Persistence of Beauveria bassiana sensu lato and Metarhizium anisopliae sensu lato in Vermont (USA) forest soil

Persistence of granular formulations of Beauveria bassiana sensu lato, strains GHA726 and ERL1170, and Metarhizium anisopliae sensu lato, strains ERL824 and ERL701, applied to forest soil was documented. One formulation was prepared by producing fungi on millet and at application adding cooked, dried millet to provide additional nutrients for fungal growth. The other was made by producing fungi on corn starch ‘packing peanuts’, a biodegradable packaging material, adding additional packing peanuts as a nutrient base. The trial was conducted in three sites in a forest predominating in sugar maple (Acer saccharum) in Underhill, VT, USA. In each site, 33 dominant sugar maples were randomly selected. A 1-m² plot located 1 m from the bole of the tree was established. Soil samples were collected before application, and 1 h, 2, 4, 8, 12 and 16 wk post-application. Fungal levels were estimated based on number of colony-forming units (CFUs). A significant increase in CFUs was observed 2 wk post-application in plots treated with isolates GHA726, ERL1170 and ERL824 as a result of fungal reproduction on nutrients in the formulations. Evidence of greater persistence for the B. bassiana than the M. anisopliae isolates was observed, based on CFU levels over time. Isolates formulated with millet proliferated more than those on packing peanuts. Use of entomopathogenic fungi in forest soil has potential for managing soil-dwelling insect pests such as pear thrips, Taeniothrips inconsequens, and the black vine weevil, Otiorhynchus sulcatus, though the type of formulation and the specific isolate used must be selected carefully.