Livestock owners are familiar with black flies, which cluster in the ears, on the chest, at the midline of the underside of the body, and elsewhere livestock. Humans can be plagued by these tiny bloodsucking flies, too. Flies in this family of Simuliidae are called buffalo gnats (due to their humpbacked appearance) or turkey gnats. They are black, grey or yellowish and black and lay their eggs in moving water. Immature forms develop on water plants, so they are most common near moving bodies of water such as streams, rivers, and irrigation canals. After several generations in the summer, black flies overwinter as larvae attached to plants submerged in unfrozen water. They develop slowly over the winter and emerge as adults in the spring as temperatures warm up. Black flies use visual cues, carbon dioxide concentrations and warmth to find hosts. If populations build up and cattle or other livestock are nearby as a convenient source for a blood meal, black flies can become an economically important pest. Cattle infested with many black flies bunch together, do not feed, become anemic, and can die. Black flies are likely vectors for the vesicular stomatitis virus, too, according to Dr. Bob Hillman, former Idaho state veterinarian.
Control or management of black flies is not easy. Some livestock owners may choose to encourage populations of native fish and other aquatic predators. For horses, flysheets and fly masks with ears can help keep the flies off the animals. Petroleum jelly or baby oil smeared in the ears and on other places on the body can serve as a barrier to the bloodsuckers. There are other petroleum oilbased products with insecticide added that will work to keep the flies off your horses. Since black flies are day-fliers, stabling horses during the day and pasturing during the night is a simple change in management to keep the black fly problem at bay. For any livestock, providing run-in sheds for animals during the day can provide a refuge from the flies, which avoid dark, shaded areas. A research study in northern Alberta, Canada demonstrated that providing a simple openfronted shed was sufficient to protect cattle from black flies. Dr. Doug Colwell from the Lethbridge Research Centre in Lethbridge, Alberta noted that black flies visualize a grazing animal as a silhouette, and if the animal is in a shelter against a dark, shady background, it’s more difficult for black flies to locate their host.
Recently, areas of Idaho have had huge populations of black flies that pestered livestock and people. One report indicated they would get so thick in the sky that aerial applicators could not see to fly. In 2004, economic losses in the beef, sheep, dairy, and horse industries of several southwestern Idaho counties were estimated at well over $1 million (USDA FSA, Idaho). County budgets and private donations provided funds to control black flies using larvicide in irrigation canals and other bodies of water.
The beef cattle pest management team, headed by Holly Ferguson and Doug Walsh and based at the WSU-Prosser station, has conducted statewide surveys of flies and other insects in cattle pastures and rangeland over the past two seasons. Data indicate that black flies start appearing in May and are present throughout the summer. They have only been found at ranch locations with running water such as irrigation canals or streams running through pastures. It has been suggested that an increase in weeds in streams and canals may encourage buildup of black fly populations, because black flies lay eggs on plants in the water.