• Blowfly

    Blowfly – the problem 


Blowfly is the main external parasite affecting sheep in the summer months. More likely to occur in warmer, humid weather, the blowfly season is starting earlier and lasting longer than before1.  If left untreated, fly strike can be fatal.


Three species of fly can attack sheep in the UK: 

  • Lucilia sericata (Greenbottle) - most common species
  • Lucilia caesar - second most common species
  • Calliphora vomitoria  - quite rare


  • Agitation and dejection - foot stamping, vigorous shaking, gnawing or rubbing of the tail and breech.
  • Odour - as infestation develops a distinctive smell is noticeable. Wool becomes matted and discoloured.
  • Wool is shed - if infestations remain untreated, the affected area increases from the centre, accompanied by constant discomfort.
  • The smell of infestation attracts more flies – if unchecked, further infestations of flies can result in a quick and agonising death.


The Blowfly Life Cycle


  • Blowfly larvae need soil temperatures above 9oC to develop.
  • The first ‘wave’ of flies – from overwintered larvae - causes spring strike.
  • Flies continue to deposit many hundreds of eggs onto affected sheep, which hatch into larvae.
  • Larvae develop through 3 stages between egg and adult.
  • Stages 2 and 3 damage the sheep's skin as they feed causing ‘strike’ which leads to production losses and welfare problems.


Sheep suffering from blowfly strike

Sheep suffering from blowfly strike

3 classifications of strike

  • Body - Flies are attracted to sheep by the odours of excessive “sweating” and decaying organic matter in the fleece, anywhere  over the loins, shoulders, flanks, neck, back, throat or abdomen
  • Breech - Flies are attracted to fleece contaminated with urine and/or faeces and breech strike is particularly associated with scouring
  • Wound  - flies can be attracted to open wounds; these are often on the feet

Economic effects

  • Blowfly strike is a serious problem
  • If untreated, strike can kill or adversely affect the welfare of infested sheep
  • Farmers, as registered keepers of animals, have a legal responsibility to prevent or treat infestations within their flocks.  Failure to do so can result in prosecution for animal cruelty.


  • There are four further ways in which blowflies can undermine sheep productivity:
  1. Downgrading wool clip
  2. Reducing reproductive potential and lamb crops
  3. Increasing time to market for lambs
  4. Reducing leather quality



Blowfly strike is weather-dependent and unpredictable weather makes early and late blowfly challenge a particular problem.  There is evidence the season for risk is getting longer each year1 .Most cases occur during:

  • Warm, humid weather
  • March to November
  • Breech strike depends less on weather and more on the moisture supplied by urine and/or scouring which  encourages  larval development, so this can also occur outside the ‘traditional’ at risk periods


  • Prevention is the best form of control. Strike can be prevented using pour-ons containing the insect growth regulators (IGRs) dicyclanil, such as CLiK® and CLiKZiN®. However these products will not treat existing strike (maggots).
  • Culling of breeding ewes and rams that are continually struck could be considered, as evidence shows that  hereditary factors may exist. Culling should also be considered for  ewes with deformed genital openings and narrow breeches that result in soiling.
  • Complete shearing can temporarily reduce the risk of strike, but this risk rapidly increases as the fleece grows. Ewes can become flyblown even after shearing.
  • Routine crutching and dagging will reduce the risk of strike  if started in April and repeated every four to six weeks.
  • Tail docking may also reduce the incidence of strike.

Treatment of existing strike

Cure and protection can be attained using a synthetic pyrethroid (SP) based pour-on such as Crovect® containing  cypermethrin. Spot-on products containing the SP deltamethrin will cure existing infestations but will not protect against further attack. A diazinon based plunge dip also offers some level of protection.

Related products






1. Wall & Ellse, Global change Biology (2010)

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