Fly Monitoring

Guidance of monitoring flies around farms and other facilities from the UK's Environment Agency from its publication 'Fly management: how to comply with your environmental permit'.
calendar icon 27 May 2013
clock icon 11 minute read

The following techniques can be used as part of a one-off inspection to gain an idea of the level of infestation, or regularly as routine monitoring to build up a picture of trends in fly numbers. Operators normally carry out routine assessments as part of their proactive fly control work.

The benefits of fly monitoring include:

  • Trends in fly numbers at the alleged source can be compared with trends in numbers in complainants’ premises, possibly providing evidence of a link.
  • Monitoring data from different parts of the site, e.g. different poultry houses, or different areas on a large landfill, can be used to identify localised areas where fly breeding is occurring. This will allow specific causes to be identified so operators can use more focused or intensive control efforts.
  • Monitoring flies throughout a cycle will allow ’normal’ levels to be established. Any rise in numbers will be noticeable, so early additional control measures or treatment can be put in place.
  • Where records have been recorded over several seasons, they can predict impending fly peaks, so allowing pre-emptive fly control work.
  • Comparing fly numbers before and after particular fly control measures have been used will indicate the effectiveness of the treatment. This is particularly useful if the officer suspects that the treatment is not being used correctly or that resistance to specific chemicals is becoming apparent.

Monitoring Adult Flies at the Source

Indoor resting counts for Common House Fly

This species readily rests in numbers on structural surfaces within buildings, like poultry houses or waste transfer stations, so resting counts are often used to indicate relative population size. Typically 1-metre × 1-metre squares are outlined with white paint on internal wall surfaces, with the centre of the square at about head-height.

There may be four to six squares in a poultry house or waste transfer station. Squares should be located in areas where flies are seen to be resting, away from frequent people or vehicle movements, close to likely fly breeding areas, and where the square will not subsequently be obscured by manure, waste or other materials. The operator counts and records the number of flies resting within each square at regular intervals, for example up to twice-a-week from April to October and once–a-week at other times.

The squares should be brushed occasionally to remove dust and cobwebs, and should not be sprayed with insecticide. Experience shows that a count of less than five flies per square can be considered normal, whereas numbers above 20 per square indicate a problem to be investigated.

Indoor adhesive paper traps for houseflies

Adhesive fly papers (see Figure 1) are used to monitor lesser housefly numbers. In each building two to four 30-cm wide rolls are hung up at about head height in areas where flies have been noted. At weekly intervals, a length of paper (approx. 30cm) is pulled down from the roll, and at the end of the week, the flies stuck on the exposed paper are counted and recorded.

The paper should then be torn off the roll, covered with cling-film and retained so flies can be identified and counted. A fresh 30cm length is then pulled down ready for the coming week. Operators should carry out counts from April to October, and at some sites may be required throughout the year. Fly counts increasing to 20 or more of one species of fly on a paper in a week indicates that numbers are rising and may cause off-site annoyance.

Note: Fly numbers will vary greatly between sites. Changes in numbers indicate changes in fly activity. Remember, there is no absolute number determined as a nuisance.

Figure 1. Adhesive fly paper used for fly monitoring in a poultry farm

Open-air Scudder grid counts for houseflies at waste sites

A Scudder grid is a standard 60-cm square wooden slatted grid, which is dropped onto the surface of the refuse. After a period of 10 seconds, the flies resting on the grid are quickly counted and recorded; these are likely to include common housefly and bluebottles, so an element of identification is necessary.

Figure 2. Scudder grid in use at a landfill site

The count is repeated 10 to 15 times in areas with higher fly numbers, such as on and around the active tipping face. Counts should be carried out at times when flies are active, typically between 10.00 and 16.00hrs.

Avoid doing counts in cold, windy or wet conditions. Counts should typically be carried out two to three times per week from April to October. Regular monitoring can determine 'usual' numbers for that site and therefore any rise will be easily seen.

Open-air adhesive paper catches for flies at waste sites

At open air sites, such as landfill sites and waste transfer station boundaries, operators can use adhesive papers to monitor fly numbers. Pieces of adhesive paper (around 30 × 30cm) can be attached to a post around the site for a week, then removed, replaced and the catch counted. The limitations are that the papers cannot normally be positioned on the active tipping face as they will be damaged by vehicles. The papers also catch large numbers of non-pest species, which have to be separated from the pest species before obtaining a final count. Birds take the captive flies on papers, and adverse weather (wind and rain) will also affect the catch.

Monitoring larval flies at the source

Scrape-and-count, for common houseflies

Operators can monitor larvae by scraping the top two- to five-cm layer from the surface of the manure or waste over an area of approximately 30 × 30cm. The number of exposed larvae is quickly estimated, and recorded.

This should be carried out at four to 10 locations within each building/vessel, depending on its size and the variability of the material. Larval stages are usually found where undisturbed damp manure or waste is present, rather than on surfaces that experience a lot of movement. Drainage channels/grid drainage channels that have a waste residual within them can become 'lucrative' nurseries for flies.

