Blowfly Control in Sheep - Clarifying the Key Issues

As the primary ectoparasite affecting sheep in spring and summer, blowfly strike is unfortunately often a familiar problem on sheep farms. As soon as soil temperatures reach 9 degrees celcius, the overwintering fly larvae mature and emerge as flies, which in turn deposit thousands of eggs onto sheep.

Changing weather patterns, however, have meant that it is now not uncommon to hear of cases as early as April and as late as October. Blowfly strike can occur quickly, and with devastating results, in warm, humid weather and despite being an annual problem, and entirely preventable, over 90% of farmers have been caught out by blowfly strike in the past.

Understanding the blowfly life cycle

The lifecycle is so quick when environmental conditions are right, and the volume of maggots is so high, that significant damage from blowfly strike can occur in as little as 24-36 hours after egg-laying. Understanding the blowfly lifecycle helps to understand the importance of a truly preventative approach.

Lucilia sericata (Greenbottle) is the most common blow fly to parasitise sheep in Ireland.

Soil temperatures above 9 degrees celcius 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 areas 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. However, by the time strike becomes visible a considerable amount of damage will already have occurred and farmers spend a lot of time 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, so in a few days one case of strike can escalate to multiple animals.

Understanding the Implications of Blowfly

As a result, sheep farmers can be left facing serious welfare and productivity issues, while the financial consequences can be significant: “Ewes are expected to have 4-5 productive seasons, but a case of strike can cut that short and reduce the value of that animal. Struck lambs will often miss out on their prime period for growth and may have to remain on the farm for longer at increasing expense,” says Matthew Colston, Technical Consultant at Elanco.

• Blowfly strike is a major worry for sheep farmers and 63% agree that the blowfly season is lasting longer
• 99% of farmers have suffered financial losses as a result of blowfly strike: more than one in five has suffered losses greater than €5881
• Preventative measures can offer farmers protection and peace of mind and are always better than cure
• Recent research found that more than 9 out of 10 farmers have been caught out by blowfly strike.
• The average loss per lamb that dies from strike is €95
• Struck lambs will suffer a €11 production loss
• The labour cost to handle each struck animal is €11
• The cost of treatment is 58c per animal
• The cost of breeding a replacement ewe is approximately €235

Although fly strike is largely inevitable, one of the hardest challenges for blowfly control is predicting the start of the season each year as it can fluctuate considerably. This has been driven by changing weather patterns over the last decade creating an increasingly unpredictable and lengthening season.

Matthew Colston continues: “Your location in the Ireland will mean the difference between getting strike early April or slightly later. What we’re looking for is the soil temperature rising above 9 degrees celcius which is a good indicator of blowfly strike risk. On a south-facing slope the soil temperature can warm up to this more quickly than other areas.”

Being prepared and preventing early means, farmers can take control of this devastating disease by planning their time and resources to suit their farm. It is also the best option for the health and welfare of the flock as well. The primary aim of using a preventative is to reduce both the number of susceptible sheep and the number of flies in the environment.

“It takes a lot longer to treat a case than to apply a preventative treatment. Being prepared and preventing early is a planned event where sufficient manpower and resources are available. This is much easier to manage than an emergency treatment and will usually take less time,” says Matthew Colston.

Please follow the link below to report any instances of Blowfly Strike.

Blowfly Map

First Published 12 May 2017