A to Z of Pests

Seasonal Pest

House Fly

Musca domestica

Flies are classified in the order Diptera, of which there are possibly as many as 250,000 species. Several thousand species have medical, veterinary or public health significance.

The wings of a House fly typically beat at around 200 Hz, moving them forward at around 8 km an hour. The underside of their feet is covered in hairs, which secrete a sticky substance. This allows them to walk on smooth surfaces such as glass, or upside down on ceilings.

The order Diptera means “two wings”. However, only one pair is used for flying. The second pair are much reduced and shaped like drumsticks. Known as ‘halteres’ they serve as gyroscopes, informing them of changes in direction as they fly.

Appearance

Adult house flies are 5-8 mm in length. They have a grey thorax with four narrow black stripes running lengthways down it. The abdomen is buff or yellow in colour. The 4th vein which runs across the centre of the wing is sharply bent forward so that it reaches the wing edge just behind the vein in front, and the wing tips are slightly pointed.

The fly larvae, or maggots, have no legs and are white in colour. They are cylindrical in shape, tapering to the head, and reach 12 mm when mature.

Life Cycle & Biology

Flies have a "complete" metamorphosis, passing through four distinct stages – egg, larva, pupa and adult. The adults mate, and the female fly seeks out a suitable breeding material on which to lay her eggs. This is typically moist decaying vegetable matter e.g. rubbish tips, manure heaps and food residues in kitchens. Larval development takes 3-60 days to complete before they migrate to a dry location to pupate. 5g of residues with high protein content can support about 100 larvae to the pupal stage.

The pupa is barrel-shaped and dark red-brown in colour. During the pupal stage, the body of the larva is completely remodelled into the adult form. The adult emerges 3-28 days later. Once its wings have expanded and hardened it can fly off in search of food and a mate.

Pest Status

Because of their preferred breeding sites in fermenting material, many flies are likely to pick up and transfer pathogenic bacteria, particularly food-poisoning organisms. Flies may end up as contaminants in processed foods. They also have a nuisance value; most people object to sharing buildings with large numbers of flies.

Control

Flies may be controlled by preventative measures, such as cleaning up the food residues in which they may breed, denying access to breeding sites (by using tight-fitting lids on refuse bins and burying domestic and industrial food wastes), and by fitting fly screens (netting) to window openings.

In addition, insecticide may be sprayed onto surfaces on which flies alight, and insecticidal sprays and dusts may be applied to breeding sites.

Electrical UV fly-killers may be installed as part of a fly control programme. The insect-electrocuting type (Electronic Fly Killer) should not be installed immediately above areas where fallout of dead insects might be a problem. In such areas a unit which entraps the flies on an adhesive sheet (Luminos unit) should be used. UV fly-killers should be located where they are not visible at night (from outdoors) through open doors and windows. This is to prevent attraction into premises, of outdoor night-flying insects.