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How To Assess Fly Density on Pig Farms

Rowan Burgess |

Flies are a considerable nuisance on pig farms. They lead to a reduction in feed intake, which affects productivity, and they can also spread serious diseases to humans and other nearby animals.

Unfortunately, pig housing offers ideal conditions for these insects with warm, moist conditions and a readily available food source. 

Management and prevention strategies are essential, and assessing population density is a crucial part of these - apart from anything else, fly density levels tell you if your control measures are working.

Density assessments must be consistent, regular, and aligned with the particular fly species. Getting it right is crucial for your farm - but there's more to it than meets the eye! Let's continue.

1. Identifying Common Species of Flies on Pig Farms


The housefly is the most common species found on pig farms. Around 4mm-6mm long, they are grey with four noticeable black stripes on their thorax. Large red eyes help identify houseflies.

Attracted by manure and animal feed, houseflies pulverise solid foods into liquid as they don't have teeth, only spongy mouthparts.

Houseflies appear as soon as the warm spring weather arrives consistently and can hang around until the end of October, depending on the temperature.

Houseflies don't bite the animals and cause irritation and skin problems, but they are vectors for pathogens like E coli, salmonella and porcine reproductive and respiratory syndrome (PRRS).


Perhaps one of the easiest types of flies to identify, bluebottles have a visible metallic blue/green colour to their wings. A little larger than the housefly, they have large red eyes.

General farm waste, manure, and decomposing animal carcasses attract bluebottles in large numbers.

Filter Fly

Filter flies are one of the smallest species found on the pig farm. They are around 2mm long and have a distinctive jumping or hopping movement, which is how they travel.

Commonly seen around manure heaps in large numbers, filter flies love animal waste and organic matter.

2. Determining the Fly Density Threshold for Your Pig Farm

What Is Fly Density? 

Fly density is the number of flies present on the pig farm. It's impossible to eliminate all of them, and you'd go mad if you tried to do so!

Containing the fly population within acceptable levels is called the fly density threshold and is essential to good farm management. 

Fly density varies depending on the species and where and when you monitor.

Regular threshold monitoring is an essential element of effective insect control protocols. They confirm that management programmes are working and flag any changes before they develop into new problems.

By the time fly density becomes a significant infestation, you already have a major problem on your hands.

Factors To Consider When Determining Fly Density Threshold

  • Different fly species have specific density thresholds.
  • One-off inspections are less helpful than consistent and regular monitoring which builds up a complete and informed picture.
  • It may be necessary to evaluate the density threshold of specific species as adult insects during the larval stage.
  • Assessment should be consistent in terms of species, location, regularity and the time of day
  • Review different locations around the farm. This helps identify breeding areas.
  • Don't expect to eliminate every fly! Evaluating the density threshold is meant to help you keep the insects within an acceptable level - not an unobtainable one.

3. Conducting Fly Density Assessments

Performing the Assessments

Assessment locations vary depending on the species’ lifecycle and feeding routine. Establish the correct assessment points for your farm, and then always use these exact locations for consistent results.

Fly density assessments are carried out twice a week during the critical insect season, which is April to October and weekly for the remainder of the year.

Documenting and Evaluating the Data

Careful record-keeping is vital to producing valuable data. Record the date and time of day when the assessment occurs. Noting the weather conditions is also beneficial, as this can impact fly numbers.

Capture any changes to the area, such as moving pigs to a different location so housing is empty or relocating a manure pile. These findings can explain otherwise unusual variations in numbers.

With a fly density threshold benchmark, captured data is compared to it to confirm whether the numbers for a particular species are under control.

Accurate data also spots unusual fluctuations. It can help resist complaints from neighbours about high numbers or infestations, especially with precise time, date and location data.

The Clean Neighbourhoods and Environment Act 2005 does not contain specific levels, so there is nothing to compare fly numbers against. 

Therefore, deflecting a complaint can only be argued with recorded evidence demonstrating the management of fly density thresholds and regular assessments on the farm.

4. Implementing Fly Management Strategies

There are different fly control methods, including indirect methods, which involve farm design and layout, manure management and good husbandry, such as regularly replacing bedding material.

Direct methods involve active control of the different fly species. Control measures include:

  • Fly traps using water-activated bait in a trap bag or a simple sticky surface like a glue roll. These are economical and environmentally friendly options which are active 24/7.
  • Electric fly killers, which are especially helpful for public areas with customers
  • Insecticides which are fly sprays applied to different surfaces, many with a long-lasting residual action. These can be handheld or used in volume via a backpack sprayer.
  • Larvicides specifically designed to kill a particular species during the larval stage and stop adult flies from emerging.
  • Smoke Bombs which smoulder and send fumes to infiltrate hard-to-reach areas like corners, around and behind equipment and up into the roof

5. Assessing Fly Management Strategies

Why Is It Important to Monitor the Effectiveness of Fly Management Strategies?

Fly control programmes are a cost to the farm in terms of person-hours and product expense. Monitoring the effectiveness of fly management strategies ensures they provide value for money - and do not waste your time.

Farmers also need to know if control protocols are ineffective. This can affect the herd's health via illness and disease, increase costs and impact productivity.

Active monitoring of insect management strategies with carefully recorded data is helpful if there are complaints from neighbours about infestations. They demonstrate that proper measures are in place and that they're working.

Effective and demonstrable fly management strategies are also essential to comply with an Environmental Permit.

How Do I Know When To Make Adjustments to My Fly Management Strategies?

Suppose your fly management strategies are not keeping the density threshold on the farm to an acceptable level. In that case, this is a significant marker you should adjust before herd health is affected.

The idea of regular monitoring is to spot minor issues before they become significant problems.

If the fly density threshold for a specific species exceeds an acceptable level and there is nothing to account for this, it's time to change or increase your fly control strategies. 

If you're looking for a different control product or a change in your farm management, Dalton can help.

Final Thoughts

Strategic fly management control programmes on the pig farm are insufficient if you don't regularly assess their effectiveness. Pig farmers must know which insect species are present and how to evaluate the threshold density.

Different flies respond better to various control programmes, and much depends upon the farm's layout, the climate and the stocking density. There's no 'one size fits all' solution.

Finding the right help for your farm can be overwhelming with such a wide range of options available. Take the right approach for your farm with Dalton Engineering's insect control products. We provide various fly control options for all sizes and types of pig farms - and we're here if you have any questions.