Blowflies

Blowflies are one of the most widespread ectoparasites affecting sheep in the UK, with surveys suggesting 80% of flocks will have one or more cases of blowfly strike every year. 

Estimates of the number of sheep struck each year range from 500,000 to 700,00 with mortality rates of 2-3% (10,000-20,000 sheep per annum). The extent of blowfly strikes varies hugely from year to year, mainly due to weather conditions. Failure to treat a strike promptly will lead to reduced performance, secondary infections and, in severe cases, death. 

What is blowfly strike?

In the UK, blowfly strike is usually caused by the greenbottle fly (Lucilia sericata). Female flies are attracted by the odour of wounds and decomposing matter, such as soiled fleece or dead animals, and lay batches of up to 250 eggs at a time. The eggs start to hatch after about 12 hours and it then takes roughly three days for the larvae to mature and drop off the sheep, where they will pupate in the soil. The whole lifecycle takes about four to six weeks.

Blowfly strikes usually occur in waves. The first fly lays her eggs in a damp or dirty area on the sheep and the odour from this strike rapidly attracts other blowflies. The larvae start to feed on the living tissue of the sheep under the soiled fleece, secreting proteolytic enzymes and abrading the skin with their mouthparts. The first signs in affected sheep are often a patch of discoloured wool and agitation. 

Rapid diagnosis and treatment are essential. Increasingly, flocks are using preventative measures to minimise the risk of blowfly strike, rather than treating strike only after it has occurred. 

Blowfly strike was traditionally expected between May and September, but changing weather patterns mean lowland flocks are often at risk from March through to December. Timing of the application of these preventative measures is key to success and Elanco Animal Health sponsors an alert that can be found on the Nadis website.

Preventative strategies

• Application of a preventative treatment at the start of the main blowfly season. Product choice depends on the class of sheep, age, withdrawal periods, shearing and lambing dates etc. More than one application may be required. Download a product information table below.
• Minimise soiling of backends. Dagging, crutching and shearing, together with an effective worm control strategy, are important ways risk can be reduced. Tail docking is also used to reduce the incidence of strike in lowland flocks. 
• Treat lame sheep promptly. Flies are attracted to wounds caused by footrot and to the feet themselves; foot strikes can be especially damaging so prompt attention is required.
• Reduce the fly population. If the fly population is controlled early in the year, there is a reduced challenge as the grazing season progresses.
  • Inexpensive fly traps have been shown to reduce strike incidence by 80% in a season.
  • Prompt disposal of deadstock is essential.
  • In high risk periods, consider grazing more exposed pastures that have a microclimate less favourable to flies.

Blowfly strike can be prevented through the application of an appropriate product before a period of challenge.

Blowfly products

There are essentially three types of topical product recommended to sheep farmers for blowfly control.

1. Insect growth regulators (IGRs). These products allow the female blowfly to lay her eggs but then prevent the larvae developing, stopping the lifecycle. They prevent strike for eight to 16 weeks depending on product choice. They do not treat an existing strike.
2. Cypermethrins. These products will treat an existing strike and give protection from further strikes for six to 10 weeks, depending on the product.
3. Deltamethrin. These products only treat an existing strike. 

Plunge dipping in an organophosphate (OP) will also treat and prevent blowfly strike. However, SCOPS recommends using one of the topical products where possible to target blowfly, rather than using the broad activity of the OP unless it is necessary. If an OP is used, it must be as a plunge dip; the use of any other method of application is not legal.

Remember

• When using any products, always read the manufacturers’ instructions on dose rate, storage, package disposal and withdrawal periods. Always check the data sheet.
• Check you have the correct application equipment for the product and that it is working properly and is calibrated. 
• Make sure you know how to apply the product and have the appropriate PPE. If in doubt, ask your vet or adviser for help.