Flies have both tremendous positive and negative effects on human society. There are more than 100,000 known species of flies in the world, and they are only out-numbered by the beetle, butterfly and bee insect groups. Their presence alone is often annoying to people, but the true effects they have on humanity go largely unseen. Some flies carry very deadly diseases and infest grain and meat in large numbers. At the same time, they also are responsible for much of the pollination of plants humans consider valuable and are an important food source for many animals.
Sand flies found throughout South America, Africa and Europe are carriers of a microorganism responsible for a disease that eats away at human skin, known as Leishmaniasis, and for which there is no definitive treatment. Other parasites that flies carry can cause diseases from sleeping sickness to typhoid fever, cholera and even leprosy. The flies themselves are simply carriers of the parasites and diseases. They pick them up from rotting food, excrement, and people or animals that are already infected. They seek out these conditions as food sources and to lay their eggs.
Stored and raw foods are a common location for flies to lay eggs, as when they hatch the larva will have something to eat as they mature. Almost any kind of dried food including grains and cereals are a target for fly infestation, as well as processed foods such as crackers, pasta, nuts and dried fruits. Though flies are attracted to raw meat and decaying food and garbage, they will also infest cured meats as well as pet food and even birdseed. Any type of food product can be a target for fly infestation, if they have a way of getting to it. It is possible for them to penetrate unopened paper or thin cardboard and plastic packaging to lay their eggs as well.
Many fly species have a very negative image in human society, while an easily overlooked fact is that they are also a very important pollinator of flowers, fruits and vegetables. Any flying insect that travels from flower to flower for pollen will propagate plants. Some fly species feed on flower pollen and nectar just as bees do. At least 550 varieties of flowering plants are known to be regularly pollinated by one fly species alone, the Diptera group of flies. Research has found that flies are a leading pollinator of 87 out of 115 leading food crops worldwide.
Many temperate climate songbirds rely on insect diets that include flies. The golden plover, for example, is known to be highly dependent on crane flies; baby golden plovers have a much better chance of survival if fly populations are strong. The fly population is an important part of the diet of hummingbirds, purple martins, sparrows and so on. The nesting season for most birds is in fact exactly when insects begin to hatch, so flies are an important part of the food chain for bird species in general.