Mode of Action and Insecticidal Value of a Diatomaceous Earth as a Grain Protectant1

Abstract

An inert diatomaceous earth of Arizona origin trade named Perma Guard was evaluated as a grain protectant, insecticidal and pathological properties being studied. Dosage-mortality curves are presented for eight test species confined in wheat dusted with Perma Guard at the rate of 2 to 7 pounds per ton, Ranking in order of decreasing susceptibility these species were: flat grain beetle, Cryptolestes pusillus (Schanherr); rice weevil, Sitophilus oryzae (L.); granary weevil, Sitophilus granarius (L.); lesser grain borer, Rhyzopertha dominica (F.); saw-toothed grain beetle, Oryzaephilus surinamensis (L.); dermestid larva, Trogoderma parabile Beal; red flour beetle, Tribolium castaneum (Herbst); and confused flour beetle, T. confusum Jacquelin du Val. Only the flat grain beetle mortality attained standards of commercial kill during the two weeks' exposure period at the commercially recommended dosage (4 pounds per ton) or below. Mortality exceeded 95% at 5 and 6 pounds per ton for this species. At the peak dosages used, the mortality of the remaining species did not approach the criterion for commercial kill, The results indicate that of the two moisture levels (12% and 14%) of wheat used in this study, the lower level contributed to mortality in both weevil species, the lesser grain borer and the saw-toothed grain beetle. The dust was directly applied to five test species. Significant differences in losses in weight were observed between test and control groups for the lesser grain borer, the confused flour beetle and the red flour beetle. Overall differences in losses in weight between test and control lots of rice and granary weevils were not significant. A drastic difference between test and control first-day mortality in the rice weevil may be indicative of an unknown lethal factor in the dust. The significant differences in loss in weight were ascribed to the dust's abrasive action on the water barrier portion of the epicuticle. The toxicology of this diatomaceous dust was studied. Dissections, epicuticular staining. and histological sectioning showed little that could be construed ns epicuticular damage.