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Resistance, Residues & Safety

Resistance, residues and safety - an introduction

New lice infestations can be prevented by careful attention to biosecurity (see Preventing lice), but once an infestation has commenced, control of lice relies almost totally on the application of a chemical louse control product.

If applied carefully after shearing to all sheep on the property and strictly according to label instructions, registered louse control products should eradicate lice. However, poorly used, chemical treatments can have a number of unintended consequences.

These include (click on the links below for more information):

Pesticide resistance

Resistance to louse control chemicals is a growing problem in the sheep industry. However, when most field breakdowns of lice treatments are closely investigated, they are found to be due to poor product application, some sheep missing treatment or a new infestation.

Resistance can be selected when failure to eliminate an infestation with a chemical treatment exposes a proportion of the population to sub-lethal concentrations of pesticide. Although inadequate application by any means can be implicated, backline treatments are generally the most vulnerable to resistance development.

Resistance in lice is known to be widespread to synthetic pyrethroid (SP) compounds and resistance to insect growth regulators (IGRs) has been identified in most Australia states.

A common element in the development of both SP and IGR resistance has been repeated use of products from the same chemical group.

To preserve the effectiveness of currently available chemicals and to avoid the costs and management difficulties that result from a control failure, it is important to adopt strategies that minimise selection for resistance.

Some general principles to prevent or delay the development of resistance include:

  • Make certain that backline products are applied strictly according to label directions and that sheep are treated at dose rates calculated for the heaviest sheep in the mob.
  • Ensure that every sheep is treated evenly along the middle of the backline from head to rump.
  • If a long wool treatment is used, ensure that a chemical from a different chemical group is used at next shearing.
  • Generally, it is a good idea to rotate products from different chemical groups at consecutive treatments.
  • Don’t mix treated and untreated sheep, particularly if there is a chance that the untreated sheep may be carrying lice. Don’t forget lambs.
  • If a chemical breakdown is suspected, use the Treatment Factors Tool in LiceBoss to see if a management reason (e.g. stray sheep, incomplete muster, wrong dose rate, incorrect application) could be the cause. If no reason can be found, discuss this with an adviser or chemical company representative and be sure to treat all sheep with a product from a different chemical group at the next shearing.
  • If treatment for flystrike and lice is required in the same year, use chemicals from different chemical groups.

More detailed information about development of a resistance management program is found in the LiceBoss Note: Preventing resistance in sheep lice.

Pesticide residues

Keeping residues low enhances wool’s reputation as a pure and natural fibre and allows access to residue-sensitive markets. Most chemical residues are found in the wool grease and not on the wool fibre, but there is a growing demand for low-residue wool because of concerns over the effluent generated during wool scouring.

European Union (EU) pollution control legislation places strict limits on emissions from wool scours and it is likely that European wool processors will preferentially source low-residue wools. Some processors in China are now also measuring residue levels.

Pesticide residues in wool come mainly form lice and fly treatments. The highest residue levels generally result from long wool treatments or incorect chemical use, but some post-shearing treatments can also leave high residues.

Strategies such as treating only when lice are detected, avoiding long wool treatments where possible, and choosing chemicals that break down rapidly, will help keep residue levels low. The Wool Residue Tool in LiceBoss allows you to easily calculate the residues expected from use of any registered treatment.

For more information see LiceBoss Note: Wool and meat residues.


Although louse control chemicals are generally designed to be selectively toxic to insects, there are risks associated with their use. Understanding the health risks and legal requirements associated with louse control pesticides is a critical part of safe and effective lice control. All managers and workers who will be responsible for applying chemicals should be aware of the risks and attend an AgSafe or Farm Chemical Users course.

Chemicals can be absorbed through the skin, by inhalation of vapour or aerosol droplets and orally when handling food with unwashed hands or drinking from contaminated water bottles. The Material Safety Data Sheet (MSDS) available for each product contains specific information on these risks and general advice for safe use.

Consult the product label for the wool re-handling period before handling recently treated stock to avoid chemical exposure. Appropriate precautions, including the use of personal protective equipment (PPE) as outlined on product labels, should be taken to minimise the risk of exposure to chemicals by these routes.

All agricultural and veterinary chemicals have detailed instructions for the proper use and safety precautions on the label. Be sure to read the safety directions before opening the container. Take care to avoid splashes or oversprays of chemical or inhaling vapours and aerosol droplets. Handling concentrates during mixing poses a particular safety risk and all precautions recommended on the label should be closely followed.

Each state has legislation that directly regulates the development, sale, storage and use of agricultural and veterinary chemicals; people working with chemicals should be aware of their legal responsibilities. Everyone working in the rural industry has a duty of care and managers have a legal obligation to provide a safe workplace for employees using pesticides.

For more information about using chemicals safely see the LiceBoss Note: Use of pesticides for controlling lice—occupational health and safety.