Houseflies not only transmit diseases between flocks by picking up bacteria while feeding on manure, they also pose a potential threat to human health as vectors of salmonella – a common cause of human food poisoning.

In fact, there are over 100 pathogens associated with the housefly which can cause disease in humans and animals, according to a Poultry World report on Farmers Weekly Interactive (FWi).

Houseflies are an annoyance not only to birds and staff but to local residents as well, who, according to Barbara Bell, ADAS's specialist principal environmental consultant, can be very vocal in raising the issue.

"Most calls I receive are from neighbours complaining about the nuisance and asking for advice," she says. When asked how she addresses the problem, Mrs Bell says she recommends an integrated approach in addition to the operational systems already in place.

The key to a successful strategy is in breaking the life-cycle of flies. A well-maintained poultry house automatically presents less of an inviting breeding ground. Good biological controls and a range of various insecticides targeting different points in the fly life-cycle will disrupt breeding patterns further, although the most effective controls are those that prevent the larvae from pupating.

"Ventilation, water control and manure management are crucial in maintaining a low fly population," says Mrs Bell.

"Good drainage from the roof is essential and water leaks must be instantly attended to as fly species need a moist breeding ground. A good flow of air throughout the poultry house also helps by keeping manure dry and aerated, which means that the flies' natural habitat becomes less desirable and breeding is minimised."

By following these simple and straightforward measures, and ensuring that houses are well managed, it reduces both the need to rely on other forms of control and the consequent risk of insecticide resistance developing.

FWi reports that biological controls also form an important part of this integrated approach and include litter beetles, which aerate the manure by tunnelling through the soil, and Carcinops, which eats fly eggs and first stage larvae.

Mrs Bell explains that Carcinops, which inhabits the top two centimetres (0.8 inches) of manure, is a niche beetle that can consume up to 24 house fly eggs and 54 larvae a day. "Unfortunately, we have not been able to breed the Carcinops beetle in laboratories," says Mrs Bell. "And we cannot transfer them from house to house because of the potential biochemical hazards."

Parasitic wasps are also increasingly used in poultry houses as they attack all common fly species without troubling humans or other biological controls. The wasp egg is deposited in a fly pupa, where it kills and consumes the fly as it develops.

On emergence as an adult, the wasp will immediately start breeding. However, as the fly population decreases, so does the wasp population as it loses its source of food and constant reintroductions may be required.

With these limits to biological controls, knock-down traps, baits and larvicides are also necessary as part of an overall fly control strategy. Treatments with larvicides need to be justified using a monitoring system that closely watches for the development of fly larvae in manure, recommends Mrs Bell.

She advises checking the manure weekly in the winter and as often as two to three times a week in the summer, especially when it is unseasonably warm. Scrape off the surface layers to reveal eggs and larvae and if 10 per cent of the area examined shows signs of first or second stage larvae, then Mrs Bell advises immediate treatment.

It is important to choose a larvicide – such as Dimilin Flo (diflubenzuron) or Neporex (cyromazine) – that will not affect the beneficial species, such as parasitic wasps, but is specific to flies. Another advantage of larvicides is that they can be used to treat sources of infestation of adult flies and, when used in combination with a knockdown insecticide, can prevent these fly-ins from breeding in the manure.

Dimilin Flo belongs to a modern class of insecticides known as the benzoylurea group which, unlike other neurotoxic insecticides, work as an insect growth regulator. They interfere with the development of larvae, preventing the normal moulting process, which causes them to die.

Mrs Bell warns of caution in using insecticides, as heavy reliance on some chemicals has led to the development of resistance. It is good practice to use a number of insecticides from various groups as part of an anti-resistance strategy, such as Dimilin Flo and Neporex (cyromazine) in combination with sticky traps over the winter months.

Poorly maintained buildings can lead to high populations of fly larvae in poultry litter, concludes the FWi article.