Evaluation of biologically synthesized zinc oxide nanoparticles from Paecilomyces farinosus on the biology of sawtoothed grain beetle (Oryzaephilus surinamensis)

The sawtoothed grain beetle, Oryzaephilus surinamensis, is a major pest of stored grains such as rice, wheat, barley, and legumes. This insect is highly adaptable and poses a persistent threat in diverse climatic conditions and storage environments. The present study aimed to determine the effect of the fungus Paecilomyces farinosus on the life stages (eggs, second and fourth instar larvae, and 24-hour-old pupae) of O. surinamensis. The fungus was tested at spore densities of 3×10⁷, 3×10⁵, and 3×10³ spores/ml. The results showed that the highest mortality rates for eggs, second instar larvae, fourth instar larvae, and 24-hour-old pupae at a concentration of 3×10⁷ spores/ml were 40.3%, 58.7%, 39.01%, and 55.6%, respectively. Furthermore, the study evaluated the impact of nano-zinc oxide (ZnONPs) on O. surinamensis at concentrations of 3000, 2000, and 1000 ppm. Various characteristics of the prepared ZnONPs, such as shape, size, and maximum absorption by ultraviolet spectroscopy, were analyzed. The results indicated 100% mortality of eggs treated with concentrations of 3000 ppm and 2000 ppm. The highest mortality rates for second instar larvae and 24-hour-old pupae were 80% and 20%, respectively, at a concentration of 2000 ppm. For fourth instar larvae, the highest mortality rate was 68.22% at a concentration of 3000 ppm. The study demonstrated that ZnONPs at all tested concentrations were more effective than the fungal suspension in causing mortality of eggs and second and fourth instar larvae. Moreover, the germination rate of rice grains treated with direct spraying was not affected by any of the treatments.