Ullahau, Nature reserve on Gotland island, Sweden

Ullahau is a nature reserve on the island of Fårö, Sweden, centred on Sweden's largest parabolic dune, a horseshoe-shaped ridge of quartz and feldspar sand open to the north. The dune is about 300 metres wide in the south and rises roughly 15 metres above the surroundings to the east, enclosing a deflation plain about 1.3 kilometres wide. The 145-hectare reserve, established in 1966, is mostly covered by pine forest, with under ten hectares of open sand.

Ullahau probably began forming in the 18th century and moved about three metres per year. From the late 19th century, plantings of Scots pine, birch, alder and beach grass halted the dune's advance, which ended early in the 20th century. The nature reserve was established in 1966.

The name Ulla Hau refers to the dune field on Avanäset, and the open sand is kept bare by strong sun and wind, giving a dry microclimate where heat-loving insects live. The grasshopper wasp, a type of digger wasp, occurs in Sweden only on Gotland and Fårö, with Ullahau one of four sites on Fårö and seven on Gotland.

The reserve is accessible on foot, though sturdy shoes are recommended since the sandy terrain can be uneven. No motor vehicles, camping, fires, or collection of insects are permitted to maintain the natural conditions.

The open sand holds one of only four sites on Fårö, and seven on Gotland, for the grasshopper wasp (Sphex funerarius), whose only Swedish occurrence is on Gotland and Fårö. Bare sand, strong sun and wind create a dry microclimate favoured by warmth-loving insects such as stinging wasps.