Resistance is known to be widespread to synthetic pyrethroid (SP) compounds. Resistance to insect growth regulators (IGRs) has also been identified, although the prevalence is not known. However, when most breakdowns are closely investigated they are found to be due to a mistake in application, some sheep missing treatment, or a new infestation.

Registration of new products is very expensive and on the world scale the sheep industry is not a big market for pesticides. It is therefore important to adopt strategies to minimise the development and spread of resistance and maintain effectiveness of currently available chemicals.

Most resistance has resulted from the misuse of backline products. However, backline treatments have considerable management advantages. More detailed information about development of a resistance management program is found in Preventing resistance in sheep lice and Resistance to insect growth regulators but some general principals 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.
• As a general principal 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 treatments module in LiceBoss to see if a management reason (eg stray sheep, incomplete muster, wrong dose rate, incorrect application) could be the cause. If no reason can be found, discuss 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.