There are many types of flies, and all of them are harmful. Horseflies seriously hurt domestic animals and even humans, transmitting many diseases with their bites. The common housefly is mostly a nuisance that can infest refuse and contaminate food. Fruit flies are big destroyers of fruits and other produce, leading to millions of dollars' worth of waste every year. Whiteflies, while they are bugs rather than true flies, are still harmful, since they colonize plants and live under their leaves, causing leaves to yellow and drop, and, in extreme cases, even killing plants.
For Fly DIY pest control, Partners has a number of effective pesticide products.