Effect of the combined use of Metarhizium anisopliae (Metschinkoff) Sorokin and diatomaceous earth for the control of three stored-product beetle species

Laboratory bioassays were conducted in order to evaluate the use of the entomopathogenic fungus, Metarhizium anisopliae (Metschinkoff) Sorokin (Deuteromycotina: Hyphomycetes), against adults of three stored-grain beetle species, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae). Two fungal preparations were compared, a conidial suspension and a conidial powder. These were applied to wheat, at three dosages, 8×106, 8×108 and 8×1010 conidia/kg of wheat, alone, or in combination with the diatomaceous earth (DE) formulation SilicoSec® (Biofa Gbmh, Germany), applied at 0.5 g/kg of wheat. Adult insects were exposed to treated wheat for 24 h, 48 h, 7 d and 14 d. The mortality of R. dominica adults after 14 d of exposure to the treated substrate was 100% and 96% at the dosages in combination, for the suspension and the powder, respectively. The respective figures without DE were 94.4% and 74.6%. In contrast, against S. oryzae adults, the application of conidial suspension combined with DE was not as effective as the application of DE alone. Adult mortality of S. oryzae increased notably on wheat treated with the conidial powder. Similarly, the conidial suspension, with or without DE, was not as effective against T. confusum as the conidial powder. The progeny production of R. dominica on wheat treated with the highest suspension dosage, with or without DE, was significantly lower than that for the other aqueous fungal dosages. Moreover, significantly less progeny were produced on wheat treated with the highest dosage of powder conidia combined with DE, in comparison with the other treatments. In contrast, S. oryzae progeny production was notably reduced only in wheat treated with the highest dosage of fungal spore powder mixed with DE. No T. confusum progeny were found on wheat treated with the highest dosage of both fungal preparations combined with DE.