Synergistic interaction of Beauveria bassiana and two commercial formulations of diatomaceous earth on Trogoderma granarium (Coleoptera: Dermestidae)

Bioprotection agents, including living biocontrol agents and naturally occurring non-living substances such as diatomaceous earth, play a crucial role in controlling stored commodity pests to maintain food health. The problem with using diatomaceous earth in storage is that their recommended dose increases grains' bulk density and decreases flowability. Therefore, the present study aimed to determine the biocompatibility between two commercial formulations of Iranian diatomaceous earth, Sayan, and foreign Celite 610 with the indigenous entomopathogenic fungus isolate, Beauveria bassiana DE (Hypocreales), and to solve the problem of reducing grains' flowability using a low-lethal dose of diatomaceous earth in the mixture. For this purpose, 14-day bioassays were first performed by impregnating wheat seeds separately with the fungus and two diatomaceous earths against adults of Khapra beetle, Trogoderma granarium Everts. Then, to determine the interaction effects, five different doses of B. bassiana were mixed with LD25 of each diatomaceous earth. Based on the results, LD50 values for fungus, Sayan®, and Celite 610® were calculated at 295.3, 4439.5, and 992.3 ppm, respectively. According to the lethal dose ratio test, the insecticidal activity of Celite 610® was estimated to be 4.47 times more than Sayan®. Moreover, the interaction was synergistic in all mixtures of B. bassiana with the low-lethal dose of each diatomaceous earth except for the highest dose of the fungus, which was estimated additive. In addition, all mixtures effectively reduced progeny production during the eight-week storage period, indicating the compatibility of these two agents for co-application in the integrated T. granarium management programs.