Choose the right chemical to eradicate lice, but also consider the various withholding periods, which could restrict your choice.
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An introduction to chemical groups.
Sheep lice treatments—chemical group characteristics
Details on chemical groups.
Sheep lice control for ewes and lambs
Factors to consider when choosing a treatment
Ewe-lamb Treatments Tool
An interactive tool to assist producers determine the best course of action including choosing the right chemical to use when treating pregnant or lactating ewes.
Question and answer
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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. These pesticides will not adequately control populations containing pyrethroid- or IGR-resistant lice. Resistance to either group is not restricted to any particular geographic region. Resistance can be suspected in pyrethroid treated sheep if live lice are present later than 6 weeks post-treatment. Resistance to IGRs should be suspected if live immature lice are seen on sheep later than about 10 weeks after treatment.
Under no circumstance should products be mixed or label rates altered in an attempt to improve the effectiveness of lice control. There are many different effective products from a number of different chemical groups currently available on the market. Mixing chemicals or increasing rates will not make up for a failure in lice biosecurity or inadequacies in application technique.
When applied as a backline lice treatment, abamectin will move into the bloodstream and affect sheep worms as well. Abamectin remains an important drench chemical, although resistance in worms has been reported in all Australian States. If considering using abamectin for lice control, producers should consider the possible impact of this treatment on their drench resistance management strategy; the abamectin drench component can be at risk of faster resistance development if its use is not really warranted at the time of the lice treatment.
It is easier to eradicate sheep lice on farms with one main shearing than it is on properties where mobs are shorn at different times (split shearings).
Similarly, flock management becomes more complicated where pregnant ewes or ewes with lambs at foot need to be treated for lice. This is because backline treatments and insect growth regulator products take some time to bring about the death of all lice. During this time, if there is contact with other untreated sheep, there is the potential for lice to spread.
Where split shearings occur, it is critical that treated mobs are kept separate from untreated sheep until the untreated sheep are treated. Managing ewes with lambs at foot or ewes due to lamb soon after treatment is more complex.
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