Bites of adult females are followed by bleeding from the punctures, and later by severe itching, swelling and irritation lasting for several days. Intense black fly attack can result in reduced milk production and growth rates of cattle, and reduced conception rates of cows and heifers. Calves and other cattle not previously exposed to black flies can die within 24 hours during an outbreak. Larvae remain attached to objects in the water, feeding on microscopic plant or animal life in the water. Adults often emerge in countless numbers within a very few hours of one another.
Some species have one generation per year, other species have two or more.
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