Blowfly is the main external parasite traditionally affecting sheep in the late spring and summer months.

Changing weather patterns, however, have meant that it is now not uncommon to hear of cases as early as February and as late as November. Blowfly strike can occur quickly and with devastating results in warm, humid weather.

Despite being an annual problem, and entirely preventable, over 90% of farmers have been caught out by blowfly strike in the past¹.

Blowfly strike creates serious welfare and economic consequences. Climate variation and well-timed preventative measures are vital to ensure protection and peace of mind for farmers and their flocks.

Key points:

  • Blowfly strike is a major worry for sheep farmers and 63% agree that the blowfly season is lasting longer¹;
  • 99% of farmers have suffered financial losses as a result of blowfly strike; more than one in five have suffered losses greater than £500¹;
  • Preventative measures can offer farmers protection and peace of mind and are always better than cure.

Blowfly – meeting the challenge

With blowfly season now starting earlier and lasting longer, prevention becomes even more important to protect the welfare and productivity of a farmer’s flock.

However, whilst most farmers are aware of this, many still prefer to adopt a ‘wait and see’ approach, despite the clear benefits a preventative strategy brings.

Three species of fly can attack sheep in the UK with Lucilia sericata (greenbottle) being the most common. Soil temperatures above 9º are needed for the fly larvae overwintering as pupae to mature and emerge as the ‘first wave’ of flies.

Female flies are attracted by the odour of wounds, soiled fleece or dead animals and will deposit many hundreds of eggs onto affected sheep. These eggs will then hatch into larvae and, as they progress to stage 2 and 3 larvae, they feed on the sheep’s skin causing wounds – or ‘blowfly strike’ (myiasis).

Whilst the blowfly (Lucillia sericata) is primarily responsible, once the initial strike has occurred, other species can escalate the problem by laying their eggs in the same site – this is called ‘secondary strike’. In warm, wet weather significant damage can occur in as little as 36 hours after egg laying.

The area’s most prone to damage are the withers, flanks and the tail area, particularly after scouring. Open wounds are particularly vulnerable.

After three days the mature larvae drop off the sheep and pupate in the soil, giving rise to more blowflies. Successive waves of blowfly then emerge throughout the summer with numbers increasing with each occurrence.


The symptoms of blowfly strike range from agitation – foot stamping, vigorous shaking, gnawing or rubbing of the tail and breech – to dejection and ultimately death.

By the time strike becomes visible, a considerable amount of damage will already have occurred – not to mention the time and resources spent checking for, and worrying about, blowfly strike.

If the conditions are right for blowflies to lay their eggs, one case of strike may mean other animals have been struck or are susceptible to strike. In just a few days, one case of strike can escalate to multiple animals.

What are the financial implications?

Blowfly strike is not only a major welfare issue, but one that requires stressful and time-consuming treatment. The financial implications of treating can be very costly, dealing a strong blow to farm margins.

Average costs of blowfly strike¹:

  • £80 – average loss per lamb that dies from strike;
  • £10 – production loss per struck lamb;
  • £10 – labour cost to handle each struck animal;
  • 50p – cost of treatment per animal;
  • £200 – the approximate cost of breeding a replacement ewe.

Preventing strike – what are the options?

Prevention is the most accepted strategy for stopping blowfly strike and preventing financial losses. There are two main types of chemical blowfly strike control.

The first is Insect Growth Regulators (IGRs) – such as CLiK® EXTRA, CLiK® and CLiKZiN®. These stop first-stage larvae from developing into the destructive second-stage larvae and so prevent the disease by breaking the blowfly life-cycle.

Specific to the blowfly parasite they are termed ‘narrow-spectrum’ products. Best practice guidelines state that narrow spectrum products should be used where possible to prevent chemical resistance from developing.

This includes insecticides; for example, synthetic pyrethroid (SP) pour on products such as Crovect® or organophosphate (OP) dips.

These are classified as ‘broad spectrum’ products as the actives can be used to control many different parasites, leading to potential overuse.

The best blowfly control option to choose is largely determined by a farm’s own flock health management regime and production schedule. Some points to consider when making this choice are:

Length of protection

A priority for many, a long period of cover is more cost-effective in the long run and will mean that you don’t need to keep re-gathering sheep to retreat saving on labour, effort and money.

Treatments on the market now offer coverage for up to 19 weeks, from CLiK® EXTRA (followed by 16 weeks from CLiK®) allowing lambs to be treated early with blowfly protection lasting through to marketing.

Treatment applied to ewes after shearing will also mean that if the blowfly season is extended due to the summer and autumn weather, the ewes will still have protection.

Short meat withhold

The withhold period is also important as it affects the ability to send lambs to market when they are ready. In this case CLiKZiN® offers eight-weeks IGR protection with a short withdrawal period of just seven days.

Full fleece cover

Many products only protect the site where they are applied which can be disastrous in terms of blowfly control.

CLiK® EXTRA, CLiK® and CLiKZiN® all incorporate FleeceBind™ technology which enables them to spread and bind throughout the fleece, providing full fleece protection* and offering peace of mind.


Resistance is also an issue of growing importance and whilst it is tempting to use a product that treats a broad range of parasites, it could lead to its overuse and the build-up of resistance.

Instead, it’s recommended to use a narrow spectrum product (such as the IGR dicyclanil in CLiK® EXTRA, CLiK® and CLiKZiN®) which is targeted at the parasite that needs controlling – in this case, the blowfly larvae.

Prevention – the key to good blowfly management

Prevention is the best form of control: for flock health; financial security; and for peace of mind. The key aim is to reduce both the number of susceptible sheep and the number of flies in the environment.

One treatment with CLiK® EXTRA, applied early, will give 19-weeks cover and take lambs right through to marketing – without needing to worry about strike or retreating.

If strike does occur, farmers must be quick to act with an SP pour-on such as cypermethrin (Crovect®) or an OP dip to minimise distress, prevent production losses and even the death of affected animals.

Ongoing strike prevention should then be applied using an IGR for the rest of the flock, if they have not already been struck.

Why correct application is important

To make the most of medicines and money – and ensure length of cover is achieved – it’s essential to always use the correct applicator and follow application instructions to the letter.

Further information

To learn more about blowfly prevention talk to your animal health advisor and read Elanco’s detailed step-by-step guide.

For further information, call Elanco Animal Health on: +44 (0) 1256 353131; or write to Elanco Animal Health, Lilly House, Priestley Road, Basingstoke, Hampshire, RG24 9NL.

  • CLiK™ Pour On contains 5% (w/v) dicyclanil;
  • CLiKZiN™ Pour On contains 1.25% (w/v) dicyclanil;
  • CLiK™ EXTRA contains 65mg/ml dicyclanil;
  • Crovect™ (Youngs’ Vector™ in the ROI) Pour On for sheep contains 1.25% (w/v) cypermethrin (cis:trans/80:20).

Information regarding the side effects, precautions, warnings and contra-indications can be found in product packaging and leaflets; further information can also be found in the Summary of Product Characteristics.

Advice should be sought from the medicine prescriber: CLiK; CLiKZiN; CLiK EXTRA; Crovect; FleeceBind; Elanco; and the diagonal bar logo are trademarks of Elanco or its affiliates.

* Spreads to areas covered by fleece, other areas may not be protected, including the feet.
1. Elanco survey via NSA of 130 sheep farmers, April 2016.
2. Richard Wall and Fiona Lovatt (2015). Blowfly strike: biology, epidemiology and control, In Practice 37:181-188 doi:10.1136/inp.h1434.