Sheep Lice

Sheep lice are a significant economic problem to wool producers. Severe infestations can result in considerable fleece damage. Lice reduce the fleece weight and wool quality but not fibre diameter.

Several studies have shown that over a 12 month period the clean fleece weight may be reduced by 0.2 to 1 kg, depending on the level of infestation. The cost of treatment is  considerable, so a grower managing a 2000-head flock that is lice-free can avoid chemical costs of at least $1000 a year.

In the future, pesticide residues from lice treatments may result in a further cost penalty due to buyer discrimination.

Integrated Pest Management (IPM):

Integrated Pest Management has been promoted as a means to combat many agricultural problems, particularly those that rely heavily on the use of chemicals. Control programs that rely on the continued use of chemicals are not sustainable because of the inevitable development of resistance to these chemicals, consequences of residues, potential health risks and environmental impact.

The two main IPM elements used to deal with lice are management options and selective use of chemicals. Before implementing an IPM approach, it is vital to understand the biology of lice, including spread and detection (see DAFWA Note: 273 Sheep lice – spread and detection).

Selective chemical use:

Chemical groups:

Most registered products used to treat lice belong to the following chemical groups:

  • Insect growth regulator (IGR) (Zapp, Magnum etc.) affects the developmental stages of insects by preventing formation of the new external skeleton. This group does not kill adults, so allow six weeks to kill developing stages and up to 14 weeks for adults to die or for their eggs to become non-viable.
  • Spinosyn (Extinosad) causes nerve dysfunction in insects, and negligible human health risks and environmental toxicity. Rapid knockdown.
  •  Macrocyclic lactone (ML) (Coopers Blowfly and Lice  Jetting Fluid, Paramax) affects the nervous system. Rapid knockdown.
  • Magnesium fluorosilicate (MgFl) (Flockmaster II etc.) causes rapid and severe dehydration of lice. Moderate rate of knockdown.
  • Organophosphate (OP) (WSD Diazinon etc.)* affects the nervous system of both insects and mammals. Rapid knockdown.
  • Synthetic pyrethroid (SP) (Clout S, Vanquish etc.) affects the nervous system of insects only. Achieves fairly rapid kill of all immature and adult lice, that is, within a day of full exposure to susceptible populations. Resistance reported 16 years ago.

Fleece derangement caused by lice

* In May 2007, the Australian Pesticides and Veterinary Medicines Authority (APVMA) announced a suspension of the registered claim for diazinon to be used as a short wool dip for sheep lice and long wool jetting for blowflies.

Products manufactured before May 2007, which display the registered label claim, can still be used according to label directions but new product will no longer include this claim and must not be used for dipping or jetting.

Most of these products will expire by mid-2009. Short wool chemical treatments for lice can be applied using an off-shears backliner, or shower or plunge dip within six weeks after shearing. Applying chemicals in long wool is unlikely to eradicate lice but may aid their control.

Louse resistance:

During the late 1980s, resistance to synthetic pyrethroids (Clout S, Vanquish etc.) was discovered in several states including Western Australia. The incidence of resistance is widespread. On some properties the level of resistance is so severe that this chemical group, whether applied as an off-shears backline, dip or long wool treatment, does not provide effective control.

Results from the National Wool Residue Monitoring Survey show that Western Australia has one of the highest SP levels on greasy wool, presumably due to long wool SP use, that is, Vanquish. The use of SPs in long wool is not recommended due to the very high risk of residues and the unpredictability of its effectiveness, which may require additional long wool treatment.

 Over the past few years, more farmers have reported treatment failure using IGR off-shears backliners. In most cases, when investigated, other possible causes of a continued louse infestation were identified including:

  • introducing sheep of unknown lice status
  • split treatment of ewes and lambs
  • possible chemical misapplication to some sheep.

However, IGR resistance is suspected on some properties where lice have been detected within six months of treatment. Currently, there is no convenient laboratory test available to detect IGR resistance, so all other possible causes of treatment failure must be discounted before an investigation is undertaken to indicate the possible presence of resistance (see DAFWA Note: 190 Sheep lice – resistance to insect growth regulators).


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