THE diseases and irritation caused by flies is a major problem during the summer months. Conditions are now ripe for a surge in the population of many troublesome insect species. So farmers and their veterinary advisors will need to take controlled steps to avert an increase in conditions such as fly strike, summer mastitis and New Forest eye.

Warm weather and rain ensures wet organic matter provides the perfect breeding conditions for flies. The best way forward is to establish a comprehensive approach to fly control that can do much to reduce the distress caused to livestock. Controlling flies can produce considerable financial benefits in cutting losses. At the extreme end, DEFRA figures suggest that a single incident of fly-borne disease can cost up to £1800 with treatment and replacement costs for an in-calf heifer with a fatal attack of summer mastitis. Prevention is so much better than cure. Whilst aggressive treatment gives a good chance of saving the quarter, the udder may be severely damaged and so affect the animal’s future productivity.

On sheep enterprises flies may also cause major losses. About 80 per cent of English sheep farms experience some cases of blowfly strike each year. Up to one million individual sheep may be affected and there are about 12,000 deaths.


Blowfly strike is a familiar and unwelcome summer disease of sheep in Britain. Each year fly strike causes serious welfare problems and lost productivity. The primary cause of blowfly strike in Britain is the greenbottle, Lucilia sericata. Adult blowflies lay their eggs on the sheep, attracted by areas of wool which are wet or contaminated with faeces. Each adult female lays about 200 eggs at a time and can do this every few days throughout its life. Maggots hatch from the eggs. Newly hatched maggots are about 1 mm in length but they grow quickly to reach over 1cm in as little as three days. The feeding activity of these maggots at the skin surface rapidly results in damage to the skin and the development of a wound. When they are fully grown the maggots drop off the sheep and burrow into the soil. The maggot turns into a pupa and, after about two weeks, an adult fly emerges. Existing strikes rapidly attract further egg laying by other blowflies. In the winter the blowflies remain as maggots in the soil until the temperature has risen sufficiently to allow the maggot to develop to the pupal stage. Adults will then begin emerging from the soil in about late April or early May. The entire life-cycle of the blowfly from egg to adult takes about 3-5 weeks and there are about 3 or 4 generations of blowfly per year. The problem of blowfly strike is highly seasonal; the first strikes are usually seen in about May/June and blowfly problems continue through to about the end of September. However, the severity and timing of strike patterns each year are highly dependent on temperature and rainfall.

Farmers choice of appropriate fly treatment will vary according to the length of period of activity required and meat withdrawal length. Although plunge dipping has traditionally been a summer activity to provide protection, it is labour intensive and may be better reserved for autumn treatments and scab control. Pour on treatments with pyrethroid and insect growth regulators have advantages in requiring less handling and these products provide varied lengths of protection. Early treatment before the first cases of fly strike is imperative. Equally, correct and careful application of these products maximizes their effect.

As well as flock treatment, farmers should try to identify and treat the animals most at risk. Flies are attracted to sheep mainly through their sense of smell.

So individuals with conditions such as faecal soiling of the rear end caused by heavy worm burdens or foot rot are likely to be targeted by flies. Therefore, these animals should be treated for these predisposing conditions and particular attention paid to them.