About horse flies and deer flies
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Horse flies are about 3/8 to 1 1/8 inches long, usually gray or blackish in color. The wings usually lack dark areas, but some species have wings that are entirely dark. The eyes often have horizontal stripes.
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Deer flies are smaller, about 1/4 to 1/2 inch long, black or brownish, usually with dark areas on the wings. Their eyes usually have spots.
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Females are blood-sucking and inflict a painful bite.
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Males feed primarily on pollen and nectar and are often found on flowers.
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Females deposit eggs on plants, rocks, and other objects usually in damp locations.
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The larvae live in wet areas and eat immature insects and other small animals. Larvae pupate in the mud near the edge of these areas.
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The life cycle may take from 3 months to 2 years to complete.
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Most are strong fliers and have a range of up to several miles. They are most active during the warmest parts of the day and often follow moving objects.
Deer fly (Chrysops ater). Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
Management
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The use of pesticides for control is not feasible due to the habits of these flies.
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Insect repellents and wearing protective clothing may help to prevent bites.