Horse flies and deer flies belong to the fly family Tabanidae. They represent a complex of at least 300 species, some of which are very pain-inflicting and annoying pests. Dairy cattle on pasture occasionally are severely attacked by these flies, particularly on pastures that border woodlands or wet, marshy areas. Female horse flies and deer flies cut through the skin of the animal with knifelike mouthparts. They then feed on the blood that pools around the wound. The wound continues to bleed after the fly leaves and often attracts face flies.
Large numbers of these flies can cause extreme annoyance and fatigue, blood loss, reduced milk production, and reduced weight gain. Some species have also been implicated in the transmission of tularemia, anthrax, anaplasposis, and leukosis. Female flies of each species (horse and deer flies) typically lay their eggs in distinctively shaped egg masses on vegetation near marshes, ponds, or streams. Development from egg to adult requires 70 days to 2 years, depending on the species.
Horse flies and deer flies are notoriously difficult to control. They are strong fliers that move long distances between breeding areas and hosts. Because they land on host animals to feed for only a very short time,it is difficult to deliver a lethal dose of insecticide.
Moreover, because horses represent only one of the many host animals these pests feed on, treating the animals will have a negligible impact on total fly populations. Severe horse fly and deer fly pressure is generally temporary because of the seasonality of fly activity. In some cases, animals can be moved during periods of peak activity from low-lying pastures near marshy areas to other pastures where fly pressure is lower.