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 1 cm 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.
Blowfly-strike affects over 80% of farms in England and Wales, where between a half to one million sheep are struck each year. The severity of strike problems is lower for upland than lowland farms and is lower in northern than southern parts of Britain.
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 and other factors such as pasture worm burdens, and management practices such as stocking density, shearing and lambing dates.