Toxicity Studies of So-Called “Inert” Materials With the Rice Weevil and the Granary Weevil*

Abstract

Crystalline silica was most effective of 6 materials tested against both the rice weevil (Sitophilus oryzae) and the granary weevil (S. granarius). MgC03, amorphous silica, bentonite, talc and walnut shell flour were less effective in the order named. Within the range tested, from 1-150 [mu], there is a close correlation between toxicity and particle size, the smaller particles being most effective. The relative humidity of the environment is an important factor influencing the toxicity of the crystalling silica dust. The killing effect is much more rapid at low humidities than at high, although there is a definite action in atmospheres above 95% saturated. Insects dusted with effective materials lost weight rapidly; those dusted with walnut shell flour paralleled the check insects. Observations on (1) the loss of weight of dusted insects, (2) the difference in insecticidal efficiency of the inert materials when used with and without food and (3) the action of the materials in saturated atmosphere indicate that the effect of these dusts lies in their power to cause desiccation through conducting water away from the body surface and through the mechanical irritation they cause.