Monday, July 8, 2013

Nature: Where Are the Deer Flies?

800px-Chrysops_callidusThe daily round of intense rain that has plagued the region for the past several weeks has elevated most area waterways to abnormally high levels for this time of year, impacting many forms of animals. For one group of insects, the early summer flooding is particularly devastating, yet anyone that enjoys being outside at the start of this season can only view this widespread mortality as the silver lining to the persistent rains.

From late June through mid July, deer flies can be most annoying to hikers, campers, canoeists, and individuals that work in the garden, yet this year there seems to be a definite reduction, or complete absence of this annoying pest.

The deer flies form a small group of house fly-size insects that are very closely related to the horseflies. When one of these flies lands, it can be quickly recognized by its single set of wings marked with a conspicuous black band across the middle of this transparent appendage. Like a house fly, the deer fly holds its wings at an angle to its body forming a triangle-shaped silhouette.

When a deer fly detects a human, bear, deer, or other sizeable animal, it initially flies in circles around the creature’s head to assess its value as a potential host and to locate a suitable landing spot. A deer fly is also characterized by its ability to withstand a fairly good smack after it lands on a person and begins to bite. Often the fly falls lifelessly to the ground after being struck, yet within a few seconds uprights itself and then eventually takes to the air again in an attempt to make another attack.

Like other biting flies, it is only the female that is intent on biting, as she requires a healthy meal of warm blood for the development of her eggs. To accomplish this, the female has a mouth equipped with two sets of slicing teeth that enable her to saw through the skin of a host and sever an underlying blood vessel or two. It then coats the tiny gash in the skin with saliva which contains anti-coagulating substances that helps prevent the blood from clotting as it encounters the air. The deer fly then laps up several drops of blood that flow from the wound before retreating to a place of safety.

Within a few days, the female lays a cluster of several hundred eggs on the underside of a leaf that overhangs the shoreline or a shallow section of a body of water. Different species of deer flies seek out slightly different aquatic setting when the time comes to lay eggs, as the larvae of each species prefer different underwater environments.

Immediately after hatching, the larvae drop into the water and eventually find a submerged spot in which to pass the next 10 to 11 months. After undergoing numerous molts, the larvae reach a stage that allows them to survive the rigors of winter. In spring, as the water warms, the larvae resume an active existence and eventually work their way up the stem of an aquatic plant, or move onto the shore where they transition into a pupa. During this transformative phase of their life cycle, the pupa gains the ability to extract oxygen from the air rather than the water. This is typically at a time of year when water levels are dropping as melting snow has finally been exhausted and spring rains have yielded to a dry summer weather pattern. Should the water level rise and engulf the deer fly’s pupa, the developing fly inside will eventually drown. Many organisms with an aquatic stage are well adapted to deal with high water level in the spring, but not in summer.

Also, once an adult emerges from its pupa case, it has a body that has evolved to deal with the hot and sunny days of early summer. Just as is the case with a dragonfly, the muscles that power a deer fly’s wings require sufficient thermal energy to function. The low to mid 70’s is believed to be the minimum temperature range needed to permit a sustained period of flight. Even though the low 70’s may feel uncomfortable when the humidity is high, it does not provide favorable conditions for deer flies to regularly fly. Also, because of the intensity of the sunlight around the solstice, a deer fly has evolved a set of eyes capable of functioning in bright light. On overcast days, a deer fly loses its ability to quickly spot a potential host, which reduces it nuisance value.

The heavy overcast days that our region has experienced over the past several weeks have made it difficult for those deer flies that were successful in hatching into adults to see and acquire enough internal warmth in our muggy atmosphere this year to fly. Flooding throughout the Adirondacks has been unprecedented for this time of year. That can be noted by consulting weather records and by taking a hike along a forest edge and encountering nothing but mosquitoes.

Photo: Deer fly, courtesy Wikimedia user Bruce Marlin.

Related Stories

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.

13 Responses

  1. Paul says:

    Have to consult my friend who walks along the Bloomingdale bog trail for exercising his dog (and himself). One time on a two mile jaunt he told me he killed 65 deer flies. Several hits to the head killed 5 or 6 in one shot! Wonder what this summer is like. Where I have been it seems like plenty of flies are surviving, can’t really see a difference. But the mosquitoes are out in force for sure!

  2. Bill Ingersoll says:

    With all due respect, I have to challenge any perceived lack of deer flies this season. In my extensive backcountry explorations in the southern Adirondacks, there has been no shortage of deer flies. I have been encountering them for several weeks, and their numbers seem to be normal.

    In my observation, black fly populations have been below expectations for several seasons. Maybe I’ve just been lucky, but I have not encountered the swarms that I did up to a few years ago. Not that I’ve been disappointed…

    Mosquitoes, on the other hand, were horrendous in the backcountry for the last 2-3 seasons. This year, despite the rain, their numbers seem to be more normal.

    • Paul says:

      Bill it is funny where I have been in the northern Adirondacks I have seen hoards of mosquitoes. I mean it stinks! Deer flies are abundant enough but maybe somewhat lower than I normally see in one riverside area that I travel that is usually swarming with the darn things. Maybe there are some localized differences? The rains have certainly been delivered in some pretty localized downpours at times.

      Question: Do we have some species of mosquito that are more likely to feed during the day that we used to? I know that in some places down south (Maryland for example) they were mostly a evening pest now they are an all day nuisance.

