Blackfly (Simulium Spp.)

Flies That Love Humans

When you decode the Greek construction of "anthropophilic," it sounds like a good thing: "human loving." When the term is applied to biting insects, however, anthropophilic takes on a more annoying, or even sinister, connotation.

Some of the most annoying, if not quite sinister, of biting insects, are the black flies that love human beings in the late spring and early summer in North America. blackflyThere are more than 40 species of black flies on the continent. Not all are anthropophilic. One species, for example, has never been found on anything but loons. Other species are generalists that will prey on any blood-carrying creatures, including humans.

Black flies lay their eggs in the flowing water of streams and rivers. The larvae that hatch from the eggs attach themselves to rocks and plants in the stream bed, where they feed on detritus, bacteria, and algae and just about anything else they can filter out of the water flowing past them. As adults, virtually all black fly species get nourishment from flower nectar. Males feed exclusively on nectar.

Females of many black fly species, however, require the protein from a blood meal in order to form and lay eggs. When they are in a maternal mood, they sample the breezes for excess carbon dioxide, a sure sign of animal respiration. Following the trail of carbon dioxide and breath odors, they track down their intended victim.

Once they get within about 15 or 20 feet, the flies switch to visual mode and home in on a landing site. As they get closer, black flies sense the heat of their prey. Once the fly lands, it switches back to its sense of smell to determine whether it is in a good location for blood.

All this loving of humans is not applied universally. Some people claim they are unusually affected by black flies ... or unusually loved, I suppose. Research done by Dr. James Sutcliffe, an entomologist at Trent University in Ontario, is proving them right. By stationing brave volunteers in a field and counting the black flies that descend on them, Dr. Sutcliffe and his student, Steve Schofield, have documented the fact that some people truly are more lovable to black flies. The reason seems to be largely that those people exhale more carbon dioxide than most. With every breath, they advertise, "Here I am!"

Dr. Sutcliffe, and many other researchers around the world, are trying to figure out exactly how black flies and other biting insects find their victims, so they can thwart the process. Fighting black flies is a big deal, affecting the economics of farming and springtime tourism in parts of Canada and the United States. When female black flies go looking for blood in swarms, they are annoying in the extreme. In some parts of Canada, black flies can be so thick and relentless that they drive cattle and dairy cows to distraction. Because harassed bovines may not eat enough to put on weight, black flies are a major factor in determining the northern limits of livestock farming in Canada.

Black flies in North America are not known to transmit any diseases to human beings, but in other parts of the world, the flies' love for humans is more destructive. Onchocerciasis, also called River Blindness, is a disease caused by a parasitic worm carried by black flies in Africa and parts of Latin America. As its common name suggests, onchocerciasis can cause blindness. Millions of people are affected by the disease, and fear of it leads to the abandonment of large areas of fertile land near rivers.

Because of the health and economic impact of black flies, many methods of control have been developed in this century. The most effective is probably the application of DDT to rivers and streams to kill the larvae before they turn into biting adults. Though DDT is banned in the United States because of its widespread toxicity and persistence in the environment, it is still used in Africa. Pesticides that are more specific to black flies and less harmful to "non-target" species have been developed and are also in widespread use in Africa and North America.

About twenty-five years ago, a bacteria was discovered in Israel that has extraordinary effect on black fly and mosquito larvae. This bacteria is a special form of Bacillus thuringiensis, commonly called Bt and used to control insects in agriculture and backyard gardens. Application of the special strain of Bt to streams typically causes the death of more than 95% of black fly larvae within a day or so. This "biological" control is much more environmentally friendly than chemical pesticides like DDT. The bacterial pesticide has little or no direct effect on fish, amphibians or other aquatic insects and is economically competitive with pesticide application. It also disappears rapidly.

For these reasons, the Bt pesticide is now widely used in North America to control black flies and mosquitoes. It's hard to argue against the idea that the use of Bt to control biting flies represents progress, compared to DDT. But I wonder about the long-term wisdom of such an aggressive battle against black flies.

I probably won't win any friends by being "black fly- philic," but they are an important component of ecosystems. Adult black flies, when they aren't biting people, are significant pollinators of many flowering plants. In many streams, black fly larvae are the most numerous insects, sometimes found in densities of a million larvae per square meter. They play a key role in recycling plant and animalblackfly detritus. They are in turn a major source of food for fish, dragonfly larvae and other aquatic predators. One multi-year study of Bt use on mosquito larvae in Minnesota wetlands showed that while other aquatic creatures were largely unaffected in the first year of application of Bt, the repeated elimination of mosquito larvae over several years changed the fauna of the wetlands in significant ways. It seems likely that similar results would be found for black fly larvae treatment in streams.

So maybe we should think again about large scale control efforts, at least in areas where black flies don't cause human disease. Researchers like Dr. Sutcliffe are trying to find ways to attract and trap biting flies in specific areas where they cause major problems, such as pastures. For example, one highly anthropophilic species of mosquito in Africa is attracted to the odor of smelly feet. It is also -- as silly as this sounds -- attracted to Limburger cheese. It turns out that the bacteria that cause foot odor are very closely related to the bacteria that turn milk into Limburger cheese, and they give off a similar smell. Traps baited with the cheese, or with isolates of the cheese odor, may someday reduce the biting of humans. Other studies are developing more effective repellents to protect livestock and people. These efforts seem to me to be a more environmentally safe source of relief.

As for personal relief, the next time you are in an outdoor crowd and the black flies seem to be singling you out for their human lovefest, remember, they are attracted most of all to carbon dioxide. So run like crazy, but don't forget to hold your breath. 




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