Stable fly (Stomoxys calcitrans) and house fly (Musca domestica) densities: a comparison of three monitoring methods on pig farms

House flies (Musca domestica L.) and stable flies (Stomoxys calcitrans [L.]) are important pests on livestock farms and often occur in mixed populations. In order to identify when control actions must be taken, monitoring is essential. The two fly species should be monitored independently as they affect animals differently and may also differ in their responses to control measures. Here, we assess three monitoring methods on pig farms with mixed population of stable and house flies for quantifying fly abundance: spot cards, sticky traps, and a visual index. We compared estimated values from the spot card and a visual index method, which did not discriminate between fly species, with the number of each species caught on the sticky traps. Our results indicate that stable flies contribute less than half of the spots on the spots cards, compared with house flies. Thus, to monitor house flies, use of spot cards could be justified if the number of stable flies is low. However, the contribution of stable flies and house flies to the visual index differed between sites. Whether the visual index or sticky traps best represent the true fly densities is unknown. However, until proved otherwise, the visual index should primarily be used when the two species are registered separately. Threshold values for house fly control, as defined using spot cards (Pennsylvania, USA) versus visual index (Denmark), were compared and found to differ. Although this might be due to differences in tolerance between the two locations, unequal use of monitoring methods and differences between farms is also possible and highlights the need for a common monitoring technique for house flies internationally.