Variation in density of cattle-visiting muscid flies between Danish inland pastures

The density of cattle-visiting flies (Muscidae) and the load of black-flies (Simulium spp.) were estimated in twelve and eighteen inland pastures in Denmark in 1984 and 1985 respectively. No differences in the geographical distribution pattern of the predominant cattle-visiting Muscidae were recorded, whilst the relative abundance and density of the species and the total fly-load varied considerably between pastures. In most cases the mean loads of Haematobia irritans (L.) and Hydrotaea irritans (Fall.) on heifers varied significantly in relation to site topography and shelter. These crude site variables explained 65-98% of the variation in densities of horn flies and sheep head flies observed between pastures. Highest densities of Hydrotaea irritans were primarily associated with permanent, low-lying, fairly sheltered grassland sites, whereas the density was low in temporary, dry, wind-exposed pastures. A comparable relationship was found for Haematobia irritans. With Haematobosca stimulans (Mg.) and Morellia spp. no relation between grassland environment and fly density was detected.