Blowflies are the main external parasite affecting sheep in late spring and summer. Blowfly strike can hit fast and hard, especially in warm, humid weather and it can have devastating consequences for sheep welfare and farm productivity. While it is a yearly threat and one that’s entirely preventable, many farmers still get caught out.
What causes strike?
The greenbottle fly is the main culprit. These flies don’t live on sheep, but they do use them as a place to lay eggs, particularly in dirty or damaged areas of the fleece. This is why keeping sheep clean and healthy is key.
Once eggs are laid, they moult through a couple of stages to a mature larvae (maggots) within hours. These maggots feed on the sheep’s flesh, causing severe tissue damage. It can take just 36 hours from egg-laying to serious harm.
Key risk factors include:
- Soiled fleeces caused by scouring (from worms and coccidiosis)
- Dirty tails or rear ends
- Foot rot or open wounds
- Wet and warm weather conditions
What to look out for
Sheep with strike often isolate themselves from the flock and may nibble or bite at their sides. The fleece may appear damp or discoloured. On closer inspection, affected areas smell foul and are often crawling with maggots.
The maggots release ammonia, which quickly gets into the bloodstream. This causes toxic shock and can lead to rapid death if not caught early.
Prevention: The best defence
Preventing blowfly strike saves sheep, reduces stress and protects your bottom line. Here’s what you can do:
- Monitor regularly: Check sheep often during fly season (late spring to autumn)
- Crutching and dagging: Start in early April and repeat every 4-6 weeks
- Worm control: Reduce scouring by maintaining a good worming plan and avoiding sudden change in diet
- Footcare: Treat foot rot and any wounds quickly to stop odours that attract flies.
The goal is to reduce both the number of flies in the environment and the number of at-risk sheep.
Treatment and product options:
Insect Growth Regulators (IGRs): Used only for prevention, not treatment. They don’t kill flies or stop eggs being laid, but they prevent the maggots from maturing. Options include:
- CLiKZin – 8 weeks’ protection
- CLiK – 16 weeks’ protection
- CLiK Extra – 19 weeks’ protection
These products must be applied properly to work. Speak to your in-store RAMA (Registered Animal Medicines Advisor) for guidance.
Synthetic Pyrethroids:
Used for both prevention and treatment. These kill flies, maggots and larvae, making them ideal for treating struck sheep. Common options include:
- Crovect
- Country Flyoff
- Dysect
Again, application method matters, especially as the process differs to prevention versus treatment. Your RAMA can advise on best practice.
Summary
Blowfly strike is a serious welfare and economic issue, but it’s manageable with the right approach. As we head into peak season, Northern Ireland is currently at medium risk. Getting ahead of the first wave of flies by using preventative treatments early will reduce pressure later in the season.
Plan ahead, stage vigilant and take action early.
For more information, visit your local Fane Valley Stores and speak to one of our Registered Animal Medicine Advisors.
