The eye gnats or the eye flies (Hippelates spp) are very small (1.5–2.5 mm long) flies that frequently congregate around the eyes as well as mucous and sebaceous secretions, pus, and blood.
In the desert and foothill regions of southern California, adult Hippelates flies are present throughout the year; they are annoying from April through November. During the peak months, they are noticeable in the early morning and late afternoon. They gather in deep shade, such as among densely planted shrubs or in the shade of a dwelling. The eggs are ~0.5 mm long, fluted, and distinctly curved. They are deposited on or below the surface of the soil. The larvae hatch and feed on decaying organic matter, including excrement. The larval stage lasts 7–11 days. During the winter months, the larval and pupal stages may persist for many weeks. Pupation occurs close to the surface of the soil and lasts ~6 days. The entire life cycle lasts ~21 days. The adults are generally strong flyers, flying both with and against the wind.
Some species are attracted to the genital organs of mammals; for example, H pallipes clusters around a dog’s penis. These gnats quietly approach their mammalian hosts. They usually alight some distance from their feeding site and then crawl over the skin, or fly intermittently and alight, thus avoiding annoyance to the host. They are persistent and, if brushed away, quickly return to continue engorging themselves.
They are nonbiting flies; however, the labellae have spines that scarify host tissue and allow entrance of pathogenic organisms. Hippelates flies often hover around the body orifices of calves, yearlings, pregnant heifers, and lactating cows. They feed on lacrimal fluid, fatty body secretions, milk droplets, and on secretions at the tips of the teats of animals. Hippelates flies also serve as vectors for Trueperella pyogenes (summer mastitis) and Moraxella bovis (pinkeye, see Infectious Keratoconjunctivitis).
Repellents, such as those recommended for mosquitoes, provide temporary relief from eye gnats. Applications of insecticides on a community-wide basis (as would take place with mosquito abatement) may provide temporary control of adults, but more adults invade the treated area after the insecticide has dissipated.