The use of entomopathogenic nematodes (EPNs) as biological control agents is impaired by their short shelf life. In this study, in vitro liquid-cultured infective juveniles (IJs) of Steinernema yirgalemense and S. jeffreyense were formulated at high concentrations using diatomaceous earth (DE) formulation of 900,000 and 700,000 IJs/g, respectively, to investigate storage stability and infectivity at different temperatures for 8 weeks. To prevent contamination during storage, antimicrobial agents Nipagin and Procydin were added to the formulation. The two nematode species differed in their survival and infectivity at different storage temperatures. The mean percentage survival decreased over time with all formulations (including the control without microbial agents), though the survival rate was higher at 14°C than it was at 25°C, with room temperature of 67-71% and 42-56%, respectively, after 8 weeks of storage. For the combined effects of antimicrobial agents and DE, Procydin managed to delay the growth of fungi longer than was achieved with Nipagin. However, Procydin lowered the survival rate and pathogenicity of both nematode species against the larvae of Tenebrio molitor. At a lower concentration, Procydin might have the potential for use in EPN formulations directed toward inhibiting fungal growth, though requiring further investigation.