Back to Other Articles In This Category

Which lice treatment will you use next time?

By Deb Maxwell1 and Peter James2

1ParaBoss Operations Manager
2Queensland Alliance for Agriculture and Food Innovation (QAAFI)

July 2015

 Link to the Lice and Flystrike Products Tool 

What governs your choice of chemical group for lice treatments? Cost is clearly one factor, but what about resistance?

Even completely effective chemicals with no history of lice being resistant can fail to eradicate lice if not properly applied to every sheep. Make sure you first choose an application method that can be used effectively on your property (strictly according to label instructions) and ensure that every sheep is treated.

But many lice treatments are unnecessary. If no lice are present at shearing and you are reasonably confident that lice have not been introduced in recent months, then treatment is not required.

There are seven groups of chemicals registered for treating lice as listed below. The different actives in each group are shown in brackets:

  • Neonicotinoid (imidacloprid, thiacloprid)
  • Spinosyn (spinosad)
  • Organophosphate (OP: temephos, diazinon)
  • Macrocyclic lactone (ML: abamectin, ivermectin)
  • Magnesium fluorosilicate/sulphur (MgFSi plus sulphur plus rotenone)
  • Insect growth regulator (IGR: diflubenzuron, triflumuron)
  • Synthetic pyrethroid (SP: cypermethin, alphacypermethrin, deltamethrin)

Unlike worms, where you can carry out a drench resistance test to see the extent to which drenches work on your property, this is usually not practical with lice.

However, resistance is only known to have developed to synthetic pyrethroid and insect growth regulator pesticides and only in some sheep lice populations.

The bottom line is that choosing a product from one of the first five groups on the list should always result in an effective treatment—if all sheep are treated correctly.

However, the IGRs and SPs are generally the cheapest option, especially for backline applications. If you have used them successfully in the past they may continue to work for you.  However, continued use of these groups year after year will almost inevitably lead to development of resistance and consideration should be given to rotating with another chemical group.

Regardless of your chemical group choice, the development of resistance to the chemicals by the lice is a continuing risk. In any population, whether it is lice, worms or flies, there may be one or a few resistant pests. Each treatment application is an opportunity for those resistant pests to survive, reproduce and increase the proportion of chemical resistance within the population. 

In the past there were a limited number of chemical groups available for lice treatment, particularly in backline products (first SPs, then IGRs). The sheep industry is now in a good position to have seven different chemical groups available and six in backline products. This provides the opportunity to rotate between chemical groups to prevent, or at least delay, the development of resistance to more recently introduced products.

While you may get away with continued use of one group for a number of years, LiceBoss recommends rotating the chemical group used each one or two years. In this way, any resistant lice that have survived a previous treatment will be killed by a new compound from a different chemical group.

Remember, changing a product may not mean you’ve changed the chemical group. Check the label to see which active/group it contains. The Lice and Flystrike Products Tool also allows you to find a commercial product based on its chemical group, as well as its application method and the pest targeted: lice and/or flies. The Lice and Flystrike Products Tool also gives a typical price per sheep for each product (price currently unavailable, being updated) as well as wool harvest interval, sheep rehandling interval, withholding period and export slaughter interval.