This story was originally published in August 2019.
Emerging in full force mid-summer, deer flies and horse flies are among the most intimidating creatures in the Northeast. Their bites are painful, and once they’ve home in on a target, they’re nearly impossible to shake.
When pest experts talk about these types of flies, it sounds like they’re discussing monsters or aliens from a Hollywood thriller.
“Their mouthparts are described as knife-like or scissor-like,” said Jim Dill, a pest management specialist for the University of Maine Cooperative Extension. “They basically slash their way in[to your skin]. They have a saliva that acts as an anticoagulant. And when you start bleeding from the wound, they lap it up.”
Dill is working on a project that involves capturing and identifying different deer flies and horse flies throughout Maine. So far, he has been fascinated by the variety he’s found.
The difference between a deer fly and horse fly
“Deer fly” and “horse fly” are common names used to describe hundreds of species in the Tabanidae family. In the United States and Canada, about 350 different species of these flies exist, and they vary greatly in size and appearance.
“Off the top of my head, I don’t know how many different species there are in Maine,” Dill said. “But there’s a lot, and some of them are kind of regional. For example, down on the coast, you hear of the ‘greenheads’ on the beaches.”
One of the species Dill found had yellow and black stripes, similar to yellowjacket. Another species is so big — about 1½ inches long — that Mainers often refer to it as a “moose fly.”
“One under each arm would carry you away,” Dill said.
Though the common names — deer fly, horse fly and moose fly — are sometimes used interchangeably, people generally use the term “deer fly” when describing species in the genus Chrysops, said Howard Russell, entomologist at Michigan State University. While “horse fly” is typically used to describe species in the genus Tabanus. Both genuses are in the Tabanidae family.
“What’s very cool about them is their incandescent eyes,” Russell said.
“I have at least 15 to 20 species where I live,” he added. “I think I’ve been bitten by them all.”
In addition to having a painful bite, deer flies and horseflies are fast, strong flyers. They are notably faster than mosquitoes or blackflies, making them a bane of runners and cyclists. And they don’t just follow you. These flies have a tendency to circle and ping off your head, a behavior that can be outright maddening.
What makes these flies so formidable?
“They’re very fast — among the fastest flying of all insects,” said Russell, though he does not know if their top speed has ever been recorded.
You can try to outrun them, but these flies hunt by sight and are attracted to movement. They are also attracted to dark colors, especially blue, according to instructions on how to make a deer fly trap by North Florida Research and Education Center.
“They can be quite territorial, too,” Dill said. “As you’re walking along, one fly can bounce around your head and after you’ve gone 10 to 15 feet, oftentimes it will leave you. But it won’t be long before you have another one.”
Unfortunately, insect repellent doesn’t seem to deter these flies much, Dill said. He believes one reason is because deer flies and horse flies often aim for people’s heads, where they wiggle under hair to find skin. And usually, people don’t thoroughly spray their scalp. Also, they do not use scent to find their hosts (as mosquitos do), therefore, it doesn’t matter if the repellent masks your scent.
Interestingly, it’s only the female flies that bite. The males feed on nectar and pollen, Dill said. And the larvae feed on aquatic predators, including mosquito larvae.
“In that way, they’re kind of a mixed blessing,” Russell said.
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