Protect your flock in the blowfly season ahead

Blowfly strike is a serious disease thought to affect over 80% of farms in the UK, resulting in major productivity losses. It is primarily caused by the green bottle fly (Lucilia sericata), which lays batches of around 200 eggs into decomposing matter. Carcasses, dirty backends, foot rot lesions and open wounds all have a high likelihood of attracting egg-laying flies.

The larvae emerging from the eggs will then feed on the sheep before migrating to the soil, causing serious welfare issues and even death. Therefore, preventative measures and prompt and effective treatment are vital to protecting the animals from harm and safeguarding farm productivity.

The majority of blowflies affecting sheep overwinter as maggots in the soil. When the weather warms up sufficiently, they pupate and eventually emerge as adult flies, which will soon feed and start to lay eggs.

Fly emergence typically begins in warmer southern areas and progressively moves to more northern and upland regions. At present, the relatively cold spring means that the majority of maggots are only just turning to pupae and the first emergence of adult flies is unlikely until the end of April or early May.

Unfortunately, a small proportion of the blowfly population often ends up overwintering in sheltered spots such as barns or sheds, in which cases early emergence is possible even before the forecasted date. Therefore, early vigilance for blowfly strike is advised from April onwards.

Reducing the risk of fly strike on your farm

There are several steps farmers can take to considerably reduce the risk of blowfly strike in their livestock. The actions recommended by NADIS are:

1. Carrying out a risk assessment

Farmers are advised to conduct a thorough risk assessment to determine the level of risk in their flock. NADIS has developed an interactive tool to help farmers and veterinary professionals quickly and easily carry out risk assessments for each major endemic livestock diseases.

Upon signing up with an email address, farmers will be required to answer a series of biosecurity questions and will subsequently receive a personalised list of risk factors for their farm. By doing so, they will be able to identify risk priorities in their herd or flock and deal with these effectively.

In partnership with Elanco, NADIS has also developed a live Blowfly Alert that predicts the emergence of blowflies based on Met Office Data. Farmers can also help monitor blowfly activity in the UK by reporting cases here.

2. Managing the fly population

Reducing the blowfly population early in the year has the greatest impact on the fly challenge during the grazing season. Female flies can lay up to 3000 eggs in a 3-month period, which can cause populations to grow exponentially.

Inexpensive fly traps have been shown to effectively reduce the incidence of strike by up to 80% in a season. The location of muck heaps and how they’re managed also impact on the spread of blowflies, and farmers are encouraged to consult their veterinarian for advice.

The prompt disposal of deadstock is another effective way of keeping blowfly populations at bay. Finally, farmers should also consider grazing sheep on more exposed pastures that are less favourable to the flies.

3. Reducing fleece contamination

Since flies are attracted to dirty backends, removing soiled fleece by dagging, crutching and timely shearing are all important measures. Moreover, tail docking lambs is a debated but accepted procedure to reduce strike in lowland flocks.

Keeping worm burdens under control can also help minimise scour caused by worms, with Nematodirus infection being particularly important early in the season. Farmers should work with their veterinarian to develop an appropriate faecal egg counting and parasite control plan.

Last but not least, recognising and treating lame sheep promptly can help reduce strike incidents caused by foot rot.

4. Applying preventative treatments ahead of the high-risk period

To dramatically reduce the risk of fly strike, farmers are encouraged to apply preventative treatments before risk levels rise to medium. Farm vets can advise on the most appropriate product to use based on labour resources, the age of lambs during the risk period, withdrawal times and anticipated slaughter dates.

Insect Growth Regulators (IGRs) are a targeted narrow-spectrum product for preventing blowfly strike and as such are the gold standard in disease prevention.

Since the correct application of a suitable product is essential to ensure maximum protection for the flock, farmers are advised to check the datasheet before treatment and seek clarification from their vet if needed.

Farmers should also keep in mind that applying a product on a rainy day, or dagging/crutching and shearing in the weeks after application may reduce the protection period.