Insecticidal effect of diatomaceous earth on grains infected with fungi

The coexistence of fungi and insects in stored grains often results in synergistic effects that accelerate grain deterioration and increase mycotoxin contamination. Diatomaceous earth (DE) is widely used as inert dusts for insect control, but their efficacy on grains already infested by fungi has not yet been evaluated. In this study, the effect of a commercial DE formulation was examined on wheat kernels that were either fungus-free or infected by fungi (mainly Fusarium graminearum with minor proportions of Aspergillus and Penicillium ). Bioassays were conducted with the lesser grain borer, Rhyzopertha dominica (F.), the rice weevil, Sitophilus oryzae (L.), and the granary weevil, Sitophilus granarius (L.), under two DE concentrations (500 and 1000 ppm). Results indicated that DE completely suppressed progeny production of S. oryzae and S. granarius in fungus-free wheat, but the presence of fungi moderated this effect, with even low levels of fungal infestation supporting progeny survival. Conversely, kernels fully infected by fungi (100%) resulted in complete mortality of both Sitophilus species, suggesting a threshold beyond which fungal development dominates over insect survival. For R. dominica , progeny production was limited regardless of treatment. These findings demonstrate that fungal presence strongly influences DE efficacy, highlighting the complex interactions between fungi, insects, and inert dusts in stored grain ecosystems. Considering that fungi can both reduce and enhance insect survival depending on their prevalence, the integration of DEs in pest management should also take into consideration fungal infestation levels to optimize efficacy. • DE fully suppressed Sitophilus spp. on fungus-free wheat, but efficacy declined when fungal contamination was present. • Fusarium graminearum was the dominant fungus and strongly influenced both insect survival and DE performance. • Rhyzopertha dominica produced low progeny across all treatments, showing limited sensitivity to fungal levels or DE concentration. • Fungal presence can either impair or protect insects from DE, indicating that fungal infestation level should be considered in DE-based grain protection strategies.