Formation of microsclerotia in three species of Beauveria and storage stability of a proptotype granular formulation to control the African black beetle (Heteronychus arator) in New Zealand pastures

The entomopathogenic fungus Beauveria spp. has not previously been described as forming microsclerotia (MS), which are compact, melanised agregates that have been recognised in plant pathogenic fungi as overwintering structures. MS were successfully produced in shake flask cultures from three different Beauveria species (B. bassiana, B. brogniartii and B. pseudobassiana) using liquid media containing a low carbon to nitrogen ratio and replacing glucose with a complex carbon source. Maximum MS yields after 10 days fermentation was 8.46 x 103 MS/mL obtained with B. pseudobassiana AgR-F704, a New Zealand fungus isolated from the African black beetle (Heteronychus arator F.). Using this isolate, MS were produced using two C:N compositions, formulated with diatomaceous earth and air dried and montored for germination and storage stability. Germination of MS granules varied with C:N ratio and storage temperature. MS produced using a higher carbon concentration maintained 100% germination after six months of storage at 4°C, but when stored at 20°C, the germination rate was reduced to 68%. In contrast, MS produced using a lower carbon concentration showed reduced germination (0 to 18%) after storage for more than two months at either temperature and had lost viability completely after six months. Conidia production from surviving MS was estimated at 109 conidia/g of granules. Results demonstrated that MS produced in the relatively higher carbon medium were capable of surviving under low moisture, ambient temperature storage and able to produce approximately 103 conidia/MS after germination; all of these attributes suggest that MS could be used as a new sustained release strategy to control soil-dwelling insects.