In the present study, bioassays were conducted to evaluate the insecticidal efficacy and to highlight the factors that contribute to the effectiveness of diatomaceous earths and zeolites, which are mined by the company “Olympos”. The different milling of diatomaceous earth’s rocks (grinding, diatomaceous enrichment, powder granulation), has led to the creation of five types of formulations (type 1, type 2, type 3, type 4 and type 5), with different insecticidal effect. Moreover, one amorphous zeolite formulation, with the only treatment to be its conversion from rock to fine powder, was tested for its insecticidal activity. Different mortality percentages, with the lowest of them to refer to Tribolium castaneum, Tribolium confusum and Rhyzopertha dominica, with Sitophilus oryzae, Oryzaephilus surinamensis and Cryptolestes ferrugineus to follow in order from the more tolerant to the most susceptible, were observed among the types of the formulations tested. At the same time, diatomaceous earth types 1, 2 and 3 showed low efficacy in all species, in contrast to the high mortality which achieved with the formulations of types 5 and 6 as well as the one of the zeolite, when the powders were applied in soft organic wheat. The same phenomenon was not recorded in the bioassays with barley, rice and maize, as none of T. confusum, R. dominica and S. oryzae appeared to be affected.
The effects of relative humidity and temperature on the insecticidal efficacy seems to have a major role, since when relative humidity increases or temperature decreases, their effectiveness was negatively affected. The adherence ratios for the tested types 5 and 6 of DE and for the zeolite formulations were generally high against S. oryzae and R. dominica, even after 5 months of application to soft wheat and maize. Bioassays using type 5 and 6 of diatomaceous earth and the one of zeolite on cement and steel surfaces, have shown very encouraging results for a successful disinfection by using these dusts in empty warehouses or silos, as surface treatments. Significant variations among the bulk density of soft and hard wheat, maize and rice were recorded, when different doses of all the types of DE and of the zeolite were applied in them. Although the higher doses of the most effective types 5 and 6 of DE and zeolite showed an reduction in the bulk density of soft wheat, the lower doses of types 1, 2 and 3 increased the bulk density of the same cereal.