Stable Fly (Stomoxys calcitrans)
Photo: Pavel Krok (Wikimedia Commons)
Biting flies are a fact of life in Canada. From spring snow melt until autumn freeze-up, mosquitoes, deer flies, horse flies, moose flies, snipe flies (Symphoromyia sp.), black flies and no-see-ums (Culicoides sp.) turn forests, wetlands and tundra into buzzing menageries of pain. Oh, how many litres of blood I have donated to the sanguivores of the north! I've lived and worked all over Canada's boreal forest, and the flies there are as bad as can possibly be imagined - actually sometimes they're worse. Presently though, I live in southern Canada, almost as far south as you can go and still be in the Great White North, 700 km from the boreal forest, but even here, on the shores of Lake Erie, the flies still torment me. There are mosquitos, and deer flies, and horse flies, including a particularly large species that I like to call the Darth Vader Fly (Tabanus atratus), but they're nothing like what I'm used to from in north country. Instead, the shores of the Great Lakes harbour yet another villain, one that's every bit as tenacious and often as abundant as its boreal couterparts: the Stable Fly (Stomoxys calcitrans).
Now I've been dealing with Stable Flies all my life, as a minor annoyance on any given fishing or canoe trip. But only recently have I found a place where a normal day in late summer or early fall entails braving swarms of thousands of the little bastards. That place is Long Point, on the north shore of Lake Erie, and I just happen to live there. After spending a summer feeding Stable Flies with generous helping of my blood, I wanted to understand why there are so many flies on Long Point.
Stable Flies are yet another unfortunate addition to the long list of Old World species that have been introduced to North America. That's right, they're not native, so if it wasn't for some damned fool who imported the little monstrosities, we'd be able to enjoy our summers with one less entomological menace. But alas, humans have a particular knack for ruining everything. The Stable Fly likely came to North America in association with livestock as early as the 1700's, and as its name suggests, it's associated with stables, barns, and farms in general. And its on farms that Stable Flies become a serious pest. They are blood suckers, and they can take so much blood from livestock so as to cause anemia, weight loss and reduced milk production. If that wasn't enough, they can potentially transmit lethal diseases like anthrax-causing bacteria, Bacillus anthracis.
Stable Flies are obligate sanguivores, females require not one, but at least two complete blood meals to produce eggs. Males also bite, something that sets Stable Flies apart from almost all of our other biting flies, in which it's only the females that take blood. Indeed, Stable Flies are oddities within their own family, Muscidae. Most muscids suck up their food using soft, spongy mouthparts, a House Fly (Musca domestica) is a good example. Among our muscids, only the Stable Fly and another livestock pest (that doesn't attack humans), the Horn Fly (Siphona irritans) bite. Stable Fly bites are particularly painful because they don't inject their victims with anesthetic, like mosquitos so courteously do. Feeding primarily on livestock which, other than floppy ears and a whipping tail, have no way to keep the flies at bay, meaning that Stable Flies don't have to exercise subtlety when they bite because there is nothing their victims can do to stop them anyhow. That's probably also why Stable Flies take their time sucking up blood: it takes about four minutes to consume a complete blood meal.
Thankfully, as anyone who has toiled among Stable Flies will know, these insidious insects fly low, focusing their bloody attacks on the legs and feet, generally sparing the rest of one's body (so dressing appropriately can be a simple and effective defense). But they also, from time to time, fly really high, they've been collected 1 km up and probably go much higher when swept away in weather systems. It's during these high-flying forays that Stable Flies disperse from their breeding sites, the rotting vegetation and manure of active feedlots and barnyards. Rotting vegetation elsewhere, like that which washes up on Great Lakes beaches or in wetlands can also be a source of Stable Flies, but it appears that livestock operations are by far the most important breeding grounds. During these dispersal events, they can form untold concentrations along the shores of the Great Lakes, including literally right in my own backyard. The reason they accumulate specifically along the Great Lakes may have to do with localized weather patterns, lake breezes.
Lake breezes are familiar to all who live on the Great Lakes. It's the lovely wind that blows onshore throughout the day, moderating summer temperatures, and making the intense humidity of July and August bearable. It's a localized phenomenon that can be thought of as a conveyor belt of air swirling above the lake and shoreline. The lake breeze begins as the morning sun heats the land adjacent to the lake. As it warms, air over land rises, often carrying with it a morning flight of Turkey Vultures (Cathartes aura), hawks, and an assortment of insects, including Stable Flies. The warmed, rising air, once it reaches a certain altitude, flows out over the lake. Vultures and hawks can exit the rising air masses at any time, but Stable Flies often remain trapped, so are pulled out over the lake with the flowing air. Now over the lake, the air begins to cool, descending and carrying with it those same flies. The rising air over land causes an area of low pressure to form, so that higher pressure lake air flows towards the shore, filling the low pressure void. This completes the cycle, the air that was once heated over land and that subsequently cooled and descended over the lake, finally flows back toward to shore. Those same Stable Flies, now well-travelled, ride back to the beach on the lake breeze, and there they accumulate.
Stable Flies aren't the only insects that get caught up in lake breeze cycles. Perhaps even more noticeable are lady beetles. I've been inundated at my Lake Erie home with countless thousands of Multicoloured Asian Lady Beetles (Harmonia axyridis). I've seen their colourful little carcasses wash ashore in unbelievable profusion, the ones that didn't make it; and I've seen every piece of driftwood and debris for kilometres of beach covered in the ones that did. Diabrotica sp. beetles also seem to have a propensity for riding the wind. Seldom is it that I can sit on the beach after a swim and don't find at least one Diabrotica sp. nearby. Other species may congregate on beaches too, not brought there by the lake breeze, but instead to feast on the concentrations of Stable Flies. In late summer and autumn, for example, migratory dragonflies, like Common Green Darner (Anax junius), take advantage of the abundance at Long Point. Let them eat their fill I say. By that I mean the dragonflies, not the stable flies!
