Bioassays with diatomaceous earth formulations: Effect of species co-occurrence, size of vials and application technique

Two series of laboratory bioassays were carried out in order to evaluate the effect of the simultaneous presence of two insect species inside the experimental vials and the effect of different sizes (50, 65, 90, 145 mm in diameter) of experimental vials on the efficacy of diatomaceous earth (DE) formulations as wheat (Triticum durum Desf.) protectants. Three DE formulations were tested; Insecto, SilicoSec and one DE enhanced with pyrethrum, PyriSec. In a third series of bioassays the influence of application technique of DE formulations in wheat and maize (Zea mays L.) on their insecticidal efficacy was also evaluated in the laboratory. Two DE formulations were tested; SilicoSec and Protect-It. The application technique was based on the duration of admixing (0.5 or 3 min) and surface treatment (without admixing) of DE formulations in grain. In all series of bioassays three species were tested; Sitophilus oryzae (L.) (Coleoptera: Curculionidae), Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) and Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae). Significant differences were noted in mortality levels of the tested species among the three DE formulations and among doses for the simultaneous presence of two insect species in all combinations and the different size of vials. In contrast, no significant differences were found in the mortality levels among the adult densities of any of the insects tested. The progeny production was low for all tested species. The increase of diameter of vials used in the bioassays decreased the mortality of S. oryzae and T. confusum adults significantly whereas no significant differences were noted for R. dominica adults. The increase of dose and exposure interval reduced differences of R. dominica adult mortality in admixture durations in the case of wheat. For S. oryzae, no significant differences in adult mortality in maize were found among admixture durations while significant differences were detected in wheat.

► Increasing vials' diameter decreased the mortality of Sitophilus oryzae and Tribolium confusum. ► Increasing vials' diameter did not decrease the mortality of Rhyzopertha dominica. ► Increasing DE dose moderated R. dominica mortality in admixture times in wheat. ► Increasing exposure moderated R. dominica mortality in admixture times in wheat. ► S. oryzae mortality in maize was not affected by admixture durations.