Many types of fly can be classified as a pest; they’re usually rather keen to gain access to professional kitchens and other food preparation areas and because they’re so mobile, they’re high risk – moving from waste material to fresh goods can introduce serious illnesses to humans. Different species will concentrate on particular food groups (fruit flies, for example, are so named because they tend to lay eggs in and eat fruits, rotting the flesh in the process) and their life cycle varies depending on the type.
A sudden increase of houseflies buzzing around indoors can be an indication that a dead animal – a mouse or rat, perhaps – is in the vicinity; it’d be worth checking around to be sure this isn’t the cause.
Wasps have their own information page here
The harm they do
The carriers of disease and micro-organisms, flies can be the root cause of food poisoning in homes or catering establishments and, to this end, prevention is far better than cure. In some environments, such as recycling facilities, there are directives obliging the operator to have active fly management processes in place at all times.
Removal
Treatments to remove flies will vary significantly on the species and function of the building or rooms affected but, in most cases, an air-borne insecticide is used to ‘knock down’ the adult population and then the source of the problem found and dealt with.
In a commercial environment, by far the best ‘cure’ is to minimise the risk of flies entering the space in the first place: window and door screens should be installed and never propped open. Within the controlled space, electronic fly killers – special lights that attract the fly towards a high-voltage grid of wires – are usually used to eliminate any flies that do happen get past the perimeter. In other environments, such as homes, it is more common to use less sophisticated techniques, track the number and types of fly that are present and then apply the most suitable approach to remove them.