Flies and Fly Problems20 May 2013
Information on the types of flies found on farms that may need to be controlled from 'Fly Management: How to Comply with Your Environmental Permit' from the UK's Environment Agency.
Just over 7,000 species of true flies (Diptera) are known to occur in the UK. Of these, around 10 species have the potential to cause regular and significant problems on and around waste management facilities and livestock sites.
Main Fly Species and Identification
Correctly identifying the fly species at a site, or reported at complainants’ premises, is critical to:
- clarify whether the complainant’s flies are the same as those at the alleged source
- establish appropriate monitoring techniques
- establish appropriate prevention and control techniques
With appropriate training the adults of most of the main fly pest species can be identified by eye, with the help of a x10 hand lens. Table 1, below, provides a brief overview of the common fly species which generate complaints and can be associated with livestock or waste management facilities.
Table 1: Fly species
In general, fly larvae occur in damp, decaying organic waste. However, each species will have its preferred niche in terms of temperature, moisture levels and the nature of the organic material. In the UK, there are two main areas in which fly problems regularly occur:
Waste management industry
Common house flies and bluebottles have always been associated with putrescible waste (this includes food and green waste) particularly during warmer weather. Infestation typically starts at the point of waste generation, when eggs are laid on waste in domestic or trade waste bins.
The longer the period of time before the waste reaches its final disposal point (landfill, composting, incineration) the greater the opportunity for fly problems to develop. In recent years the move towards fortnightly collection of domestic refuse, the introduction of a variety of waste processing techniques, and the reduction in the number of landfill sites and amounts of waste have increased the potential for fly infestation.
Rearing poultry (particularly for egg production), pigs, cattle or other livestock inevitably creates quantities of manure, which is vulnerable to fly infestation. The potential for problems is greatest in husbandry regimes where the manure remains within the animal house for extended periods (such as some free-range poultry laying systems). In recent years, the rapid growth in large-scale free-range egg production has resulted in more frequent problems with lesser houseflies, which prefer the cooler environment in free-range houses.
Fly Breeding and Development
The life cycle of most flies has four main stages: egg, larva, pupa and adult, shown in the diagram below (which refers to common housefly).
Figure 1. Common housefly life-cycle
In general, the adult female will lay eggs on a suitable surface for larval development; typically damp, decomposing organic materials. The larvae will hatch out and feed on the substrate, and when fully grown will search out a drier area in which to pupate. The adult fly will emerge from the pupa, mate, and continue the cycle.
The duration of the cycle is very dependent on the temperature of the larval environment, as shown in Table 2.
Table 2. Example of the effect of temperature on the rate of common housefly development
The higher the temperature, the more quickly flies develop and increase in numbers, and so the greater likelihood of problems. Most other fly species will develop slightly more slowly than the common housefly.
At temperatures below around 12°C, development will cease for most housefly species, while at temperatures above about 45°C, houseflies and their immature stages will be killed. As waste decays the microbial action generates heat, which can mean the in-waste temperature is warm enough for flies to breed even in winter months if waste turnaround isn’t adequate.
Although most adult flies stay close to their breeding sites (manure or putrescent waste), a proportion will disperse away and may cause problems at receptors. Houseflies are capable of dispersing over distances of several kilometres, although problems seldom occur at distances greater than two to three kilometres from the source. Significant problems likely to cause unacceptable nuisance levels tend to occur within 500 metres of the source. Regulators will look at the extent of fly breeding at the alleged source rather than how far flies have dispersed to ascertain the extent of the problem.
Dispersal factors can vary, but high levels of fly breeding at the source are what normally appears to result in high dispersal levels. Dispersal appears to be greater in calm, warm weather. A specific event, such as opening of poultry houses in preparation for removing manure, allows rapid dispersal which can cause a sudden increase in fly complaints.
Dispersing flies are difficult to find. Even where there are a large number of flies at a source, and concurrent problems with flies in nearby premises, flies are seldom visible in numbers in the intervening areas.
Problems Caused by Flies
The persistent presence of flies gives rise to a range of issues:
People find the continued presence of numbers of flies in their home or workplace irritating and unpleasant. Where there are breeding sites nearby, residents or employees may experience hundreds of flies in their homes or workplace. Where there are no breeding grounds nearby, one or two flies would be normal. The annoyance is often increased because houseflies are difficult to control with insecticides, and are particularly attracted to kitchens and humans.
Adult flies are often active on putrescent and microbially contaminated substrates. As a result, their external surfaces and gut will become contaminated with a broad range of pathogens. If these contaminated flies subsequently come into contact with people, livestock or foodstuffs, there is the potential for disease transmission.
In the tropics a major source of infections in humans can be traced back to houseflies. Although fly numbers and the opportunities for contamination in the UK are typically much less, a small risk still remains.
The physical presence of flies may lead to contamination issues. Flies may become incorporated into food products during manufacture, and have been found within the packaging of eggs being delivered for processing.
Fly spotting (fly faeces and vomit) on eggs can lead to their rejection or downgrading. The presence of large numbers of flies in an area might also interfere with the effective operation of other nearby businesses, such as vehicle repainting.
Of the flies regularly associated with waste or livestock sites, only the stable fly (Stomoxys calcitrans) feeds on blood. It normally feeds on large animals such as pigs, cattle or horses but may also bite people.
Egg producers with persistent fly infestation resulting in poor quality eggs may find that their accreditation to various food quality schemes are suspended, or that customers such as supermarkets decide to terminate their commercial agreements.