Thursday, July 24, 2014

Adirondack Insects: The Stable Fly

Stomoxys_calcitrans_on_aloe_veraDuring summer, both residents and visitors of the Adirondacks should be required to spend time on the water, preferably in a canoe, kayak or guideboat in order to experience the serenity and magnificence of slowly and silently moving across our fresh water environments. However, traveling over the surface of most waterways in a small, open, human-powered craft from July through mid August does have its cost, as people in such boats are occasionally subject to the harassment of a small, fast-moving fly inclined to bite on the upper part of the foot, or the lower section of an uncovered leg, particularly around the ankle. This unwelcome pest is most likely a species of stable fly (Stomoxys), a genus of flies belonging to the same family as the common house fly.

Stable flies are slightly more robust, yet smaller than the house fly, and are a little lighter in color. Additionally, stable flies are far more wary, as they are quick to burst into flight when something approaches them. Hitting one with a fly swatter is a far greater challenge than smacking the more sluggish house fly with the same long-handled instrument. And lacking a fly swatter in a boat can lead to great frustration, as it is impossible to kill these pests with any other object, other than the spray from a can of highly potent pesticide.

Like a vampire, the stable fly needs a meal of blood to properly function, which is unusual for a member of the housefly family. Enabling it to penetrate into a creature’s skin, the stable fly has evolved a sharp, piercing mouth piece that can be quickly inserted and withdrawn from the skin. Unlike other biting flies, such as the deer fly and black fly, the stable fly rarely fills its stomach in a single encounter. In most instances, this noxious bug extracts only a limited amount of blood before quickly taking to the air, only to return a minute or two later for another quick drink. It has been estimated by researchers that the typical stable fly eventually gets the blood it requires for its nourishment after biting a host 4 to 5 times. Also, the stable fly is unique among all other biting flies, as both the adult female and male attack warm-blooded creatures when in need of nourishment.

Like the deer fly and horse fly, the stable fly depends on its eyesight to find prey, which is the reason why this pest is only active during the light of day. Once it spots a sizable moving object, like a deer, fox, or woodchuck along the shore, or person exploring the water’s edge, the stable fly quickly travels to investigate, as its flying ability is extremely well developed. An individual in a canoe may be followed by one or several stable flies the entire length of a lake, as this bug is known to travel in excess of a dozen miles pursuing a potential meal, and waiting for an opportunity to land on a host that appears distracted.

Researchers believe that the stable fly is programmed to target the places on an animal where the skin is soft and the capillaries are close to the surface. This would be the belly of a dog or fox, the ears of a varying hare, and the ankles of a deer or human. However, if you are in a kayak without a shirt, this flying pest may land in the middle of your back or your shoulders where it is difficult to swat. Also, once one lands in a canoe, it frequently selects a spot near the bottom and waits there until it can strike at the person’s ankles.

The common stable fly prefers to reside in pastures, where it has access to domestic livestock, however, this fly is also capable of existing along the shores of wetlands where damp soil, rotting wood, and clumps of grass occur in moist conditions. A few other species live in wetlands, especially in the marshy sections around lakes, however, most of these insects are not well researched, and very little is known regarding their life history.

The common stable fly, however, has been studied, and after its meal of blood, the female lays her cluster of eggs in a patch of moist organic debris that offers the larvae an abundance of nutrients. A clump of excrement from a raccoon, fox, mink or otter would be ideal, however, many of these flies settle for a chunk of rotting wood or a patch of moist leaf litter.

Because it takes a larva well over a month to change into a pupa during periods of very warm weather, many fail to develop to this stage of the life cycle at summer’s end. With the arrival of cold weather, a larva enters into a state that enables it to withstand the freezing conditions of an Adirondack winter. Should the larva reach its pupa stage, it remains in that state throughout the winter, and continues its development into an adult the next summer when temperatures near the peak for the season.

There are many occasions when people on the water, or strolling along the shore, fail to encounter this unique insect. It is not as abundant and as wide spread as most of our other flying insect pests; however, you know one when you encounter one, as their persistence at trying to bite your ankles makes this bug a noteworthy member of the wildlife community here in the Adirondacks.

Photo by Wikimedia user Fir0002.


Tom Kalinowski

Tom Kalinowski is an avid outdoor enthusiast who taught field biology and ecology at Saranac Lake High School for 33 years. He has written numerous articles on natural history for Adirondack Life, The Conservationist, and Adirondack Explorer magazines and a weekly nature column for the Lake Placid News. In addition, Tom’s books, An Adirondack Almanac, and his most recent work entitled Adirondack Nature Notes, focuses on various events that occur among the region’s flora and fauna during very specific times of the calendar year. He also spends time photographing wildlife. Tom’s pictures have appeared in various publications across the New York State.




9 Responses

  1. Paul says:

    Thanks Tom.

    These “ankle flys” will nail you even in a motorboat, and they are good at hunkering down below the gunnels as you try and flush them out with a good breeze. Yes, they are also really hard to kill.

  2. Jim S. says:

    Times fun when your having flies!

  3. marco says:

    On occasion, they have been known to swarm a single boater. They will follow you for several miles while you paddle furiously to get away from them. Like the smaller blackflies, they often are detered by tucking your pants into your socks, but they can bite through light weight clothing. Nope, they are no fun.

  4. Steve Hall Steve Hall says:

    “God in his wisdom created the fly,
    and then forgot to tell us why.” Ogden Nash

  5. Marisa Muratori says:

    They are clever and quick, but they can be swatted dead, which gives one a feeling of satisfaction and accomplishment. Also, I find that a good herbal bug spray will keep them at bay for a while…an hour or so before reapplication is necessary.

  6. Mary Warner Marien says:

    These flies have been very active for about a month. They are drawn to the deep yellow-orange Rudbeckia (A western variety–not the usual dark-eyed Susans)planted in front of our camp. The flies are so interested in the flowers that they never bite us. If disturbed, they quickly fly back to the plants. Mary Lou

  7. Jay O'Hern says:

    It gives me great satisfaction to zap a stable fly with my electronic fly swatter.

  8. Mike says:

    These flies are the worst! I had always called them barn flies, which was close to their actual name. They are incredibly persistent, and like he says hard to kill. I find that dozens will fill my canoe this time of year, if wearing shorts they torment me the entire time!