Research Targets Horn Fly Scourge.

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Considered a filth fly, horn flies (Haematobia irritans) feast on a cow's blood, leave to lay their eggs in manure, and then fly back to their beef buffet. Eggs hatch after about two weeks, and a new generation begins. The flies pierce the hide and inject an anticoagulant to help free the flow of blood. Cattle swing their heads, slap their tails, and twitch their skin in attempts to stop the biting.

"Seeing an animal with horn flies is a really discouraging situation when you think about that animal," says J. Derek Scasta, associate professor of rangeland management at the University of Wyoming. "Cattle producers suffer production losses because the animal is losing blood, but also because of these annoyance avoidance behaviors. Every time that animal picks its head up and swings, it's not taking a bite of grass, so grazing time decreases."

He says other scientists have found such cows produce less milk, which lowers weaning weights. The biting also slows growth rates of the younger animals, such as calves or yearlings.

Some animals will be bothered more than others in any group. "They may all look similar as far as you and I can tell, but there will be some cows infested more than others. We're...