06 May 2025

Blowfly strike: Be prepared for the season ahead

A yearly and costly occurrence on sheep farms, Teagasc Sheep Specialist, Ciaran Lynch highlights the importance of having a control plan in place early in the season to mitigate the risks associated with blowfly strike.

The severity of blowfly strike in any given flock depends heavily on both the prevailing weather and specific farm conditions, and it can become a major issue without an effective prevention strategy in place.

Understanding blowfly strike

Blowfly strike often begins in soiled areas of the fleece, with flies drawn to moisture, organic material, or any type of wound. The tail end is a common target, but strikes can also develop on the back, flanks, underbelly, feet, or any area with broken skin, such as shear cuts or injuries from rams fighting.

Early signs include animals appearing restless or separated from the group, often seen biting, kicking, or rubbing at the affected area. In warm, humid conditions, the risk of infestation rises dramatically, particularly in sheep with scour, footrot, or open wounds, as these create ideal breeding grounds for blowflies.

Three key preventative actions

Have an effective internal parasite programme in place on your farm
Dag, crutch, or shear any sheep with dirty or heavily soiled fleeces.
Treat any signs of strike immediately and effectively.

Prevention and treatment options

The best strategy is always prevention, there are several options available:

1). Plunge dipping

This traditional method remains effective when done correctly.

Use the dip at the recommended concentration.
Replenish the solution regularly, following the manufacturer’s instructions.
Each sheep should submerged in the dip for at least 60 seconds.
The length of time sheep are in the dip correlates directly with how long protection lasts.

2). Topical pour-ons

These are now the most commonly used treatments on farms and can offer protection from 7 to 19 weeks, depending on the product.

Apply to clean fleece only, avoiding soiled or damp areas.
Ensure dosing guns are calibrated to deliver the correct volume.
Even application to designated body areas is critical for efficacy.
Be mindful of meat withdrawal periods, which can range from 7 to 40 days depending on the product.

There are two main categories of pour-ons:

A). Insecticidal Pour-Ons
- These pyrethroid-based products (e.g., cypermethrin: Ectofly, Vector) offer short-term protection (6–8 weeks) and can also kill larvae if applied directly to an active strike site.
B). Insect Growth Regulators (IGRs)
- Products like Clik, Clik Extra, and Cliczin use dicyclanil to interrupt the blowfly life cycle. They prevent stage-one larvae (harmless) from developing into damaging stage-two larvae. However, they do not kill existing stage-two or stage-three maggots, so timing is critical. Protection duration varies by product, from 8 to 19 weeks.

3). Shearing

Removing the fleece reduces the ideal environment for maggots to develop, offering a temporary reprieve. However, this protection is short-lived, particularly in high-risk weather. Sheared sheep should still be included in your control plan with one of the above treatments applied as needed.

Treatment and planning checklist

Prevention is key: Implement control measures before the onset of the high-risk period.
Correct application: Follow manufacturer instructions meticulously for all treatments.
Know your product’s cover duration and withdrawal period
Monitor and reapply: Be aware of each product's duration of protection and reapply as necessary

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