The cost and labour involved in treating for lice is substantial, so it pays to be aware of resistance and to use practices likely to slow its rate of development.
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Resistance, residues and safety—Pesticide resistance
An overview of resistance to louse control chemicals.
About resistance, why it is important, and how it can be slowed.
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Resistance can be defined as exposure to a low dose of pesticide that enables some lice to survive, and these have genes that may allow them to survive higher doses that would normally kill all lice. Continued use of the same chemical or chemical group allows the resistant lice to survive, breed and increase in numbers until they make up the majority of the population.
Slowing the spread of resistance helps to control costs by avoiding control breakdowns and the need for extra treatments (extra treatments also increase the level of residues in the wool).
Sometimes, when resistance is present, treatment suppresses lice, but does not completely eradicate them. These suppressed infestations are difficult to detect and increase the chance of lice spreading between flocks, particularly on purchased or agisted sheep.
Preserving the efficacy of currently available compounds is also important as the costs of developing and registering new products continues to increase. New groups of lousicides are almost always more expensive than their predecessors, in turn increasing production costs for woolgrowers.
Knowing which chemical group your lice control products belong to is critical to resistance management—the group is the class of chemical to which the particular active ingredient belongs.
Remember that treatments to prevent flystrike also expose any lice present to pesticides. It is important to use products from different chemical groups when treating for flystrike and lice in the same year and to consider flystrike chemicals when determining a resistance management plan. Fortunately, the two main chemicals used for flystrike control, cyromazine and dicyclanil, do not have any effect against lice and will not contribute to selection for resistance in lice.
It is also important to rotate between chemical groups when sheep are treated more than once in a year (long wool treatment followed by off-shears treatment), and to rotate between chemical groups in consecutive years.
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