Rainy Weather Reduces Deer Fly Populations

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According to the Adirondack Almanac, the 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.

When a deer fly detects a human, bear, deer, or other sizable 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 transformation 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.

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.

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