Entomopathogenic fungi are important biological control agents of insect pests; however, storage stability is one of the main challenges for their commercialisation. In this study, osmoprotective substances were added to formulation prototypes of a native Beauveria bassiana isolate as a strategy to prolong the efficacy during storage. A three-month accelerated thermo-stability study was conducted as selection criteria. In vitro efficacy (corrected mortality) tests were considered in addition to the microbiological response variables commonly reported in the literature (germination and active ingredient concentration – plate count of colony-forming units). The results allowed to select trehalose as the best osmoprotective substance. This prototype had a lethal concentration (LC50) on Diatraea saccharalis (Lepidoptera: Crambidae) larvae of 2.8×105 conidia mL−1, and its efficacy remained higher than 90% when stored at 6°C and 18°C. Mathematical models were constructed according to efficacy data, the shelf-life of the selected prototype was 36.6 months when stored at 6 ± 2°C, 26.0 months at 18°C, and 5.3 months at 28°C. The obtained results emphasise the importance of shelf-life evaluation based on efficacy data of biological activity in addition to microbiological variables that may not be directly related to efficacy. The current study conclusively demonstrates a relationship between temperature and shelf-life.