Three parental strains ofLecanicilliumspp. and eleven hybrid strains previously obtained by protoplast fusion were evaluated for their virulence to different life stages ofTrialeurodes vaporariorum.Egg treatment yielded no statistically significant difference on the hatching rate of eggs as opposed to the mortality of subsequent hatched larvae which was over 80% by 4 hybrid strains and the commercial product Vertalec(R). Among the strains, hybrids 2aF1 (4.6 x 10(4) conidia/ml) and 2aF43 (5.7 x 10(4) conidia/ml) showed one order lower concentration of LC(50)value compared with Mycotal(R)(6.7 x 10(5) conidia/ml). Mortality of 1st instar larvae infected through direct inoculation and hatched larval mortality through egg inoculation resulted significantly in lower LC(50)values in most of the tested strains. In bioassays against 4th instar larvae, Vertalec(R)provided the highest mortality (65.2%) with other strains producing mortalities between 11.1% and 43.0%. Four hybrid strains of 2aF4, 2aF30, 2aF43 and 2aF31 produced significantly higher mortality against adults than Mycotal(R)and Vertalec(R). Our results demonstrated that many hybrid strains exhibited higher pathogenicity than that displayed by their parental strains, suggesting that protoplast fusion can be useful to produce strains with greater potential for the development of powerful microbial control agents.