. It has been found that dusts do not depend for heir action on MY chemical reactivity, some highly inert substances such as diamond and carborndurn being very effective. A physical process is therefore indicated.Dusts do not penetrate into the respiratory system, and although large amounts are hgested these seem to have little harmful effect.It has been demonstrated that effective dusts increase the rate of evaporation of water from iasectS to the air, and that the relative killing powers of different dusts run p a d e l with their capacities for promoting evaporation. The results are in harmony with the view (proposed by &ex workers) thot inert dusts kill insects by inducing death by desiccation.

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