The repeated use of any one sheep louse treatment can lead to lice resistance to the active ingredient in that treatment. This results in some lice surviving a dose of the chemical.

The continued use of the same chemical allows resistant lice to breed until they make up the majority in the population, rendering that treatment ineffective. To minimise resistance, it is necessary to rotate lice treatments with different modes of action.

The products available to use in louse treatment rotations in New Zealand belong to these chemical classes:

It is essential that farmers know and understand the mode of action and resistance status of the products they are planning to use in a louse control programme. There is widespread resistance to synthetic pyrethroid compounds in New Zealand and Australia.¹,²,³ High level resistance to insect growth regulators such as diflubenzuron and triflumuron is widespread in Australia ⁵,⁶, while the louse resistance status of insect growth regulators in New Zealand is unknown. Previous product usage and performance must be considered before including these active ingredients in a louse control programme.

If consecutive use of the same chemical group is required, careful attention should be paid to achieving lice eradication post-treatment to help prevent the establishment of a resistant lice population.

If separate treatments for flystrike and lice are required in any one year, use products with different modes of action for each treatment. If a long wool lice treatment is applied, a product with a different mode of action should be used at the next shearing.

To formulate a louse control programme, contact your local PGG Wrightson Technical Field Representative. To purchase lice management products, view July's Stock n Save online at store.pggwrightson.co.nz.

SUPPLIED BY ELANCO

Registered pursuant to the ACVM Act 1997, Seraphos #A004265, Cypercare® #A005284, Wipeout® #A004558, Vanquish® #A005997, Zenith® #A006102, Magnum® #A007704, Zapp Encore® #A010400, Cyrazin® KO #A010635, Cyrex™ #A009917, Expo™ Extinosad Pour-On #A010205, and Extinosad™ #A008206. Always read the registered label before use.

References:

1. Levot, G. (2008). Speed of action and in vitro efficacy of spinosad against sheep body lice, Bovicola ovis (Schrank) (Phthiraptera: Trichodectidae), resistant to pyrethroid, organophosphate or insect growth regulator insecticides. Australian Journal of Entomology 47, 251–255.
2. Wilson, J.A. et al. (1997). A preliminary investigation into resistance to synthetic pyrethroids by the sheep biting louse Bovicola ovis in New Zealand. New Zealand Veterinary Journal 45(1):8–10.
3. Johnson et al. (1992). Resistance to synthetic pyrethroid pour-on insecticides in strains of the sheep biting louse Bovicola ovis. Australian Veterinary Journal 69(9):213–7.
4. Heath, A., & Levot, G. (2015). Parasiticide resistance in flies, lice and ticks in New Zealand and Australia: mechanisms, prevalence and prevention. New Zealand Veterinary Journal 63, 199–210.
5. James et al. (2008). Resistance to insect growth regulator insecticides in populations of sheep lice as assessed by a moulting disruption assay. Medical and Veterinary Entomology 22, 326–330.
6. Levot, G.W., & Sales, N. (2008). Resistance to benzoyl urea insecticides in Australian populations of the sheep body louse Bovicola (Damalinia) ovis. Medical and Veterinary Entomology 22, 331–334.