Historically we could forecast the blowfly strike risk period with a reasonable degree of certainty but as our weather patterns become increasingly unpredictable, many more of us are being caught out earlier than ever.

The impact on the welfare of those affected is clear, but are you also aware that the estimated cost of replacement, following loss to strike, is £209 for a lamb and £184 for a ewe?1

Understanding Blowfly Strike

Blowfly strike is caused by the green bottle fly, Lucilia sericata.

The green bottle fly is attracted to odours such as those associated with urine, faeces or wounds.

One female fly can lay 3,000 eggs which can hatch into maggots as quickly as 8 hours.

They use their scratching mouthparts to feed on the skin surface, creating wounds, which in turn attracts more flies.

Risk Factors

A number of factors increase the risk of fly strike occurring, even where preventative medicines are used, including:

  1. Organic material- if the fleece has faecal contamination or urine soaking, this attracts egg-laying female flies and reduces how effective preventative medicines are. The warm and humid environment created, particularly by urine soaking, favours maggot development.
  2. Open wounds- whether traumatic or associated with lumpy wool (bacterial infection), open wounds also attract flies. As preventative products tend to concentrate in wool and/or hair, open wounds are not well protected and remain high risk for strike until healed.
  3. Thick fleeces- maggots need a hot and humid environment to support their development. Even in weather which seems cooler to us, the micro-climate in the fleece may still be warm and humid- this is why strike risk reduces significantly post-shearing.
  4. Weather- warm and wet weather will lead to the conditions which support maggot development. During drier spells, eggs may remain in the fleece ‘waiting’ for the arrival of rain and humidity to stimulate hatching.


Preventing blowfly strike requires a multi-faceted approach and no single solution can completely eradicate risk. In order to significantly reduce the likelihood of strike, the following measures should be implemented:

  • Annual shearing. Shearing results in a 95% reduction in the incidence of strike2
  • Regular dagging/crutching of soiled fleeces. Urine and faecal contamination significantly increase risk, even where preventative products have been applied
  • Docking- where appropriate, and when done in conjunction with the law. Docked lambs are 5 times less likely to suffer from fly strike.
  • Preventative medicines- significantly reduce the risk of strike occurring- remember to ensure you have the correct applicator and follow the instructions.
  • Check animals frequently during the high risk period- strike can occur rapidly
  • Promptly dispose of any carcasses which act as a fly breeding ground
  • Reduce scouring (good nutrition, control of parasites)
  • Monitor any wounds closely until fully healed

Which preventative product should I use?

This will vary depending on factors such as the duration of cover required and withdrawal periods which fit with your management system.

Registered Animal Medicines Advisors (RAMAs) undergo intensive training and assessments, in addition ongoing continuous education, to ensure that they are best placed to advise you on many aspects of animal health.

Please contact one of our qualified RAMAs and they would be delighted to discuss your flock health plan and determine which blowfly strike preventative product, is most suitable for your flock.

This content is provided by Downland, retailers of Vectocert 1.25 % w/v Pour-on Solution for Sheep

Vectocert® 1.25% w/v pour on solution for sheep, contains cypermetherin, POM-VPS. May be prescribed by any Registered Qualified Person (RQP – a veterinarian, a pharmacist or an appropriately qualified RAMA). For further information see the SPC, for advice farmers should contact their animal medicines supplier RAMA, veterinary surgeon or Downland Marketing Ltd, Warwick Mill, Warwick Bridge, Carlisle CA4 8RR. Use medicines responsibly: www.noah.co.uk/responsible.

1 Fly strike in sheep: updates on the cost of control. K Lihou, F Lovatt and R Wall. In Practice, April 2023.

1 Lihou, K., & Wall, R. (2019). Sheep blowfly strike: the cost of control in relation to risk. animal. https://doi.org/10.1017/S1751731119000831

2 https://bvajournals.onlinelibrary.wiley.com/doi/10.1136/inp.h1434