Horse fly and deer fly, which include yellow flies and green heads are in the family Tabanidae. This group of biting flies is a serious pest to cattle, horses, other forms of livestock, pets, and humans. They get their name because they will happily take a blood meal from deer, elk, and other large grazers and carnivores.

Know Thy Enemy and How to Destroy Them
These are true flies, and each has subfamily has many species within their general family – Tabanidae. While they share many similarities, they also differ from each other. A greater understanding of deer and horse fly helps people to decrease the populations of these terrifying pests.

They Bite for the Same Reason: The females both require a blood meal to complete their reproductive cycle. It only makes it worse to know that the blood they suck from you is helping them to reproduce and next year, there will be more females looking at you as though you are prime rib, and you are.

Similar in Appearance: Biting flies also have beautiful and brightly colored eyes. They are usually big, bulky flies that can reach to 1.25 inches in length. Horse flies often have dark bodies and darkly colored wings. Deer flies, which are often smaller have transparent wings dark lines or bands.

While they are similar looking, they do have differences. Deer fly, for example, has wings that are A-shaped whereas horse flies have wings that fold straight back when at rest. Horse flies also tend to be slightly larger with bigger eyes and a slightly wider body. Either will make you run for cover.

Their Mouthparts are Lethal: The female horse and deer fly have skills, and they use those skills to hunt and feed on their prey. Their drive to drink blood comes from their need to reproduce. As such, they are relentless in their quest to feed on warm-blooded mammals. People fear these obnoxious pests because of the razor sharp, dagger-like mouthparts which they plunge through your skin to sever the blood-rich capillaries. The evolution of both deer and horse fly created an efficient tool that turns flesh into a buffet of blood for large mammals. As painful as that sounds, it is not the bite that causes the most pain. That severe, sharp, stinging sensation is the result of the anticoagulant that they use to keep your blood flowing. The bite, itself is painful, but their saliva is worse.

They Both Spread Disease: During the biting process, both deer fly and horse fly can spread several types of disease such as anthrax, filarial worm, and equine infectious anemia. In an article produced by the University of Kentucky, entomologist Less Townsend estimates that each bit from a deer or horse fly removes about 1-cc of blood. To put that into perspective, the human body has about 5 liters of blood, which is equal to 50,000 cubic centimeters. Townsend estimates that a small terror cell of biting flies that number between 20-30 hungry females can extract a quart of blood in just ten days. That’s about 1/5 of the blood in your body.

In bad years, when there are lots of hungry female biting flies, their constant biting can negatively affect livestock. Farmers report decreased milk production, injuries, and disease thanks to biting flies.

They Are Hard to Control: Horse flies, deer flies, yellow flies, and green heads are difficult pests to control. Many are resistant to pesticides making chemical controls almost worthless. They also do not have many natural predators. So, they have free reign to terrorize people. Big spiders capture some of these driven females but not many. Plus, who wants to live with big spiders?

One method that works well is a special type of fly trap called the Fly Cage. Because deer and horse fly find prey in similar ways, the trap uses their tenacious habits to attract and then kill the females. Both deer fly and horse fly seek out large dark shapes that move. This is why deer, horses, cattle, goats, dogs, and humans make such easy prey. All are largish, slow moving, and dark. Big bodies present many opportunities to feed. Only man has limbs that present a danger to the flies. No matter, though, they will risk a swat to feed.