THE MERCK VETERINARY MANUAL
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Face Flies

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Face flies, Musca autumnalis, are so named because they gather around the eyes and muzzles of livestock, particularly cattle. They may also be found on the withers, neck, brisket, and sides. Their mouthparts are adapted for sponging up saliva, tears, and mucus. Face flies are usually not considered blood feeders because their mouthparts are a sponging type and not piercing or bayonet-like, as are those of Stomoxys calcitrans. However, they follow blood-feeding flies, disturb them during the feeding process, and then lap up the blood and body fluids that accumulate on the host's skin. Face flies are found on animals that are outdoors and usually do not follow animals into barns.

Face flies are found on range cattle throughout southern Canada and most of the USA. The mouthparts consist of sponging labellae, and there are four longitudinal stripes on the abdomen. Although similar in appearance to the common house fly, face flies can be differentiated by the closeness and angles of the interior margins of the eyes and by the distinctive coloration of the face and abdomen. Speciation requires the skills of a trained entomologist.

Cattle are the principal host of the face fly in the USA, but face flies will also feed on horses and probably sheep and goats. The face fly is a pest of range cattle; it does not develop in feedlot situations and thus is not a parasite of confined cattle. The eggs are laid in fresh cattle feces in rangeland situations and hatch in ~1 day. The yellowish larvae develop in 2–4 days and, when mature, leave the manure to pupate in the surrounding soil. The complete life cycle from egg to adult requires 12–20 days, depending on climatic conditions. The diapausing adult overwinters within buildings and other protective places.

Face flies annoy the host and ultimately interfere with the host's productivity. Females feed on facial secretions, such as tear fluid, nasal mucus, and saliva, to obtain protein for egg development. The irritation around the host's eyes stimulates the flow of tears, which attracts even more flies.

Face flies also feed on other fluid sources, such as blood from wounds and milk on calves' faces. Because face flies have small, rough spines (prestomal teeth) on their sponging mouthparts, they can cause irritation and mechanical damage to the eye tissue of the host. The feeding activity of face flies enhances transmission of Moraxella bovis (see Infectious Keratoconjunctivitis). Face flies can also serve as intermediate hosts for Thelazia spp (see Eyeworm Disease) and for Parafilaria bovicola (see ParafilariaInfection).

Adult face flies are morphologically similar to house flies. These two species can be differentiated only by minor differences in eye position and color of the abdomen. Speciation requires the skills of a trained entomologist. In general, if a medium-sized fly is found feeding around the eyes and nostrils of a cow or horse, it is most probably a face fly.

Control of face flies is difficult. Much effort has been made using various insecticides and application techniques, such as dust bags, mist sprays, and wipe-on formulations. Also, insecticides and insect growth regulators are used as feed additives. However, results are usually less than satisfactory. The introduction of insecticide-impregnated ear tags has provided somewhat better control, but generally, seasonal face fly reduction of only 70%–80% has been achieved, even with two tags (one in each ear) per animal.

Last full review/revision August 2013 by Charles M. Hendrix, DVM, PhD

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