Study of the Entomophthorales infection in selected black fly populations from two outlets of a lake was conducted at the Reserve Faunique du Saint-Maurice (Quebec). Entomophaga near limoniae infected Simulium verecundum/rostratum, whereas Erynia curvispora predominantly parasitized Simulium decorum, and Erynia conica infected Simulium venustum and the Simulium verecundum/rostratum and Simulium vittatum complexes. This is the first report of the presence of Erynia conica and Entomophaga near limoniae in North America and Erynia curvispora in Canada. Entomophaga near limoniae in black flies represents a new association. / A successful method for rearing Simulium rostratum, and the in vitro production of fungal conidiospores, produced a sufficient supply of material to study the cuticular invasion process of Erynia conica in the laboratory. Both Erynia species exhibited a diurnal periodicity in the formation of secondary conidiospores in darkness in vitro. This mode of development was influenced by the pH of the medium. The cyclic pattern in secondary conidia formation by Erynia conica disappeared with exposure to light, which suggests that photoperiod may be a factor contributing to host infection. Only secondary type 2 conidia of Erynia conica produced germ tubes that invaded the cuticle of the proper host, Simulium rostratum as opposed to the primary conidia. The infective unit exhibited delayed germination and formed neither appressoria nor invaded the cuticle of the non-host, Simulium decorum, which may explain the host specificity observed in the field study. Cuticular lipids triggered appressoria formation and penetration pegs on the host black fly and did not seem to inhibit fungal invasion on the non-host. / The effects of selected physical factors on the development of the infective unit of Erynia conica were determined. Germination and sporulation in vitro were influenced by medium pH, environmental temperature and charge of the substrate, but both processes were independent of substrate hydrophobicity. Germination and sporulation occurred throughout the pH and temperature ranges tested, with both achieving maximum at pH 7.5-8.0 and 10-20$ sp circ$C. Temperature range of the germination of the conidiospores in situ was similar that in vitro, which corresponded to the temperature variation in the field. Production of invasive structures was noted for in situ experiments only and was more temperature sensitive than was germination.

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