Fly and lice uncovered

Fly and lice are both external parasites that can affect sheep (and most other species too!) and their treatment and control is ongoing and challenging.

The first step is to understand their lifecycles and habits, then identify and target their weaknesses:


Adult females lay eggs on warm smelly parts of the fleece and lay around 2000 eggs in a six week lifespan. They are attracted to odour from wool grease, dags, scours, urine, infected cuts and feet. The eggs hatch in 12 hours and first stage maggots feed on the surface of the skin. A day later, they moult and become second stage maggots - these are the damaging ones. A day later, the maggots moult again into third stage maggots which feed and then drop off and seek shelter. Meanwhile, more flies are attracted to the open wound by the smells created by the first maggots, so a cycle is set up meaning death is inevitable if left untreated. Juvenile fly will remain as maggots or pupae depending on the conditions under the carcass or several centimetres below the soil surface. The whole life cycle is completed in as little as four weeks in mid-summer, whereas much longer in cooler months. 


Unlike flies, the entire lice lifecycle is spent on the sheep and, in ideal conditions, they generally don't live for more than four or five days once separated from sheep. Lice cannot fly or jump and they move very slowly over the animal's body surface. They live at the base of the fibre and need close contact such as yarding, trucking, camps, mother/daughter, sire/dam interaction to spread from sheep to sheep. Each female louse lays between 10 and 30 eggs over her lifetime of about 30 days. Populations usually build up over autumn, reach a peak in winter and then decline in spring and summer. 

With the life cycles and habits in mind, we can look at ways of inhibiting the development and reproduction of the parasites. As the two parasites are quite different in their cycles and habits, it is best to consider their control separately. Having said that, the fact that many of the chemicals available for treatment kill both, it is tempting to attempt control with a one hit wonder!

To reduce the need for chemicals, and preserve the effectiveness of the available chemical products, firstly consider management techniques. For flystrike, timing your shearing, crutching and/or dagging to coincide with the fly season is the most effective control as it reduces the factors that flies are attracted to. Good parasite control and appropriate tail docking length can also reduce dagginess, as can selection for dag free stock. Some areas of a farm may have increased fly challenge and, if known, these areas can be avoided during the high risk period.  For lice, it is important to avoid introducing lousy sheep by ensuring all bought in animals are lice free, ensuring boundary fences are secure, and if possible getting neighbours to dip their flocks at the same time.

In many cases a chemical treatment will be required alongside the management practices. There are a wide range of chemicals with different actions available and the choice of product will depend on a range of factors, including:

  • Species to be controlled (lice, fly or both)
  • Length of protection
  • Equipment available (jetting wand, spray race, shower or plunge dip, pour-on)
  • Cost
  • Wool length
  • Resistance status

Timing of chemical application is critical, with each product and method of application having an optimum wool length and period of effectiveness. Moral of the story is always read the label and use as directed! We read or hear this so often in our daily lives, but the application of insecticides to sheep is probably the most complex animal health procedure a farmer can undertake - not reading the label and using as directed is by far the main reason for disappointing results.

Treatment products need to be handled with respect and it is important to take responsibility to the animals, people and environment under our care seriously.