Both solar UVA and UVB radiation impair conidial culturability and delay germination in the entomopathogenic fungus Metarhizium anisopliae

The entomopathogenic hyphomycete Metarhizium anisopliae has been used in pr ograms of agricultural pest and disease vector control in several countries . Exposure to simulated solar radiation for a few hours can completely inac tivate the conidia of the fungus. In the present study we determined the ef fect of exposures to full-spectrum sunlight and to solar ultraviolet A radi ation at 320-100 nn (UVA) on the conidial culturability and germination of three M. anisopliae strains. The exposures were performed in July and Augus t 2000 in Logan, UT. The strains showed wide variation in tolerance when ex posed to full-spectrum sunlight as well as to UVA sunlight. Four-hour expos ures to full-spectrum sunlight reduced the relative culturability by approx imately 30% for strain ARSEF 324 and by 100% for strains ARSEF 23 and 2575. The relative UV sensitivity of the two more sensitive strains was differen t under solar UV from that under ultraviolet B radiation at 280-320 nn (UVB ) in the laboratory. Four-hour exposures to solar UVA reduced the relative culturability by 10% for strain ARSEF 324, 40% for strain ARSEF 23 and 60% for strain ARSEF 2575. Exposures to both full-spectrum sunlight and UVA sun light delayed the germination of the surviving conidia of all three strains . These results, in addition to confirming the deleterious effects of UVB, clearly demonstrate the negative effects of UVA sunlight on the survival an d germination of M. anisopliae conidia under natural conditions. The negati ve effects of UVA in sunlight also emphasize that the biological spectral w eighting functions for this fungus must not neglect the UVA wavelengths.