Head flies or plantation flies, Hydrotaea irritans, are nonbiting flies found in large numbers in northern European countries, especially Denmark and Great Britain, where they are pests of cattle, sheep, and other livestock. This fly resembles the house fly and is ~4–7 mm long. The thorax is black with gray patches, the abdomen is olive green, and the wing bases are orange yellow.
Head flies are a nuisance to domestic animals and people because they are attracted to the mouth, nose, ears, eyes, and wounds to feed on secretions. Unlike other Hydrotaea spp, H irritans produces one generation per year, with three larval instar stages. Eggs deposited in late summer hatch out larvae within a few days. The saprophagous stage is brief, before development to the stage that is predatory on other insect larvae. Overwintering occurs as late-stage larvae. Adults are most active from early June until late September and are common in the vicinity of thickets or woodlands in which they shelter between periods of feeding.
In Great Britain, sheep are mainly affected. Large swarms of flies, attracted by the movement of animals, congregate to feed on secretions from the eyes and nose and on the cellular debris at the grown horn base. To alleviate the persistent irritation, sheep scratch and rub their heads, resulting in raw wounds or “broken heads,” especially on the poll. Flies, attracted by the blood, settle on these self-inflicted lesions and extend the margins by their feeding activity. Sheep of all ages are involved, but breeds with horns and without wool on the head are most severely affected.
Head flies also attack people, deer, horses, cattle, and rabbits. Although no corresponding broken head lesions develop in cattle, the occurrence of summer mastitis (due to Trueperella pyogenes) and the seasonal activity of head flies are closely associated, especially in Denmark. Head flies may also be involved in the spread of myxomatosis in rabbits (see Myxomatosis).
The development, emergence, and congregation of head flies, which occur away from farm areas, preclude the traditional methods of insecticide spraying of generalized breeding sites and resting habitats. Control at the point of contact between the feeding adult insects and the mammalian hosts is also limited in value. With sheep, the retention of organophosphate compounds or pyrethrin derivatives on the susceptible head areas is of short duration, which necessitates impractical reapplications in free-ranging animals. Use of insecticide-impregnated ear tags in cattle decreases the incidence of summer mastitis, presumably by reducing transmission by head flies.
Removal of livestock from infested locations during the fly season is the only completely effective way to prevent damage. Once broken heads have occurred, the housing of sheep is the only successful method to stop further fly damage.