An alginate-based encapsulation enhances shelf life and bioactivity of the entomopathogenic fungus, Metarhizium anisopliae

Background This study evaluated the shelf life and effectiveness of Metarhizium anisopliae encapsulated in Na (sodium)-alginate matrix. Air-flow-dried and freeze-dried beads containing conidia of M. anisopliae produced in submersion in the liquid broth were fabricated and characterized. Shelf life and efficacy of fabricated beads against three species of the white grubs, namely Holotrichia serrata , Adoretus sp., and Lepidiota mansueta, were evaluated under greenhouse conditions. Results The mean size of the wet beads ranged between 1.4 and 3.2 mm. The mean size of beads that were dried through freeze drying ranged between 0.55 and 2.3 mm. However, the mean size of beads dried through air flow ranged between 0.55 and 2.35 mm. Viability of M. anisopliae was more in freeze-dried beads stored at 8 °C compared to air-flow-dried beads with 20.0 × 10 7 and 16.00 × 10 7 CFU/g, respectively, after two weeks of drying. Further, freeze-dried beads supported M. anisopliae conidial viability till 10 months with 2.0 × 10 7 CFU/g. The LC 50 values of the freeze-dried beads with M. anisopliae after 21 days of treatment were 0.35 (2.1 × 10 8 CFU/g), 0.31 (2.1 × 10 8 CFU/g), and 0.76 mg (2.1 × 10 8 CFU/g) against H. serrata, Adoretus sp., and L. mansueta, respectively. Conclusion The results of this study support the use of Na-alginate beads followed by freeze drying to maintain the viability of M. anisopliae. The effectiveness of freeze-dried beads with M. anisopliae against white grubs indicates that these beads can be an alternative option of synthetic insecticides against soil dwelling insect pests. In addition, this study offers a steppingstone for further advancement of sodium alginate encapsulated formulations for soil insect control.