University of Maryland Extension

Horse and Deer Flies

adult horse fly
Horse fly

Horse fliesĀ 

They are larger than deer flies, 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. Only the female flies are blood sucking and inflict a painful bite. Males feed primarily on pollen and nectar and are often found on flowers. Females deposit eggs on plants, rocks, and other objects, usually in damp locations. The larvae occur in wet areas and are predaceous on immature insects and other small animals. The life cycle may take from 3 months to 2 years to complete.

deer flyDeer flies

They are smaller than horse flies, about 1/4 to 1/2 inch long, black or brownish, usually with dark areas on the wings. The eyes usually have spots. Only the female flies are blood sucking and inflict a painful bite. Males feed primarily on pollen and nectar and are often found on flowers. Females deposit eggs on plants, rocks, and other objects, usually in damp locations. The larvae occur in wet areas and are predaceous on immature insects and other small animals. The life cycle may take from 3 months to 2 years to complete.

Adult flies are generally found near swamps, marshes, ponds and other areas where larvae occur. Most are strong fliers and have a range up to several miles. They are most active during the warmest parts of the day and often follow moving objects. The use of pesticides for control is generally not feasible due to the habits of these flies. Insect repellents and wearing protective clothing may be effective in preventing bites by the females.

Additional Resource

PublicationĀ (PDF) HG 26 Flies in and Around the Home

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