Development of a spray-drying technique for submerged spores of entomopathogenic fungi

The stability of blastospores or submerged conidia of entomopathogenic fungi is one of the key problems associated with the practical use of fungal biopreparations. A spray-drying technique was developed which allows the drying of blastospores and/or submerged conidia, here called submerged spores, without a significant loss of viability and with only a slight delay in efficacy. The method was tested successfully using the fungal species Metarhizium anisopliae, M. flavoviride, Beauveria bassiana and Paecilomyces fumosoroseus. For all experiments, a laboratory spray dryer was used. The optimum inlet and outlet temperatures were found to be 64 +/- 2 degrees C and 48 +/- 2 degrees C respectively. The best protective agent for submerged spores was skimmed milk powder at concentrations of 10 or 20%. The addition of sugars, especially 2.5% sugar-beet syrup, slightly improved the viability after spray-drying. Submerged spores of eight isolates of M. anisopliae, M. flavoviride, B. bassiana and P. fumosoroseus were suspended in 20% skimmed milk powder and 2.5% sugar-beet syrup, and spray-dried. Germination rates of about 90% were achieved, which were comparable to those of freshly produced submerged spores. Finally, the virulences of spray-dried submerged spores of M. anisopliae (Ma 97) and M. flavoviride (Mfl 5) were tested in bioassays using third- and fourth-instar nymphs of the African locust, Locusta migratoria. In contrast to M. flavoviride, there was no significant difference in the median lethal time between spray-dried and fresh submerged spores of M. anisopliae. The investigations demonstrated that spray-drying is possible to preserve sensitive submerged spores of entomopathogenic fungi.