Distribution: The Holarctic species is spread over whole Europe, Asia Minor, Siberia and North America (Mohrig 1969, Cranston et al. 1987, Schaffner et al. 2001, Becker et al. 2003).

Habitat: Breeding sites of O. flavescens are always open, sunlit and never shaded, containing fresh or brackish water up to 8 g/l of chlorides; larvae develop in ditches, marshes, peaty swamps, flooded meadows, in salty ground outcrops and in the watersides of lakes and ponds where submerse and straight vegetation is abundant (Service & Smith 1972, Rettich et al. 1978, Schaffner et al. 2001, Becker et al. 2003). The species prefers semi-permanent water bodies of larger size (Mohrig 1969).

Biology: O. flavescens is univoltine; eggs are laid on the soil or on plant debris in dried-up depressions, which are flooded after winter rains or in spring after the snowmelt; eggs immersed in autumn or winter do not hatch until February or March; however, eggs laid in June will hatch in the summer if subjected to intermittent immersion, for example during summer inundations (Service & Smith 1972). First larvae appear in early spring and are present till June (Trpiš 1962, Rettich et al. 1978, Schaffner et al. 2001). Larval development takes roughly two months (Schaffner et al. 2001).

Adults: Adult emergence starts early in May; imagoes rapidly increase in number to a maximum in late May and early June and disappear in July-August (Aspöck et al. 1970, Service & Smith 1972). Postponed hatchings may cause a second wave of emergences at the beginning of summer (Schaffner et al. 2001).

Females principally feed on large mammals, including cattle, horses, sheep and man; they rest in low lying growths of vegetation during the day, near to the larval sites, and bite all hosts entering the herbaceous areas (Service & Smith 1972, Schaffner et al. 2001). They are aggressive during the day, and even more at dusk and may cause a strong nuisance in Northern Europe and near the coastal areas (Mohrig 1969, Schaffner et al. 2001). In the laboratory the species is a vector of the Tahyna virus and can be infected with Tularaemia (Schaffner et al. 2001).