Bad Bugs of Summer: Horse Flies

Mosquitoes and ticks are nasty, but no family of blood feeders can compete with horse flies. They’re the most diverse group of blood-feeding animals on Earth (and, as far as we know, in the entire universe).

Whether you call them horse flies, greenheads, deer flies or bloodthirsty so-and-sos, these insects are all part of the family Tabanidae – which consists of approximately 4,450 described species (about 400 of which can be found in the U.S.). All of these species have two things in common that make them a pain (literally). One: the females consume blood to help get the protein necessary to develop mature eggs. Two: they have blade-like mouthparts that they use like scissors to cut open skin and get at the aforementioned blood. Unlike mosquitoes or ticks, which can be pretty sneaky when feeding, you know right away when a horse fly bites you – it hurts.

Horse flies are particularly problematic in the summer because that is when they breed. Males feed exclusively on nectar, and females also feed on nectar much of the time. But when it’s time to mate, they want blood, which they need in order to reproduce successfully. Summer also happens to be when humans often opt for shorts and t-shirts, making us more likely targets for horse flies. It’s a perfect storm, people.

After mating, female horse flies lay their eggs in moist environments: marshes, pond shores, creeks and even in termite mounds, waterfalls or beach dunes. The eggs hatch into predatory larvae – venomous maggots that will feed on small invertebrates and even vertebrates (such as minnows or frogs). That’s right – they’re venomous predator maggots! The larvae go into their pupal stage over the winter, emerging as adults in late spring. And the cycle begins again: bite a mammal, mate, lay eggs, die.

How do horse flies find you? We’re pretty sure that they are drawn to large, dark objects (like horses, natch) and, when they get close enough, that they are drawn by the carbon dioxide that animals exhale when they breathe. Since we can’t shrink, change color or stop breathing (without dire consequences), what can we do to avoid horse fly bites? Not much.

Insect repellents can provide some protection, but repellents aren’t as effective against horse flies as they are against mosquitoes or ticks. Wearing a white shirt may help keep horse flies away (they like large, dark objects – remember?) – but since white can also attract ticks, that may not be a great idea. So, basically, you can swat them.

But here’s one piece of good news: horse flies are not the major disease vectors that mosquitoes and ticks are. They can (and do) transmit some diseases (such as the misleadingly friendly-sounding Loa loa) – but they aren’t major players in the epidemic disease game.

And, just to show that they’re not all bad, it’s worth remembering that horse flies consume a lot of nectar. As a result, they are ecologically valuable as pollinators. Without horse flies, a lot of flowering plants wouldn’t be able to do their thing. Maybe that will provide some comfort right after one bites you.

Check out the other posts in our series on summer pests: mosquitoes, ticks, carpenter bees and black widows.

Note: Many thanks to Dr. Brian Wiegmann, professor of entomology and director for education and outreach of the National Evolutionary Synthesis Center, for taking the time to talk to me about horseflies. Any errors in the above post are mine and mine alone.