Late May and early June is the peak of black fly season in the Adirondacks, and the intensity and aggressiveness of the swarms of these small, dark-colored biting bugs varies greatly from one location to another and from one year to the next. From all indications, this year seems to be one in which there is a definite abundance of black flies in our forests, much to the delight of numerous species of insect eating birds that migrate north to feast on the seasonal abundance of bugs, but much to the dismay of hikers, campers and canoeists that want a wilderness experience free of flying insect vermin.
Black flies, also known as the buffalo gnats, form a group of true flies that are recognized by their small size, dark color, and arched back. Like many other flies placed in this category, the adult black fly depends on nectar and other sweetened fluids for the bulk of its nourishment. Only after the female mates and her eggs become fertilized does she require specific proteins contained in the blood of a mammal or bird. After a single meal of warm blood, these nutrients are absorbed and distributed to her developing eggs.
There are several dozen species of black flies in the Adirondacks, and each prefers to target different hosts at this time of year. Most strongly favor various species of mammals rather than birds, as birds tend to eat black flies. A few species of black flies are known to parasitize only a single type of host, however many investigate any larger, living entity that they detect even though they may opt to refrain from biting after crawling around on the creature’s skin for a brief period of time. Occasionally, a person outside will have numerous black flies buzzing around his/her head without landing, or biting. These individuals are likely black flies looking for a blood meal from another larger mammal, but just not from a human.
Researchers have discovered that the female black fly initially relies on its eyesight to locate a potential host. Because of this, the black fly is active during the day. Several hours after sunrise and an hour or two before sunset are especially favorable to this small and delicate creature as the air is usually more humid during this period when the sun is close to the horizon. Overcast days and times when a mass of muggy air has settled over the region are other occasions when black flies are active throughout the day. As soon as the air becomes warm and dry, dehydration can seriously impact this small fly. Hiking during the heat of the day when the relative humidity drops to under 30% is the ideal time when black fly season is at its peak.
During dry periods, black flies seek out damp settings, such as dense patches of woodlands where the air is always more humid. They also find a temporary refuge from dehydrating conditions by working their way into the damp leaf litter on the forest floor. This is why disturbing the soil during the day can stir up black flies.
Occasionally, during times of low humidity, some species of black flies temporarily migrate into the damp air that exists just above a quiet pond or lake. Should a canoeist pass through the area, he or she can quickly attract a small cloud of these tiny biting demons. The swarm may be reluctant to follow when the boater reaches shore if the air is too dry.
Like several other biting flies, the black fly also has the ability to “smell” the presence of carbon dioxide in the air. This information helps provide it with the exact location of a warm-blooded creature which produces a plume of this gas as it exhales. Breezy conditions cannot only interfere with the ability of the black fly to accurately maneuver through the air, but also quickly destroy the concentrated cloud of carbon dioxide immediately behind a deer, squirrel, dog or person. During the early morning and later in the afternoon, it is quite common for the wind to die down, which allows the black fly to again use its sense of smell to seek out a source of food.
Places where there is an exceptionally high concentration of carbon dioxide, like downwind from a campfire, can overwhelm the ability of the black fly to accurately locate a host. A fire is often an effective way of eliminating pesky black flies from a campsite, however, care should always be used when maintaining a fire, even in an outdoor fireplace.
There are several ways to deal effectively with black flies for the short period of time they are a nuisance throughout the region, however their presence should never discourage anyone from venturing into the woods at this time of year. It is just like the cold, if you take the proper precautions, you have little to worry about, except if you brought enough to eat.
Above, an illustration from The Book Of Camp-Lore And Woodcraft by Dan Beard; Below, the life cycle of the black fly (courtesy Purdue Entomology, Catherine A. Hill and John F. MacDonald, site authors; and S. Charlesworth, artist).