Parasite Watch: Treat flies now to prevent production losses

Livestock farmers are being advised to get ahead and protect stock from flies in order to prevent production losses.

When flies take hold, it is harder to get them under control, warns Zoetis vet Dave Armstrong.

“By the time you notice cattle or sheep being troubled by flies, a population explosion is already taking place.

See also: Parasite Watch: What sheep worms are rife in your region?

“Flies not only annoy your herd and flock, but they can cause major economic losses through reduced feed intake, which affects productivity,” he says.

Studies in cattle have shown fly worry can cause growth rate losses of up to 0.3kg/day and milk loss of 0.5 litre/day.

In sheep, more than 5.5kg can be lost over four to six days and wool production reduced by up to 26% in blowfly strike cases.

“Not only do they worry animals, but flies can also be involved in the transmission of diseases, such as pink eye and summer mastitis”, adds Dr Armstrong.  

“Midges can also spread diseases such as bluetongue and Schmallenberg and blowflies are important due to the damage caused by their maggot stages.”

There are three main types of flies that affect livestock – nuisance, biting and strike flies. Different fly species appear at different times of the year, meaning they are a constant threat.

“This is why you need to treat early and regularly throughout the fly season in order to protect stock from the different types of flies,” says Dr Armstrong.

Type of flies

  Nuisance flies Biting flies Strike flies
Where are they found? Nuisance flies breed in dung or decaying matter and are attracted to the mouth, nose, ears, eyes and wounds. They feed on secretions. Their presence causes irritation, scratching, rubbing and wounding. Biting flies include stable flies, hornflies, horseflies, midges and mosquitoes. They breed in dung, around water or in damp and muddy areas. Blowflies, which include the bluebottle and green bottle, are the most common species in the UK. Larvae feed on host tissue, but may also feed on decaying organic matter and carcasses.
What do they transmit? The two main species affecting livestock are face flies, which transmit new forest eye (pink eye), and head flies, which transmit summer mastitis and can cause black cap or broken head in horned sheep. They have piercing mouthparts that bite and feed on blood. They not only cause intense irritation, but they can also be vectors for disease transmission such as bluetongue and Schmallenberg. Hatched larvae (maggots) feed on host tissue, which can cause damage to the animal and, ultimately, death.
When are they present? Face flies are active from late spring to early autumn, with eggs laid in fresh dung. Flies emerge 7-20 days after eggs are laid.
Head flies are active from June until October and are present mainly in the uplands of Scotland and northern areas. They tend to have one large breeding cycle and emerge as large swarms in midsummer.
Biting horseflies are present from May to September and have one breeding cycle. The hornfly is most active from May to September and has several breeding cycles a year.
The stable fly is most active from June to September and has three or more life cycles, depending on temperature. The culicoides midge has one life cycle and is most active from late summer into autumn.
They are temperature dependent, but mainly present between May and October.


Farmers can help take steps to improve the environment to help reduce fly numbers. For example, by keeping manure dry and compacted and turning every two to four weeks, as well as covering heaps and not overfilling slurry lagoons.  

Improving ventilation in sheds to create an unfavourable fly environment will also help, as well as providing good drainage and using fly sheeting in high-risk areas. Keeping stock away from fly breeding areas such as wet and muddy areas can help reduce the risk.

In helping to control fly strike, stock should be checked regularly and shearing must be done at the correct time of the year.

“Fly strike flies will lay eggs in decaying matter, so if you have sheep or lambs with dirty tails or sheep with footrot, they will lay their eggs there,” explains Dr Armstrong.


Because of the different flies and the risks they pose, Dr Armstrong advocates protecting stock throughout the entire season.

“Treating early and treating regularly will result in greater success,” he says.

“It’s important to know what the treatment you’re using will protect for. For example, in sheep some products are insect growth regulators, meaning they stop growth at the larval stage.

“Insect growth regulators will protect against blowfly and not nuisance or biting flies, and will only protect against blowfly at the larval [maggot] stage.

“This means they cannot be used to treat blowfly infestations. However, there are products that will treat all fly species at all different stages. It’s important to speak to your vet or SQP to find out more,” advises Dr Armstrong.

Monitoring fly levels

As part of Parasite Watch, run by Zoetis, farmers involved in the scheme have been using fly traps to monitor fly populations on the farm.

Dr Armstrong explains: “As soon as you start seeing flies in the trap, you know they are going to be bothering stock and you may want to take appropriate action.”

Vet Emily Gascoigne, from Synergy Farm Health, says she has already seen a sudden increase in the strike fly population in the South West. “We are seeing a lot of fly strike across the South West at the moment. There has been a rush on shearing over the past couple of weeks,” she adds.

John Hoskin, who farms with his sons at Maiden Castle Farm, Dorset, says two out of the 10 groups of sheep on his farm have had issues with flies.

“We haven’t lost any yet, but we’ve had some that have been struck. We are shearing as soon as we can,” he adds.

Neilsen Gillard, Creeds Farm, Bridgwater, Somerset, farms 650 breeding ewes and 500 ewe lambs, which are North Country Mules and North Country Mules crossed with a Dorset. 

He has had a small challenge with flies, with maggots detected on the fleece, but not entering the skin. They have been treated with a pour-on.

Farmers can share their fly populations online by tweeting a picture of their fly traps to @sheep_farmers and by using the hashtag #parasitewatch or on Facebook at SheepfarmersUK.

Fly traps are available to buy or farmers can simply make their own.

How to use Parasite Watch

The aim of Parasite Watch is to show what is happening across the UK using an interactive map. Parasite data from each of the farms will be updated regularly, which will allow farmers to see if there are spikes in certain parasites throughout the year in their area and enable them to take appropriate action.

To use the map, click on a farm in your area and details of any parasites that have been found as well as when they were detected will be displayed. Test results will be online within hours of the test being taken.