Flystrike on a small block: when it starts, how to spot it, and what to do

🪰 The short version

Get in front of it with dags, worms and timing. Then check often.

Flystrike is maggots eating a live sheep. It moves fast, it's miserable for the animal, and it can kill. The good news for a small block is that most of the prevention costs nothing but timing: keep back ends clean, keep worms under control so sheep don't scour, and crutch or shear before the risk peaks, not after.

After that, it comes down to two habits. Know when the flies arrive on your place, which a cheap trap tells you. And look at your sheep often enough to catch a strike in its first day, not its third. On ten acres that's a quick walk past the mob, and it's the single biggest difference between a small job and a dead ewe.

📖 The basics

What is flystrike?

Blowflies lay their eggs on damp, dirty wool or on open wounds. The eggs hatch into maggots, and the maggots release enzymes that break down the skin and flesh underneath so they can feed. A strike that starts as a small damp patch can become a large open wound within days. Struck sheep lose weight, and without treatment they can die from infection and starvation.

It isn't only a welfare issue, it's a legal one. New Zealand's Code of Welfare for sheep and beef cattle has a minimum standard just for flystrike. It says all reasonable steps must be taken to prevent, or identify and manage, the risk of flystrike, and that affected sheep must receive appropriate treatment at the earliest opportunity. The code's own list of ways to do that is a good checklist: frequent inspections, treating damaged skin such as footrot, crutching, dagging or shearing before risk periods, parasite control to prevent dags, grazing management, and insecticides where needed.

🔍 Know the enemy

Which flies cause it?

Beef + Lamb New Zealand names four blowfly species behind flystrike in this country: the Australian green blowfly, the European green blowfly, the brown blowfly and the hairy maggot blowfly.

You don't need to tell them apart. Two things are worth knowing, though. First, the Australian green blowfly was first recorded here in 1984, and vets now describe it as the most prevalent and significant strike fly in New Zealand. Second, the species turn up at different times and in different conditions. That's part of the reason no single calendar date ever fits every block or every year.

🌡️ The timing

When does the risk start?

The usual answer is roughly November to March, in warm, humid weather. Blowflies like the same conditions that make grass grow. Warmth and recent rain, especially rain in summer, push the risk up. Windy, cool days push it down, because the flies are less active.

That's fine as a rough guide and not much use as a plan. In a warm, wet spring the flies are early. In a cold one they're late. And a sheltered gully on your block can be weeks ahead of an open paddock down the road.

So the useful question isn't "when does flystrike season start?" It's "when did it start here last year, and the year before?" Almost nobody knows, because almost nobody wrote it down. The fly trap section below is how you find out.

🗺️ Your own place

Which paddocks are worst?

Beef + Lamb NZ's advice is to identify the hot spots on your farm and avoid them in high-risk periods. Their list:

• Sheltered, scrubby gullies
• Patches of thistles
• Bush lines
• The lee side of shelterbelts

Anywhere warm, still and sheltered. For high-risk stretches they suggest moving sheep to higher, more exposed ground, where it's cooler and windier, and avoiding intensive grazing.

Read that with your own place in mind. Most people already know which two paddocks are worse than the rest. They just know it as a feeling, not as a rule. Write those paddocks down now, while it's calm, so the decision is already made in January when you're tired and in a hurry.

🧹 Prevention

The free half of prevention does most of the work.

A struck sheep is almost always a sheep that gave the flies somewhere damp and dirty to lay. Take that away and most of the risk goes with it.

• Crutching and dagging before the risk, not after. The welfare code notes that crutching and dagging reduce discomfort and inflammation of the skin underneath, and so reduce the risk of flystrike. Everyone wants the shearer in the same fortnight, so book early.
• Shearing timed to work for you. Short, clean wool through the risk window is harder for flies to use. Plan shearing and crutching around when your flies usually arrive.
• Worm control. Scouring turns a clean back end into a dirty one in a couple of days. Your worm management and your flystrike risk are the same problem. Our drench and worm guide covers how to keep on top of worms without breeding resistance.
• Treat damaged skin. Footrot, cuts and other skin damage give flies a place to start. The code lists treating these as part of flystrike prevention.
• Tails. Docking is mainly done to reduce soiling and dags, and with them flystrike risk. Our docking and tagging guide covers what the code says about how it's done.
• Deal with dead stock quickly. A carcass is a blowfly nursery. VetSouth lists removing dead stock alongside crutching, shearing and parasite control as part of the non-chemical toolbox.

None of this is complicated. The hard part is doing it at the right time, which is why the next section matters.

🪤 Monitoring

A fly trap: the cheapest early warning on any block.

The monitoring method Beef + Lamb NZ describes is a small fly trap baited with offal, checked weekly. When you start catching blowflies, that's your signal to act: crutch, shear, check the mob more often, consider protection, and move stock away from the hot spots.

A commercial farm might run several traps. On a lifestyle block you need one, hung in your worst paddock, and a look on your way past once a week.

What that gives you is a date. Not "it felt early this year", but a real date, on your block. Do it once and it's interesting. Do it three years running and you have something no magazine or Facebook group can give you: your own window, which tells you when to book the crutching before you need it.

👀 Early detection

How to spot a struck sheep.

