Richard Wall Professor of Zoology, Veterinary Parasitology & Ecology Group, University of Bristol said: “Blowfly abundance is now at its seasonal maximum and the very warm weather going into September means that fly activity will be high. The dry summer probably depressed the population of g.i. nematodes on pasture and reduced the quality of grass available, so remaining lambs may be cleaner than in a wet year which will help to reduce their strike risk, but any rainfall in early September is likely to be associated with a very high risk of strike in both lambs and ewes and this will persist until cooler autumn weather prevails.”

The next forecast will be at the end of September.

Latest research and statistics

Fiona Hutchings, technical vet at Elanco is urging farmers to beware of being caught out by the longer blowfly season, take control and ‘strike first’ with preventative treatments, rather than risk impact to productivity and profits.

Industry survey results shows blowfly risk awareness growing but still underprepared.

The National Farm Research Unit has released the results for its Elanco Blowfly Survey, based on responses from 150 British sheep farmers. The data reveals that despite strong consensus that preventative techniques are more effective than treatment (97%), there is still a high proportion of farms hit by strike each year. (79%)

Experts also identified several underestimated factors in effective blowfly management, including:

  • Extended and less predictable blowfly season results in majority of farmers getting caught out – even when protected.
  • The true cost of strike goes beyond treatment and is widely misunderstood.
  • Environmental factors are essential when predicting strike, but only a minority regularly check reports and trackers.

Prevention awareness

97% of respondents agreed with the following statement: “Prevention, not cure, is best for blowfly strike.” Despite this, only 51% of farms caught out by blowfly strike have adjusted their system to incorporate treatment earlier in the year – leaving almost half of farms under protected and at risk.

A significant number of farmers find the timeframe of blowfly season increasingly difficult to predict, with 96% caught out by strike in the last five years. 82% of respondents believe the season has extended in some recent years, with significant activity being measured as early as April in many years.

Fiona Lovatt, an independent sheep veterinary consultant at Flock Health Ltd, found the low use of weather forecasts, soil and air temperature (26%, 37% and 31% respectively) to indicate a wider issue: “A lot of farmers think “it’s not in my control” because of bad weather or other circumstances. But they’re unnecessarily putting limits on themselves. By asking what they can control, what improvements can be put in place, farmers have the power to make their protection far more robust.”

The cost of strike

Respondents’ estimation of blowfly costs had a vast disparity. Richard Wall, Professor of Zoology at University of Bristol and a specialist in ectoparasites, believes that many farmers underestimate the costs caused by strike: “A ewe dying of strike represents a loss of at least two hundred pounds, factoring in replacement and vaccination costs. If you take time costs into account however, and the loss of production from reduced lamb weight gains, strike actually ends up having a much more severe financial impact on a farm than it initially seems.”

Fiona Lovatt, independent vet, from Flock Health Ltd., also noted the cost on production and time by blowfly strike: “Many farmers couldn’t estimate the cost of blowfly strike according to the survey. Even those who can are often not counting in loss of production, or cost of labour, despite 86% of farms experiencing a considerable loss of time due to blowfly strike. And arguably even more significant – both financially and in terms of welfare – is the effect on the future performance of an animal that has suffered strike.”

Prevention strategy

85% responded that they use a preventative product before the fly season to ensure protection, leaving 15% who don’t take measures until a flock’s safety and productivity is already compromised.

“97% of farmers agree that blowfly poses a risk, but it’s a problem that can be prevented,” said Professor Wall. “If farmers treat appropriately and are vigilant, there is no reason to get a number of sheep attacked by blowflies. The products are out there, we know a lot about the risk factors for strike and the forecasts are available. So, it just shouldn’t happen. We are now in the risk period and farmers should always treat before the first case of strike, not wait for cases to appear.”

“The challenge in the industry is to convey the real specific impact of blowfly strike – that blowfly strike is not an inevitable part of farming and can be mostly avoided with the right management strategy” challenges Fiona Hutchings, Technical Vet at Elanco. “There are no guarantees when it comes to blowfly strike – with significant levels identified into November, an essential part of any strategy, has to ensure an early treatment that extends right through the long season.”

Awareness of blowfly strike is high, but it’s essential to translate awareness into tangible on-farm implementation. Key to this is ensuring SQPs and Vets are equipped with the latest information, and farmers are encouraged to seek their advice. The experts agree that farmers need to adopt a preventative treatment to combat blowfly strike, applying the first treatment before any cases show.

The impact in numbers

Timing of the blowfly season and longer season

  • 96% of farmers have been caught out by blowfly strike in the last 5 years and 79% have seen it every year.
  • 78% of farms have a season lasting longer than 11 weeks and 52% have blowfly longer than 17 weeks. With 82% believing the season is staying the same or getting longer. = Longer coverage required.
  • The earliest blowfly was reported in February with significant levels seen in April. 88% feel this is consistently the case or earlier than normal. Peak first strike month is May.
  • The latest blowfly report was December with significant levels still seen into the autumn September – November.


  • 97% know that prevention not cure is best for blowfly strike.
  • 51% of farms caught out by blowfly strike have changed their system to apply earlier treatment to avoid being caught in the future.
  • Where do you get information about blowfly strike?
  • 63% NADIS
  • 63% press
  • 62% strike on neighbouring farms.
  • Weather forecast, soil and air temperature were surprising low at 26%, 37% and 31% respectively.
  • 91% did not consider fly traps to be important.


  • Cost – huge disparity on cost of strike and many unable to put a cost to it indicating considerable lack of understanding of the loss of production costs of blowfly strike.
  • 86% of farms have experienced a considerable loss of time due to blowfly strike.
  • 99% of farmers have suffered financial loss due to blowfly strike (2017 figure).
  • 84% agreed that seeing a struck animal caused stress while 91% said that it impacted job satisfaction.
  • 85% said that ewe heath was significantly impacted, while 77% recognised that lamb finishing rates had taken longer.
  • 67% stated that blowfly strike had had a high impact on sheep mortality.


  • 97% agree that blowfly presents a risk to their flock.
  • 85% use a preventative product before the fly season leaving 15% not taking preventative action early, leaving themselves open to risk.

Treatment choice

  • 93% believe duration of protection important for product choice.
  • 95% require treatments to be easy to use.
  • 68% feel a short meat withhold period is important to them.
  • 58% were not price sensitive when choosing a product.