Treat stock for flies now to prevent production losses

Livestock farmers should be protecting stock against flies early and throughout the season in order to prevent production losses.


That’s the message from Zoetis vet Dave Armstrong, who warns that substantial production losses can occur when flies take hold.

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

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

In sheep, more than 5.5kg of weight can be lost over four to six days and reduced wool production 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- Face flies, which transmit new forest eye and Head flies, which are involved in the transmission of summer mastitis. Face flies are active from late spring to early autumn and Head flies June-October.
  • Biting flies - Stable flies, Horn flies, Horse flies, Midges and Mosquitoes. They are mainly active late spring until September, with the exception being the midge which is active late summer- autumn.
  • Strike flies - Blue Bottle and Green Bottle are the most common Blowfly species in the UK. They are temperature dependant, but mainly present between May and October.

“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.


Farmers can help take steps to improve the environment to help reduce fly numbers. For example, keeping manure dry and compacted and turning every 2-4 weeks, as well as covering heaps and not over filling 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. Also 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 for strike and ensure shearing is done at the correct time of the year.

“Fly strike flies will lay eggs in decaying matter, so if you’ve got 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 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.

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

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

The aim of Parasite Watch is to show what is happening across the UK using an interactive map found at