Flies
What are they?
Flies are the adult insects of the order Diptera and are characterised by having only two wings. The mouthparts of houseflies are formed into a proboscis for obtaining liquid food by dissolving the food with regurgitated enzymes allowing it to be sucked up. They go through complete metamorphosis from egg, the feeding and growing larval states, pupa and adult. In suitable conditions, the life cycle can be very rapid (as little as 10 days for houseflies) and therefore infestation can build up readily. Domestic flies are associated with poor hygiene and rotting organic matter, as that is where the eggs are laid, the larvae develop and from which the adults emerge and feed.
House Fly: Musca domestica is a relatively large fly (about 6mm long) grey/black in colour with four dark stripes along the thorax. The females will lay batches of about a hundred small (1mm) cream, oval eggs on rotting refuse and other decomposing organic material. The eggs hatch within 12 hours and the larvae will feed, typically for 7 days, in warm conditions then they will pupate in a hard dark brown oval pupa. The adult fly emerges a few days later and will look for organic matter on which to feed. After mating, females will locate suitable egg laying sites on decomposing matter and restart the cycle.
Lesser House Fly: Fannia sp. is superficially similar to the housefly but is slightly smaller. It has a characteristic zig-zag flying pattern and prefers resting on light fittings, ceilings and walls. Its generally life cycle and feeding habits are similar to the housefly, but it generally breeds in animal manure.
Damage
Houseflies are pests in that they can carry diseases which can be transmitted to people when the flies rest and feed on food. They can also cause superficial damage (flyspecking) through leaving residues of vomit and faecal material on objects such as paintings, windows etc.
Control
Where houseflies are in large enough numbers to be a nuisance, control is usually achieved through good hygiene. Removing refuse, clearing drains, cleaning areas of decomposing material will normally cure the problem. Intensive poultry houses and pig units may be a source of fly problems.
Aerosol insecticide sprays are rarely effective.
Bluebottles, Calliphora sp and Greenbottles, Lucila sp. These are large flies (10-15mm long), with a characteristic metallic blue or green colour, and droning noise in flight. They are associated with dead animals and rotting meat on which they lay their eggs and the larvae feed.
The life cycle is similar to other flies, although the eggs must be laid on dead animal matter. The larvae (or ‘maggots’ to the angling fraternity), rapidly grow to about 15mm and then move away from the foodstuff to pupate in surround soil or similar.
Control
Where bluebottles are found indoors in significant numbers, it is usually because of a dead animal (perhaps mice under the floorboards or a dead bird in a chimney) or from poor food and refuse hygiene. There will often be an accompanying odour! Finding and removing the offending carcase, cleaning and sanitising rubbish bins etc., will cure the problem.
Bluebottles are attracted to UV light and may often be found near windows.
There are a number of species of flies that congregate in large numbers inside buildings in winter to hibernate. These are known as Cluster Flies. Eggs are laid outside in fields on animal dung etc., and develop there. However, in early autumn, the adult flies are attracted to warm south and west facing walls of buildings. As the weather cools, they will push through the tiniest of cracks around windows, between roof slates or other gaps in the building to form large, clustering masses inside loft areas, pelmets, cornices etc., in order to hibernate. On warm winter days, or if heating is switched on, the flies can wake up and fly around often accumulating in window areas as they try to return to the outside. Often only certain buildings or rooms will be affected, and invaded year after year.
Cluster Fly: Pollenia rudis is the most common cluster fly. It is about the same size as a house fly and easily identified by its large, reddish eyes, the thorax covered with yellow-gold hairs and its habit of folding its wings over its back when resting.
The adult flies lay their eggs on damp soil, rotting leaves etc., and the emerging larvae seek out and develop inside earthworms.
They pupate in the soil and the merging adult flies feed on nectar of flowers. In warm weather, up to four generations of flies may be produced. In late summer, large number of flies can ‘cluster’ as explained above, often giving off a sickly-sweet smell.
Green Cluster Fly: Dasyphora cyanella resembles the common greenbottle fly. The larvae grow in dung and the adults cluster in the same manner, often in association with the common Cluster Fly.
Autumn Fly: Musca autumnalis resembles a small house fly. The larvae develop in cow dung etc., and are commonly associated with cattle.
Damage
Cluster flies are primarily a nuisance insect when they invade the living areas of buildings, however the large number of dead flies that can accumulate in the hibernating areas can provide food for Carpet Beetles.
Some carpet beetle infestations in historic collections have been shown to originate in cluster fly debris on window sills, loft areas etc., where they have not been cleaned up regularly.
Control
As cluster flies develop in the surrounding countryside and may fly half a mile to hibernate, controlling the sources is impossible.
Sealing up windows, cracks etc., and underlining roof slates etc., to deny access is the only effective measure to prevent the flies entering a building. However, it is often not possible or practicable. In severe cases, insecticide can be sprayed around the edges of window to kill insects before they get in.
Where cluster flies are found in a building, vacuuming them up and destroying the bag is often effective for small numbers. Large accumulations normally need professional treatment using insecticidal smokes or fogs. In severe cases, external walls can be sprayed with insecticide to kill landing flies before they enter.
It is important that dead cluster flies are removed by vacuuming regularly as they can attract other pest insects.