Resistance of sheep lice to pyrethroid insecticides is widespread in NSW. Organophosphate resistance remains rare. Resistance to insect growth regulator lousicides is unconfirmed.

Resistance may be suspected on a history of treatment failure after correct application of the correct amount of lousicide at the correct time, either:

• as breakdowns within 3-9 months of application by ineffective wet dipping of short wool sheep, or
• as breakdowns within 3-4 months of application of backline treatment in sheep treated off-shears or in short wool, or
• where sheep in long wool have been thoroughly treated with a pyrethroid and failure to control infestation is obvious within 6 weeks.

NB. Long wool application of pyrethroids may leave undesirable residues at shearing.

Faulty application of treatment should be considered where infestations are localised to one side or part of the sheep, or where one mob or line is infested and other mobs are free of lice.

Diagnosis

Examination of sites of, and degree of, infestation in at least 4-5 affected sheep by counting lice in 10 partings on each side of the infested sheep (each side examined in 2 lines of 5 partings); demonstration of lice resistance using an in-vitro laboratory test.

NB. It is more productive to inspect a larger number of sheep rather than search a smaller number of sheep more thoroughly.

A thorough examination of the dip operation and thoroughness of wetting is recommended, particularly in the case of shower dipping failures with insecticides.

Specimens required

Testing for resistance is not routinely available; it requires controlled temperature and humidity and is a laboratory based test available at the Elizabeth Macarthur Agricultural Institute (contact Dr Garry Levot). Submission of live affected sheep would be required.