      Are these “Asian Tiger Mosquitoes” making their way north??

      • Tom Kalinowski says:

        Hi Paul: Mosquitoes are very sensitive to a dry atmosphere. This is why they tend to remain in shady places, especially damp forested areas during the day when the humidity is low. Since we have had numerous days over the past few weeks when the humidity has been extremely high, the mosquitoes were able to come out and not be concerned with the problem of dehydration. Chances are that if you encounter a mosquito during the day, it will be in a damp, humid setting, and not in the drier air in the sun.

    • Tom Kalinowski says:

      Hi Bill: In the southern Adirondacks the temperatures might be just warm enough to stimulate the deer flies compared to the higher elevations in the more northern sections of the Park. What was the weather like when you encountered them? (Sunny or overcast?, Temps in the 70’s or 80’s?) I have had several encounters with deer flies, but all have occurred during those brief sunny periods when the temperature climbed to near 80. I certainly have not experienced anything yet like last year. It is interesting that you also mentioned the lack of black flies, as I encountered more black flies this spring than in the past 10 years. They were very concentrated during the last week in May and then seemed to disappear. Lastly, I have to agree with you on mosquitoes. There doesn’t seem to be any more of them around than usual. Thanks for your comments and for reading the Almanack.

      • Bill Ingersoll says:

        “What was the weather like when you encountered them? (Sunny or overcast?, Temps in the 70′s or 80′s?) I have had several encounters with deer flies, but all have occurred during those brief sunny periods when the temperature climbed to near 80.”

        Those are pretty much the conditions I would EXPECT to see deer flies! They don’t come out at night, after all, when the weather is cool.

        The point is, you have anecdotal evidence, I have anecdotal evidence, but there is not any science–so there is no real story here.

        Last year, I stopped at a DEC trailhead near Number Four to go for a hike. The deer flies were so bad they were swarming my car before I even came to a stop in the parking area. Fearing certain death, I continued up the road to Twitchell Lake. There the deer fly population was nowhere near as intimidating.

        The point is, there are a number of variables to consider–factors for why one species of insect might be more populous in a certain location at a certain time and under certain conditions.

        If I had known someone was wondering where the deer flies were, I would have happily saved all the ones I killed this weekend. They would’ve been easy to pack out.

        • Local Yokel says:

          “but there is not any science–so there is no real story here.”

          You are ridiculous Bill. This is a blog post, not a peer reviewed journal article. Mr. K is a respected naturalist, educator and author who has actually lived in – not just visited- the northern Adirondacks for more than 30 years. Given his familiarity with the area and his professional expertise, I’d trust his anecdotal evidence in this case. I’m betting he’s compared his observations made this season to those made daily over the last couple of decades.

          Mr. K, you were a wonderful teacher, and I always enjoy your columns. However, I humbly suggest that before you write another blog post you submit it to Bill Ingersoll so that we all may rest easy knowing it meets his standards of scientific rigor.

  3. Eagleye says:

    wow. local yokel needs to take a chill-pill. what’s up with that ?
    I too very much enjoy Mr. K’s articles, and his biology is spot-on, but I fear more wishful thinking in case of deer flies than reality, unfortunately for us outsiders !
    Keep up the good work Tom.

  4. bill says:

    If you are missing them, please come up to the Town of fine area. You can take some back with you….we have plenty to spare

  5. TR says:

    I camped and fished at Little Tupper this past June 17-21. Expecting the normal hord of deer flies and black flies, my stay was amazingly pest-free. Even the number of mosquitoes was minimal. Best spring Adirondack trip ever.

  6. Larry Hall says:

    Mr Kalinowski:

    I live in the Town of Salisbury located in north central Herkimer County and – although I’d like to spend more – spend a considerable amount of time in the forest in northern Herkimer and Fulton County and Hamilton as well. I’ve noticed a reduction in the number of deer flies this summer thus far as well as a reduction in Black flies also. That is not to say they are absent or scarce, just fewer where I’ve been so far this year. Naturally the bugs took longer to really become pesky because of the cooler spring that we had in the areas I travel.

    For the rainfall amounts we’ve received here the mosquitoes seem like they have been worse, but they’re hot enough. I pay attention to these things because I’m a birder and I hate to use insect repellent if I can get away without it.

    I was in the Stillwater area and Ferd’s Bog last summer and the deer flies were so abundant that clouds of them would follow the car. Thus far this year I haven’t seen them to this degree. In any event they are still here – just fewer.

    Really appreciated your explanation and consider the science you applied to be ” true science “. You’re obvious expertise which came to be through academic study and life’s experience is very reliable and I appreciate you sharing it with others such as myself. Thank you.

  7. barkeater says:

    It’s summer in the Adirondacks, the bugs suck, that’s just the way it is and the reason for DEET. Happy hiking!

  8. You might be interested in my deer fly blog ( I trapped nearly 13,000 deer flies during the 2012 season on my property in SE Michigan. This year the fly numbers are way down. Not sure if this is due to my trapping efforts or to the fact that last spring and summer was so dry in Michigan (near drought conditions). In any case, the numbers are much much lower than they’ve been in the 23 years we’ve lived on our property (a heavily wooded 30-acre parcel that is part of a bigger, more extensive span of woods).

Wait! Before you go:

Catch up on all your Adirondack
news, delivered weekly to your inbox