Hot humid weather creates ideal conditions for blowfly strike. Fortunately, the fly population is still relatively small because of the cool conditions in May, but it is starting to increase in size quickly.

This means strike risk will rise through July, spreading from south to north with the chances of strike increasing particularly quickly in lambs and any ewes remaining unsheared.

Thought to affect more than 80% of farms in the UK, blow refers to the laying of eggs by flies and strike is the damaged caused by the larvae (maggots).

The Blowfly Alert in conjunction with Elanco predicts the emergence of blowflies based on Met Office Data. and the latest report for Scotland shows how the very wet weather over the last few weeks has increased the risk of strike in ewes that have not yet been sheared.

Wet weather has also increased the survival of worm eggs shed onto pasture, increasing the risk of infection for lambs; worm infection and resultant scouring in lambs is an important strike risk factor. However, at present the lower than average temperatures keep fly activity relatively low. Warmer temperatures into July will nevertheless start to increase strike risk quickly.

Blowfly strike causes serious discomfort to sheep. Flies are attracted to the faecal stained fleece; resulting in frequent tail swishing, nibbling at the back end and disrupted grazing.

The cause

In the UK, strike is caused primarily by the green bottle fly, Lucilia, which seeks decomposing matter to lay her eggs. Carcases, dirty backends, foot rot lesions and open wounds are all good candidates for egg laying sites.

Female flies lay batches of 200 eggs at each oviposition, so fly populations will quickly increase over a short period of time in the summer months.

As well as having a major impact on productivity costing in the region of £2.2m per year, blowfly strike has a serious impact on sheep welfare

Losses are incurred from:

Welfare

Loss in productivity (weight loss and decreased milk yield)

Fleece damage

Deaths

Treatment costs; including product, labour and time

Disrupted grazing occurs when sheep are affected by blowfly strike, this in turn leads to weight loss/decreased weight gain

Disrupted grazing occurs when sheep are affected by blowfly strike, this in turn leads to weight loss/decreased weight gain

Flocks should be carefully checked at least once a day throughout the blowfly season to look for any signs of blowfly strike. It is often necessary to handle animals and part the fleece to fully appreciate the extent of disease.

Early signs of strike

Irritation

Nibbling at tail head

Increased swishing of tails

Rubbing

Further signs of discomfort in lame animals

Nibbling of tail head in this lamb affected by strike is affecting its grazing, which in turn will lead to decreased daily live weight gains.

Nibbling of tail head in this lamb affected by strike is affecting its grazing, which in turn will lead to decreased daily live weight gains.

Footrot lesions may also attract flies to lay their eggs. In these cases the lameness will be exacerbated.

Signs of severe strike

Discoloured/damp fleece

Fleece loss

Separation from flock

Sick animals

Death (due septicaemia from secondary bacterial infection and release of toxins)

Prevention/treatment

In order to prevent Blowfly strike, the following steps are recommended:

Application of a preventative product in advance of the main risk period for flies

Discuss with your vet or SQP the most appropriate product, based on labour resources, age of your lambs during the risk period, withdrawal periods and anticipated slaughter dates

Reduce dirty backends – dagging, crutching and timely shearing are all important.

Tail docking lambs is a debated but accepted procedure to reduce strike in lowland flocks.

Control worm burdens. Discuss with your vet an appropriate faecal egg counting and parasite control plan.

Treat lame sheep promptly. Flies are attracted to wounds caused by footrot

Manage the fly population: reducing the fly population early in the year has the greatest impact on the fly challenge during the grazing season

Inexpensive fly traps have been shown to reduce strike incidence by 80% in a season

Prompt disposal of deadstock

In high risk periods, consider grazing more exposed pastures which are less favourable to the flies.

Long fleeces and tails, as well as dirty back-ends (probably due to worms) all increase the risk of blowfly strike