Lice infestation is of major economic importance – costing tens of millions of dollars annually. Infested sheep become restless, with scratching and rubbing, resulting in reduced weight gain, damage to the fleece (up to 24 per cent reduction in value) and an increased likelihood of flystrike.

Adult lice (Bovicola ovis) are very small (1 - 1.5mm long) and move along wool fibres feeding on wool grease and skin debris – in bright light they remain very close to the skin. Eggs are laid on wool fibres, then hatch and complete three nymph stages within two to five weeks, before maturing into adult lice.

The entire life cycle occurs on the sheep and takes about five weeks. Lice numbers start to build up in autumn, peaking in winter/spring, and drop away toward summer. Lice only leave the sheep when they come into close contact with other sheep, so normally spread slowly within a flock, except when sheep are in poor condition.
Total lice eradication is possible – but it is more feasible to aim to prevent new infestations and minimise the impact of any existing lice. Shearing can remove up to 80 per cent of lice, after which timing of chemical treatment ‘dipping' is crucial.
There are four main groups of ‘dips': Insect Growth Regulators (IGR), Synthetic Pyrethroids (SP), Organophosphates (OP) and Spinosad. It is critical to choose the most appropriate product and method for your property and situation, and apply it according to the label.
• Full Saturation: concentrated products are diluted with water then applied either by plunge (full submersion) or high-volume/high-pressure shower dipping. The optimum time is two to six weeks off-shears, when shearing cuts have healed and there is sufficient wool and grease to ‘hold' the chemical. Any later fleeces will be very difficult to soak to skin level.
• Pour On: ready-to-use (un-diluted) products are applied through a pour-on gun, usually with a ‘T Bar' nozzle. Most effective when applied immediately off-shears.
• Jetting: Using concentrates diluted with water is only effective if it achieves saturation, so is most applicable to hand jetting.

The critical aspect is getting the correct amount, of the right chemical, applied at the correct concentration onto the sheep's skin and wool to ensure the lice are in contact with a lethal dose. Wool length is a major barrier to lice treatments. If it is too long there will be under-dosing. Treat all sheep on the property to prevent cross contamination. Contact with the neighbours' lousy sheep will undermine your control programme. Ideally, ‘quarantine dip' all sheep brought on to the property and routinely check sheep for lice every summer and/or autumn, just before shearing.