A strike caught in its first day is a small job. Caught on its third, it can be a vet bill or worse. NZ vets describe these signs:

• Restless behaviour: stamping feet, twitching the tail, getting up and lying down, turning to look at or nibble one spot, rubbing.
• Wool that looks wrong: a damp, dark or blackened patch (that can be discharge from the wound underneath), or wool coming away.
• Off on its own: separating from the mob, seeking shade.
• Later signs: dull and depressed, off its feed. By this stage the strike is well established.

Where to look: the breech and tail area first, then over the shoulders, where wool stays wet. Also check the feet of any sheep with footrot, and around cuts or any other wound.

🩹 Treatment

What to do when you find one.

This is the general approach NZ vets describe. Follow the label of whatever product you use, and call your vet for anything beyond a small, early strike.

1. Get the sheep in and away from the mob so you can see what you're dealing with.
2. Clip the wool around the struck area and a margin beyond it, so you can see the edges and the dressing can reach the skin. Take care not to cut the skin.
3. Remove all the maggots, checking under the clipped edge and in any pockets. Some will be deeper than they look, which is why you clip a margin around the strike.
4. Apply a dressing labelled for treating flystrike, as the label directs. Dressings have withholding periods too, so note it.
5. Give food, water and shade, and keep checking the sheep daily until it has healed. While the mob is in, check the rest of them too.

Call the vet if the strike is large or deep, if the sheep is dull or off its feed, or if you're not sure. Vets may give antibiotics and anti-inflammatory pain relief, and they can tell you whether the animal will recover. The welfare code's general rule applies: signs of ill-health must lead to timely action, and significant injury or ongoing pain needs veterinary advice.

⚗️ Fly products

Fly products: two rules, and no recipes.

This guide doesn't name products. The right one depends on your stock, your region, what's in the bottle and what still works where you are. Your vet or farm store can help you choose. Two rules from Beef + Lamb NZ apply whatever you use.

• Use a different chemical to treat an active strike than the one you use to prevent strike. Over-reliance on chemicals leads to resistance, and the same product for everything is how it builds.
• Know the withholding period before you use it, not after. It's on the label. It matters whether the animal is going to the works or into your own freezer.

Two practical points from vets. Preventive products work less well when they're applied wrongly, when the fleece is heavily soiled, or when heavy rain follows application, so read the label for conditions. And prevention products and treatment products are often different things: a product that protects clean wool for weeks is not necessarily the right thing for an open wound.

For the full detail on the fly life cycle and how to read a chemical label, the standing reference is the free Managing Flystrike and Lice guide from Beef + Lamb NZ, Merino NZ and the Sheep and Beef Cattle Veterinarians.

🐕 Other animals

Not just sheep.

Sheep are by far the most commonly affected, but vets point out that flystrike can happen in any animal with an open wound or skin damage. Rabbits, long-haired dogs and cattle with skin lesions such as skin cancer can all be struck. On a mixed block, the same rule applies to everything: a wound or a dirty back end in warm weather needs checking every day until it's clean and healed.

📲 Keeping track

Keeping track of it.

Flystrike runs on a yearly clock, and that's exactly what memory is bad at. The things worth writing down:

• The date the trap first caught blowflies, every year.
• Your hot-spot paddocks, so the decision is made before January.
• When you crutched and sheared, and which animals.
• Every strike: which animal, where on the body, the date, and what you used.
• Every fly product: what it was, the active ingredient, the date, and when the withholding period clears.

That's what My Block Hand is for. Sheep have Shearing and Crutching / dagging as care types, set to come round again on their own. Fly strike treatment is a treatment type for any animal or the whole mob, and it records the product and works out the date the withholding clears. A weekly trap check can be a repeating job, so it turns up without you having to remember it. And each animal's history keeps its strikes, so you can see which ones get struck again and again.

🚧 Where this guide stops

What this guide cannot do for you.

This is general information for lifestyle-block owners, written from Northland. It doesn't recommend any product, and it can't see your sheep. Fly pressure, weather and resistance vary a lot from place to place. For a struck animal that's more than a small, early strike, or for a plan that suits your block, talk to your vet.

📚 Further reading

Where the information on this page comes from.

• Beef + Lamb New Zealand — Preparing for flystrike this summer: the four blowfly species, integrated pest management, hot spots, offal-baited traps checked weekly, moving sheep to higher ground, and using different chemicals for treatment and prevention.
• Beef + Lamb New Zealand, Merino NZ and Sheep and Beef Cattle Veterinarians — Managing Flystrike and Lice: the standing NZ reference on the fly life cycle, chemical groups and reading a label.
• MPI — Code of Welfare: Sheep and Beef Cattle (2018, in force at the time of writing): minimum standards for managing flystrike, for shearing, dagging and crutching, and for injury and disease.
• Vetlife — Fly and lice: the Australian green blowfly as New Zealand's main strike fly since 1984, risk factors, and the main prevention approaches.
• Anexa Vets — Flystrike: how maggots damage the skin, behaviour signs, and flystrike in animals other than sheep.
• VetSouth — Watch out for flystrike this season: the November to March risk period, signs, treatment steps, why preventive products can fail, and struck ewes at scanning.
• Northvets — Preventing and treating flystrike in sheep (PDF): signs, where strikes happen, and treating a struck sheep.