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Online learning: Treatment—Pesticide resistance

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.

Structured reading

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Resistance, residues and safety—Pesticide resistance
An overview of resistance to louse control chemicals.

Pesticide resistance
About resistance, why it is important, and how it can be slowed.

Question and answer

For those who prefer a problem based approach to learning, answer the following questions.
Each of the questions below links further down the page to the answers.

Questions:

  1. What is resistance?
  2. In which chemical groups has resistance been found, in Australia?
  3. Why is it important to know about and slow the development of resistance?
  4. Why is it important to know the group of a lice pesticide?
  5. How can the development of resistance be slowed?
  6. What should you do if resistance is suspected?

Answers:

You can also click on each question below to go to WormBoss pages with related information.
 

1. What is resistance?

Resistance can be defined as a genetic change in response to exposure to a pesticide that enables lice to survive 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.
 

2. In which chemical groups has resistance been found, in Australia?

  • Synthetic pyrethroids (SPs)
  • Insect growth regulators (IGRs)
  • Organophosphates (OPs)  — but very rare; only one strain of lice with reduced susceptibility to OPs has been reported.

3. Why is it important to know about and slow the development of resistance?

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.
 

4. Why is it important to know the group of a lice pesticide?

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 chemicals. 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.
 

5. How can the development of resistance be slowed?

  • Make sure the dose rate is correct
  • Apply chemicals strictly according to label directions
  • Don’t expose treated sheep to untreated lousy sheep
  • Rotate products from different chemical groups
  • Consider flystrike chemicals
  • Avoid using long wool treatments where possible
  • Where a long wool treatment has been used, ensure that a chemical from a different group is used after the next shearing
     

6. What should you do if resistance is suspected?

  • Carry out a complete review of your lice control program, including the treatment method used and the biosecurity program. The Treatments Tool and Short Wool Tool in LiceBoss Tools can assist with this.
  • If a long wool treatment is being contemplated, consult the LiceBoss Long Wool Tool to determine whether it is economically justified and which chemicals can be used.
  • If applying a long wool treatment, use a product from a different chemical group to the last treatment used on this mob.
  • If a long wool treatment is used, it will not eradicate lice; all sheep will need to be treated after their next shearing. Use a chemical from a different group for the post-shearing treatment.
  • If problems continue to arise and no reason can be identified, seek professional advice.

 


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