Two naturally-occurring strains of the fungus Beauveria bassiana have potential as biological control agents for both house fly and stable fly. These strains were originally isolated from field-collected house flies. The two fungal strainsin two different formulations, a dust and a water solutionwere tested against adult house and stable flies. The spores were applied to plywood surfaces and the adult flies were exposed to the treated surfaces for three hours. Fly mortality occurred within 7 days of exposure to either strain. The dust formulation was better than the wet formulation in causing infection and mortality. House flies were killed at most of the dosages tested, while stable flies were not as susceptible. In these studies fungal virulence (the ability to infect and kill flies) decreased with time, so repeated B. bassiana applications may be necessary to achieve good adult fly control.
Since house flies readily breed in animal bedding, B. bassiana was also tested against house fly larvae in manure and sawdust bedding mixtures. Fungal spores were thoroughly mixed with larval medium (fresh cow manure, sawdust bedding and water), then fly larvae were added to the medium. However, the fungus was only marginally effective against the larvae, and only at high doses.
Adult flies normally rest on the walls and ceilings of dairy barns, so treating building surfaces may be a convenient way to reduce fly populations. Such applications would also reduce the chance of exposing beneficial insects to the spores (B. bassiana also kills fly pupal parasites and probably other natural enemies). However, dust formulations (that worked best in these experiments) do not stick to vertical surfaces very well. Further studies to improve formulations for application to vertical surfaces are necessary before B. bassiana could easily be used as a biological control agent of flies.
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