Comparative Tolerances of Various Beauveria bassiana Isolates to UV-B Irradiation with a Description of a Modeling Method to Assess Lethal Dose

Abstract

The tolerances of 20 Beauveria bassiana isolates derived from host insects worldwide to UV-B irradiation were assessed quantitatively in multi-dose bioassays. Conidial suspensions of the isolates smeared on glass slides were exposed to the gradient UV-B doses of 0.1-1.6 J cm(-2) (D), which generated from 0.75 to 10.17 min irradiation of weighted 312-nm wavelength at 2.0-2.61 mW cm(-2). Irradiated conidia were then incubated for 24 h at 25 degrees C under saturated humidity. The ratio of germination at each dose over that in the blank control was defined as survival index (I (s)). For all isolates, the I (s) - D observations fit well with the survival model I (s) = 1/[1 + exp(a + bD)] (0.94 < or = r (2) < or = 0.99) generated widely spanned lethal doses of 0.154-0.928, 0.240-1.139, and 0.383-1.493 J cm(-2) for their losses of 50%, 75%, and 95% viabilities, respectively. These were far below the solar UV-B dose of 2.439 J cm(-2) measured in a sunny day during the summer. The large variation of UV-B tolerance among the isolates indicates a necessity to select UV-tolerant candidates for formulations applied to insect control during summer. The highly efficient bioassay method was developed to measure accurately the UV-B tolerances of fungal biocontrol agents as lethal doses.