Ecology of Metarhizium anisopliae in soilless potting media and the rhizosphere: implications for pest management

Wholesale container-grown ornamentals are often maintained at the nursery for at least two growing seasons and are subject to infestation by black vine weevil (BVW), Otiorhynchus sulcatus F., for several months each year. Therefore, a potting media amendment aimed at controlling BVW needs to persist for an extended period of time. These studies were conducted to determine the persistence and ecology of Metarhizium anisopliae (Metchnikoff) Sorokin incorporated into peat and bark-based potting media with and without a crab meal amendment in container-grown Picea abies 'Nidiformis.' Rooted cuttings of P. abies were planted into potting media amended with M. anisopliae (1 g of formulated product/L; ∼6 log10 CFU/g dry potting media). The fungal population in bulk potting media was quantitatively determined using selective media at 14, 21, 28, 35, 49, 63, 77, 91, 105, 119, 143, 175, 203, 231, 258, 287 and 342 days. The fungal population in the rhizosphere was quantitatively determined at 203, 231, 258, 287, and 342 days. M. anisopliae colonized the rhizosphere of P. abies and the fungal population in the rhizosphere was significantly greater than in the surrounding bulk media. M. anisopliae persisted in the peat and bark-based potting media at 6.22 and 5.74 log10 CFU/g dry potting media for 342 days, respectively. Bioassays using bark and peat-based potting media inoculated with M. anisopliae at 6 log10 CFU/g dry potting media resulted in 93.5% and 97.5% infection of last instar BVW, respectively. P. abies roots inoculated with M. anisopliae infected 76% of 2nd-3rd instar BVW. Inoculation of roots with M. anisopliae represents a novel method for delivering entomopathogenic fungi and would greatly reduce application costs. Factors associated with fungal biology outside the host may be more important than virulence in a laboratory bioassay.