Persistence and efficacy of Beauveria bassiana against the house fly, Musca domestica L., on typical structural components of poultry houses

Entomopathogenic fungi have been shown to have great potential as alternatives to chemical pesticides for control of flies in layer production facilities. When utilized as an internal residual spray (IRS), persistence of the spray residue is a critical element for ensuring long-term efficacy.  Conidial longevity depends on interplay between abiotic factors, isolate choice, substrate type and delivery system. Oil-formulated conidia of Beauveria bassiana were sprayed onto a range of typical structural components of poultry houses and contractor grade plastic sheeting as a potential wall-coating material and examined for persistence and efficacy with repeated fly exposures at 1, 7, 15 and 30 days post-spray application.  All treated surfaces resulted in 100% fly mortality within 6-10 days 1 day after spray application. However; efficacy declined rapidly following subsequent exposures. Conidia on surfaces that were not exposed to flies remained viable for up to three months, maintaining ≥75% viability for two weeks regardless of substrate types, application methods, or production batch. We investigated the impact of different fly densities on the persistence and infectivity of conidia in spray residues, and demonstrated that house flies both deactivate and remove fungal conidia from spray residues in a density dependent manner. While this poses a challenge to management of flies with fungal-based biopesticides, further research will establish an operational relationship between fly density and spore deactivation and removal rate to optimize application parameters and examine spray efficacy under simulated field conditions.