## 4 Discussion  
Our data indicate that there are specific formulations that were effective at a relatively low concentration, when applied on surfaces. For the DE formulations tested, [PERSON] and [PERSON] [28] described specific modifications that could be performed in order to improve their insecticidal value; these modifications mainly included sieving, drying and smashing of the DE particles. In principle, smaller particles are more effective than large ones, but can negatively affect the bulk density of the grains on which they are applied [15; 16]. Moreover, the shape of the diatoms can be also related with their insecticidal effect, and \"flat shaped\" with greater number of pores may have a higher wax adsorption from the insects' cuticle [14; 16]. Still, the way that the insects are in contact with the inert dust particles that are applied in surfaces might be different than that of the particles that are applied on grains, where feeding from the treated substrate also takes place. The comparison of the two surfaces, taking the entire mortality data into consideration, suggests that the materials used are equally effective on both concrete and steel, after a week of exposure. It is generally considered that most contact insecticides are more effective on steel than on concrete, since the latter is porous and alkaline, which are factors that contribute to the dissipation of the insecticide [31; 32; 33; 34]. For instance, [PERSON] et al. [35] tested four organophosphorous insecticides on concrete and galvanized steel and found that the efficacy of these active ingredients against three posids species was higher on steel than on concrete, due to the fact that they are quickly hydrolyzed in the alkaline concrete surface. However, there are cases where the tested compounds are equally effective on different surfaces [36; 37; 38]. In fact, [PERSON] et al. [39] reported that spinosad was more effective on concrete than galvanized steel for the control of _T. castaneum_ and _T. confusum_. The fact that the inert materials evaluated here had no difference in their efficacy between the two surfaces tested can be considered as a positive characteristic, given that these dusts can be used in different application scenarios, without loss in efficacy. Moreover, for some of the species tested, the \"speed of kill\" of adults was comparable with other insecticides that are used in surfaces [32; 37], while mortality was found to be more of a species-mediated parameter. Figure 6: Mortality (%) of _T. confusum_ adults after exposure for 1, 3, 7 and 14 d at three different doses [0 (control), 0.5 and 1 g/m\({}^{2}\)] of DEs and zeolite applied on concrete surfaces (boxplots followed by the same letter are not significantly different, according to the Dunn test at \(p\) < 0.05. Where no letters exist, no significant differences were noted). From the species tested here, we can conclude that both _S. oryzae_ and _R. dominica_ were far more susceptible than _T. confusum_ to all formulations. From the most to the least susceptible, these three species can be classified as _R. dominica_\(>\)_S. oryzae_\(>\)_T. confusum_. Earlier studies have confirmed that the species of the genus _Tribolium_ are probably the most tolerant stored product beetle species to inert dusts [6; 8; 40]. Interestingly, _R. dominica_ was the most susceptible among the three species tested, despite the fact that this species is less agile than the other two [41; 42]. Both _S. oryzae_ and _R. dominica_ are primary colonizers and can easily infest sound grain kernels, so they are most likely to be found in bulked grains, and not in surfaces, as compared with _T. confusum_, which is mostly found in processed commodities and processing facilities, such as flour mills. The application of these dusts as \"barriers\" to limit the spread of primary colonizers to infest additional areas with bulked grains has been found to be a viable solution, given that the presence of inert dusts has a repulsive action in some stored product insect species [1; 8; 43].