Hoping for a Deer-Fly Drought

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. Their beefier cousin the horse fly is another in the one-too-many category. All it takes is a single deer fly chewing on your face while you try to land a fish or set up an extension ladder, to make your eyes cross in pain and frustration.

Deer and horse flies are in the family Tabanidae, a large group of aquatic insects comprising more than 4,000 species worldwide. Fortunately we only have about 100 species of deer flies and 200 of horse flies in the U.S. As with nearly all biting insects, female deer and horse flies do all the chomping, slicing through skin with their scissors-like mouthparts, and then sopping up the blood. They do this in order to mature their ovaries so they can lay between 100 and 800 eggs (depending on species) at the edge of a pond, marsh, or even a temporary mud hole. The larvae, which look like short, brown worms, are easily found (should you want to) in ponds and marshes in the near-shore ooze. Mind the leeches.

While it seems like they must breed nonstop all summer, deer and horse flies take a full year to complete their life cycle, and some take as many as 2 to 3 years. The larvae are predators, eating small invertebrates. Apparently, some species of very large horse flies eat frogs and toads. I never want to meet the adult of that species. all species of black flies emerge at once in the spring, which is overwhelming, but then they conveniently die off by summer. Various species of deer and horse flies emerge at different times of the season, making for a summer full of them. Where deer flies are concerned, a summer like this is as close to a break as we’ll ever get, because their biology makes it impossible to control them with insecticides.

The nature of an irritant is that its presence is readily noticed, but its absence often doesn’t register. Because last year was so wet, these guys had way more real estate 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, but it is a safe bet that very few people will notice. I know there will be fewer because this year has been very dry.

In a dry year, many traditionally wet places shrink in size or dry up completely. Ponds and marshes retreat; ephemeral streams evaporate earlier than usual. The early species of deer flies which laid their eggs in vernal ponds or on the edge of an ephemeral stream will not be passing on their genes. Turns out “survival of the fittest” depends on the weather. Larvae in the family Tabanidae need water to survive, and as ponds shrink and vernal pools dry up, the larvae shrivel and die.

DEET and other repellants are not very effective against them, so we have to use other survival tools. Wearing a hat will help, and you can even get ones with a face veil. I picked up one at a dollar store. You can go even further if you deploy sticky patches, which you can find at sporting-goods outlets, on your hat. Deer flies seem to be attracted to the color blue, so don’t wear that. You can make use of their color fetish. Take something blue, such as a 5-gallon bucket, and coat it with a sticky product like Tanglefoot. You won’t put a big dent in their population, but it is very satisfying to see how many deer flies you can collect that way.

Many songbirds like swallows and flycatchers depend on deer and horse flies to fatten themselves up before their fall migration. Dragonflies and damselflies eat loads of these flies, in addition to feasting on mosquitoes and black flies. Dragons and damsels are also aquatic, and even as underwater nymphs (immature forms), they eat deer and horse fly larvae. Sure wish they’d eat more.

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

Written by:  Paul Hetzler