Effects of in vitro passage of Beauveria bassiana on virulence to Bemisia argentifolii.

Effects of repeated in vitro subculture on the virulence and morphological characteristics of entomopathogenic fungi appear to vary considerably among isolates and species. Some reports indicate a loss of pathogenicity in isolates of Beauveria bassiana, Metarhizium anisopliae, Nomuraea rileyi, Paecilomyces farinosus, and Verticillium lecanii (Kawakami, 1960; Schaerffenberg, 1964; Nagaich, 1973; Fargues and Robert, 1983; Morrow et al., 1989), whereas other isolates apparently retain their virulence after repeated transfer (Ferron et al., 1972; Hall, 1980; Hayden et al., 1992). Potential loss of virulence is highly relevant to the maintenance of strains in general and in the production of strains for comparative bioassays. It is particularly critical for commercialized strains that undergo several routine conidium-to-conidium cycles prior to large-scale fermentation. The potential for virulence loss has obvious implications for the quality of product resulting from a particular fermentation batch; maintenance of virulence throughout the mass-production process is essential to ensure the consistent quality of the marketed material. The objective of the current study, therefore, was to assess effects of repeated in vitro passage on the virulence of the B. bassiana GHA strain, the active ingredient in the commercial products BotaniGard and Mycotrol (Mycotech Corp., Butte, MT). Vidal et al. (1997) and Fargues et al. (1997) initially passed fungal isolates through insect hosts in an attempt to ensure that full insecticidal activity was restored to each of the test strains prior to bioassay. In this way, differences detected in their relative pathogenicity for the target pests could be attributed to real interstrain variation rather than a function of prior culture conditions. The B. bassiana GHA strain was originally isolated from a chrysomelid beetle and deposited as ARSEF201; it was subsequently passed through grasshoppers in 1990 and later (1995) through Bemisia nymphs (S.T. Jaronski, unpublished data). The preliminary culture used in the current study was derived from a single spore isolate of the Bemisiapassaged GHA strain (S.T. Jaronski, unpublished data); to ensure the virulence of the initial culture used in the trials, the fungus was first passaged through