Tuesday, July 31, 2018

Deer Flies—Away!

adult deer fly Toothaches, difficult break-ups, and traffic accidents. With some things in life, if you have one, you have one too many. This applies to deer flies, those hard-biting pests with a knack for moving in at the instant your hands are full. And the same goes for their beefier cousin the horse fly.

Deer and horse flies are in the family Tabanidae, a group of aquatic insects comprising over 4,000 species worldwide. Fortunately, we “only” have around 100 species of deer flies and 200 of horse flies in our region. It is the female deer and horse flies which slash you with their scissors-like mouthparts and sop up your life-blood to mature their ovaries. After a nice bloody Mary, or Tom or whoever, they will lay 100 to 800 eggs at the edge of a pond, marsh, or temporary mud hole. The larvae are easily found (should you want to) in ponds and marshes in the near-shore ooze. Mind the leeches.

While it seems they must breed nonstop, deer and horse flies take a full year to complete their life cycle. The larvae mostly eat small invertebrates, though some species of very large horse flies reportedly eat frogs and toads. I never want to meet one of those full-grown. All types of black flies emerge in spring together, but then conveniently die off by summer. But various deer and horse fly species emerge throughout the season, making for a summer full of them.

The nature of an irritant is that its presence is readily noticed, but its absence often doesn’t register. Because last year was very wet, these guys had loads of watery real estate in which to lay eggs, and we are paying the price for all their success. Next year, though, there will be far fewer of these bloodthirsty flies because this year has been so dry. However, it is a safe bet that very few people will notice.

In a dry year, many traditionally wet places shrink in size or dry up completely. Larvae in the family Tabanidae need water to survive, and as ponds shrink and vernal pools dry up, the larvae shrivel and die. The early deer flies which have already laid their eggs this season will not be passing on any genes. Turns out “survival of the fittest” depends on the weather.

Their biology is such that chemical control is impossible, and DEET and other repellants are not very effective against them, so we need other tools. Wearing a hat will help, and you can even get ones with face nets. You can go even further by deploying sticky patches, sold at sporting-goods outlets, on your hat. Deer flies seem to be attracted to the color blue, so be advised.

Songbirds like swallows and flycatchers depend on deer and horse flies to fatten themselves up before fall migration. Other aquatic species like dragonflies eat loads of these flies, and even as nymphs (immature forms), they eat deer and horse fly larvae. Sure wish they would eat more…

Here’s to a great remainder of this summer, and one a bit less fraught with deer and horse flies next year.

Photo of Adult Deer Fly by Wikimedia user Bruce Marlin.

Related Stories


A Canton, NY-based arborist, educator and writer, Paul Hetzler had intended to be a bear when he grew up, but failed the audition. He settled for an educator position instead, and serves as Horticulture and Natural Resources Educator at Cornell Cooperative Extension of St. Lawrence County.

His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine. He is the author of Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World.

You can reach Paul at the Cornell Cooperative Extension office in Canton at (315) 379-9192.




17 Responses

  1. Matt M says:

    Mix 1/2 Lemongrass essential oil with 1/2 water in a small spray bottle. Works well at keeping deer flies away.

  2. drdirt says:

    thnx Paul,
    Those flies sure can limit the enjoyment of any Adirondack adventure. Reading some info. on them makes me want to lay on a rock and feed those new generations of eggs.

  3. Bluto says:

    Worst of all are those little biting house flies that prefer your feet when you’re trying to enjoy a trip in your canoe.

  4. Paul says:

    great year if you’re a deer fly, bad if you are a human! As bad as I have ever seen them in some places. was killing two or sometimes three with each swap at the back of my neck last weekend. swarming my car even! Paul, why do they do that? CO2 in the emission?

    • Not sure why they swarm cars, actually. They also swarm my water buckets left out in the sun, so maybe it is the reflected heat… They seem to like dark colors, and are attracted to motion. To be safe, dress in white, stand still, and only inhale.
      Wishing you a good remainder of the summer!

      • Paul says:

        Thanks. It’s true, as things cool down with the vehicles the flies seem to chill out and quit the swarming. They are attracted to the motion. Seems like you try and run and they get even more nuts! Swing your arms around to swat them, more bugs. Can’t win.

    • Boreas says:

      I visited Bloomingdale Bog / Oregon Plains a week or so ago and never left the car. They were very interested in mirrors and windows, so I think a lot of the swarming is due to the fact that they are also attracted to any deer fly activity. When reflective surfaces are involved, it doubles their numbers. Probably why they swarm my head so much…

      • Paul says:

        Yes, the mirrors are the spot that they seem to congregate. Nice and close to you when you get out!

        Deer must be spending lots of time in the water!

  5. Luke says:

    I play at a golf course that has a couple holes near low swamp, and the deer flies can be horrendous while you’re walking. I’ve tried all the sprays, clothes dryer sheets, even a fake dragon fly on my hat. Nothing seems to deter them. I do use the adhesive fly paper, Tred Not, on the back of my hat (full disclosure, I have nothing to do with the company), which you can buy on-line, and that at least traps them while they’re buzzing your head. My record is 38 flies for 18 holes.

  6. Bill Quinlivan says:

    Drove through Moose River Plains yesterday. At one point the deer flies were attacking my truck in droves. I don’t know how or why the campers were there putting up with them. And so much for the attraction to blue, my truck is gray. Did see a large Blue group tent though — Yikes!

  7. Matt sisti says:

    I’ve been in the Adirondacks since I was a kid and always noticed the deer flies following side view mirrors. I asked an old timer why that was and he offered an interesting response. He said the main target of a deer fly is, surprise, a deer. The problem is they can’t bite through the thick hair that covers most of their body with one exception, the top of their ears which has minimal hair. He believes deer flies associate the side view mirrors on a slow moving vehicles with ears on a deer. Interesting and fun thought

  8. Stephen Daniels says:

    Ole Time Woodsman Fly Dope on your hat. Deer flies will not come near you.

  9. Todd Eastman says:

    The upside of deer flies are that they encourage record fast hiking and running times…

  10. Catherine Kinne says:

    I wondered if this year was the worst fly season for end of aug to early Sept. at Indian Lake? My husband and I rented a motor boot Weds 9/5 and went from the Marina and boated up to Jessup river. Wow I had forgotten how persistent the flys were! We had two dogs in the boat with us. Our boat was slightly taking on water. Maybe it had it’s own mini fly eco system? We so enjoyed the view but we did notice not many boats on the water. Ok that was normal after Labor day but something didn’t seem right. We all agreed not to turn back and get our moneys worth for the rental.. We must of looked silly from the occasional boater as we were constantly swatting flys holding the dogs. thank goodness for the towels.! Even wet they offered some reprive. Hmm best place to be was in the water to avoid the persistent sting. I can’t imagine the NY state camp sites were full.
    What is the difference between the late 70’s and now. Has global warming if there is such a thing a factor? Did the state spray years ago? Are there less fly predators I just don’t remember being this bad.
    My family camped for two weeks at a time. We did have a screen tent but it wasnt always necessary it was more shelter for when it rained. Perhaps August was the new July? I always remembered July being worse for flys. Well I hope to be back next year and hope its not as bad. I would love to hear if other people experienced this,
    Best Catherine

Leave a Reply

Leave a Reply

Your email address will not be published. Required fields are marked *