Horse flies increasing in numbers, livestock and humans at risk for bites

SAN ANTONIO (KTSA News) — If you think mosquitoes are a nuisance, you might want to fasten your seatbelt.

You have likely heard of horseflies, but were you aware that these giant flies eat blood?

A quick glance at the calendar shows it is horsefly season. Further, heavy rainfall from Tropical Storm Beryl could cause an increase in horsefly populations in several parts of Texas.

Sonja Swiger, Ph.D., Texas A&M AgriLife Extension Service entomologist in the Texas A&M Department of Entomology, Stephenville, said she expects reports of horsefly infestations to increase over the coming weeks.

Despite the name, horseflies are not host-specific to horses. They will feed opportunistically on available human and animal hosts.

“Like mosquitoes, only female horse flies bite because they need the host’s blood for egg production,” Swiger said. “Horseflies only bite and feed once every few days, but their need for blood meals makes them relentless biters.”

What makes horsefly bites especially savage, Swiger said, is they cut their host’s skin with knife-like mouthparts and then consume the blood that pools around the wound.

Horseflies are larger than common flies, such as stable or horn flies, growing up to an inch long. Some are all black while many have colored patterns on their abdomen and wings. They also have very colorful eyes that vary with species.

Swiger said the relentless pests typically stay in shaded areas along tree lines. They consume carbohydrates in the form of nectars and honeydew, but females will range away from cover to hunt hosts for blood meals.

Female horseflies typically lay eggs over winter and in early spring in shady, semi-aquatic to moist areas, including around the edges of ponds or water tanks, she said.“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. Maggots are also very predatory and eat each other, so there will only be one larva in a 10–20-foot radius.”

Horseflies have a maximum range similar to most flies – five to 10 miles – though most fly populations remain relatively close to the breeding ground where they emerged, she said.

Swiger said the best defense for livestock is to move them from the infested area into a barn or to cover them with lightweight summer sheets designed to stop biting flies and mosquitoes, she said.

Traps specifically designed for horseflies can reduce numbers in infested areas, she said.