Good monitoring locations are those where this material is present for extended periods, such as the manure pit in a poultry house or the tipping face of a landfill site, allowing a series of counts to be taken and trends to be established. In premises where there is a high turnover and removal of substrate (belt removal systems in poultry houses, or well-run transfer stations), then routine monitoring of larvae may be inappropriate, and monitoring will be based on adult counts.

Counts should be repeated up to twice-a-week from April to October. Consecutive counts should not be carried out on exactly the same area of manure or waste. One or two larvae would be considered 'normal', if more than five are found then action is needed.

Sample-and-count for lesser houseflies

The majority of severe problems with lesser houseflies occur almost entirely in free-range poultry layer units. Counting larvae in-situ is not appropriate for lesser houseflies because of limited access to the manure in free-range poultry houses, the difficulty in seeing the young stages within the manure, and the difficulty in separating larvae from pupae. Instead, operators can use a long-handled trowel or similar to scrape a sample of around 300g manure from the top five- to seven-cm of the pit surface, which is then put into a white bowl or tray. They should then check each sample and count and record the number of larvae and pupae present.

At best, the operator should obtain and check around four manure samples per week per poultry house. Samples should be taken from the manure pit wherever there is access. If possible, operators should try to get both drier (from edge) and wetter (from under drinkers) samples.

Again, very few larvae or pupae should normally be found. If the number is high, action is needed.

Monitoring adult flies at complainants’ premises

Monitoring should be carried out in indoor locations where the flies regularly occur in numbers. This may be a porch, outbuilding, conservatory, kitchen etc. The Agency does not recommend monitoring houseflies outdoors owing to the diversity of the catch, and the risk of catching birds or bats on adhesive papers

Figure 3. Adhesive fly papers in a resident’s home

Traditionally, long adhesive fly papers are used, available from DIY or farm supply stores. They are cheap, easy to use and catch flies well, although they can be difficult to handle and store after use. Single-sided adhesive fly papers should be transported in cardboard boxes and can then be stuck on white paper to identify (where possible) and count flies.

Officers should take photographs of papers as they may be required for evidence. Fly papers should be replaced weekly throughout the infestation, and continue to be used after numbers have dropped so that 'usual' numbers can be recorded. For enforcement cases, it is essential to be able to state what is 'usual' in that area. Experience has shown that fly papers can attract up to 500 flies, and that a number of 50 or more of one species in a week indicates a significant breeding ground nearby.

Alternatively, more specialised adhesive fly catching products devices are available. Although these appear to catch fewer flies, the devices may be conveniently labelled and stored after use, and re-examined later as required. One or more adhesive devices should be hung up in each premise. Adhesive devices should be labelled and changed weekly. The catch should be identified, counted and recorded by the investigating officer.

Liquid-baited fly traps are not suitable for fly monitoring as the traps cannot be stored after use and the flies quickly decompose, making them difficult to identify.

Interpreting fly monitoring data

How many flies at the source constitute a problem?

There is not a fixed relationship between the number of flies at a ‘source’, and the risk of nuisance in neighbouring properties. The risk of nuisance occurring will depend not only on the number of flies at the 'source' but also on other factors, such as the distance from the 'source' to the neighbours, the attractiveness of the neighbour’s premises for the flies and the weather. For example, one poultry site may have very high fly numbers but not cause any problems because the neighbours are too distant, while a similar poultry site situated only 100 metres from neighbours will have to work very hard to prevent nuisance.

Through fly monitoring and feedback from the regulator, individual sites may develop an understanding of what fly levels on site correspond with complaints from neighbours. This enables a threshold fly count to be set for that site, above which complaints are likely to occur. Fly management measures at the site should then be timed to prevent fly numbers exceeding this threshold.

How many flies in a complainant’s property constitute a nuisance?

As with other issues such as noise or odour, there is no rigid definition of how many flies represent a nuisance. Experience shows that some households will complain vigorously about five flies in their home, while other households will quietly tolerate thirty flies. Defra’s guidance on interpreting the Clean Neighbourhood and Environment Act 2005 says that:

‘There are no objective levels at which a statutory nuisance exists or may be caused. In general, in domestic premises, it is likely that the threshold will be very low and control actions might be taken in cases of few house flies.

As a guideline, an occupier will normally experience some irritation if there are five or more ‘flying’ house flies present in any one room at any one time on three successive days. If house flies are monitored with baited traps, sticky ribbons, or spot cards, a collection of more than 25 in any 48-hour period may indicate grounds for distress.’

In practice, there is a significant difference between something being an irritation and a situation constituting a nuisance in law. Therefore, officers should focus monitoring and investigation in properties with an unequivocal problem.

Defra’s figure of more than 25 houseflies caught on fly papers in 48 hours identifies properties with a significant problem. However, for regular monitoring, weekly visits are more practical than 48-hour visits, and in practice a catch of more than 50 houseflies per paper per week would indicate a premises with a significant problem as outlined above.

Investigating officers should also hang fly papers in premises where no fly problems are reported to get an idea of 'normal' background fly numbers.

In general, claims of fly nuisance relate to indoors. Claims about fly nuisance outdoors are much more challenging because of the numbers and diversity of other non-pest insect species. Complainants about flies in gardens rarely identify the species involved.

May 2013

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