News & Advice

Keeping flystrike under control

Nov 2, 2021 | Dry stock, Dry stock animal health & welfare, Sheep

Fiona McLeod, Veterinarian, Anexa Vets Te Kauwhata

Flystrike in New Zealand is mainly caused by four species of blowflies. Flies lay their eggs in wool and the maggots that hatch begin to eat the sheep’s flesh. The Australian green blowfly does not need dags or a wound to invade; they will lay eggs on a completely clean-fleeced sheep. The secretions of the maggots can cause ammonia poisoning days after the first maggots hatched and will attract more blowflies.

How does flystrike affect stock productivity?

Flystrike causes significant production losses in the form of lost weight gains, lost wool, damaged carcasses and death. Each year 3 to 5% of the national flock are struck, and at least 250,000 lambs die from flystrike. Even minor strike can result in severe appetite loss and therefore weight loss. It can take up to six weeks to recover lost weight and eight months for the fleece to recover. Wool growth and quality will be reduced and severe scarring causes downgrading of pelts. Flystrike also has a significant impact on fertility as ewes can be 80% less fertile if they are struck around mating.

What can we do to avoid flystrike?

Insect growth regulators (IGR’s) interrupt the lifecycle by inhibiting the growth and reproduction at the maggot stage. Resistance to Zapp and Exit has spread to the wider Waikato and King Country regions. Cyrex and Vetrazin are still good IGR’s to use in jetting races but it is important to follow the label directions. There are many cases of apparent “product failures” where sheep have been under dosed by volume or poor coverage is being achieved so the results are also poor. These products need six to eight weeks’ fleece growth for the product to adhere to. Clik spray-on is an excellent alternative to jetting. It is licenced for use off-shear and gives up to 18 weeks’ protection. Heavy rain following treatment with any product will cause stripping and reduce the time of protection.

If you would like some advice on how to prevent flystrike on your farm or which products are best to use, give us a call, we’re here to help.

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