Persistence of augmented Metarhizium anisopliae and Beauveria bassiana in Finnish agricultural soils

The persistence and penetration into soil of surface-applied unformulated conidia of two isolates of Metarhizium anisopliae and one of Beauveria bassiana at sites with clay, peat and two kinds of sand as their soil types and at depths of from 0 to 20 cm was studied in 1988-1991 under conditions characterized by permanent snow cover and frozen soil in the winter time. At 0-5 cm depth, M. anisopliae persisted throughout the experiment at all sites, clay being the most and peat the least favourable soil for persistence. Clay and one of the sandy soils were the least and peat the most conducive soil to penetration of M. anisopliae from the surface to deeper soil layers and persistence therein. Differences in persistence were evident between the two M. anisopliae isolates in the sandy and peat sites, but not in the clay site. Three years post-application there were still enough infectious propagules of M. anisopliae in soil of all sites to infect over 80% of the Tenebrio molitor larvae used as baits in samples taken from the cylinders of all soils. All the augmented propagules of B. bassiana disappeared during the first winter after application in clay and one of the two sandy sites, but some persistence one year post-inoculation was evident at 0-5 depth in one of the sand soils and at 0-5 and 5-10 cm depths in peat.