Horse Fly Population Higher Than Normal, Hard to Control

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Horse flies can inflict serious wounds and introduce diseases to livestock. Livestock should be moved from infestation sites immediately to relieve and protect animals because chemical controls have shown minimal success abating horse flies, and many are not labeled for horses. (Texas A&M AgriLife Communications photo by Katie Hancock)

High populations of horse flies, a stubborn and difficult biting fly to control, have caused problems for Texas livestock producers this year, said a Texas A&M AgriLife Extension Service expert.

Dr. Sonja Swiger, AgriLife Extension entomologist, Stephenville, said reports of horse fly infestations are higher than she’s ever experienced during her 11-year career.

“It’s the worst year I’ve seen,” she said. “We don’t have much data on seasonal horse fly populations to compare, but we can speculate that all the rain we’ve had the last few years has created the right environmental conditions for higher horse fly numbers.”

Swiger said numbers should begin to wane because temperatures have risen to more inhospitable levels. But the pest can resurge with later summer rains or in the fall.

Despite the name, horse flies are not host-specific and will feed on opportune human and animal hosts, she said.

Like mosquitoes, only female horse flies bite because they need the host’s blood for egg production, Swiger said. But unlike mosquitoes, horse flies cut the host and consume blood as it drips from the wound.

Horse flies only bite and feed once a day, she said.

Swiger said horse flies typically stay in shaded areas, such as along tree lines. They consume carbohydrates, such as nectars and honeydew, but females will range from cover to hunt hosts once a day.

Horse flies typically lay eggs over winter and in early spring, she said. Eggs are typically laid in shady, semi-aquatic to moist areas, including around the edges of ponds or water tanks.

“They can be in any number of locations, and that makes them hard to treat effectively,” she said. “The larvae are maggots, but they look like maggots on steroids. They are also very predacious and will feed on each other.”