Pesticide Resistance for Sheep

Resistance to SPs was detected in the mid to late 1980s and is now widespread.

Resistance to the insect growth regulator (IGR) pesticides diflubenzuron and triflumuron has been confirmed and has been shown to be responsible for lice control breakdowns in at least some instances. As diflubenzuron and triflumuron have the same mode of action, if resistance is present to one, then it will also be present to the other. The current extent of this resistance is unknown.

With backliners or drenches, the wrong dose rate often results from underestimating the weight of the heaviest sheep in the mob or using an applicator gun that is delivering the wrong dose. It is important to weigh the heaviest sheep in the mob to determine the right dose rate, and to check that the applicator gun is delivering the correct dose.

With dips or showers, under-dosing can occur if the volume of the dip sump is underestimated or the mixing rate is wrong. With products that strip (see LiceBoss note: Plunge and cage dipping), if the dip is not reinforced and topped up according to the label instructions the concentration may fall below the concentration required to kill all lice. This is especially likely to occur near the end of the mob and instructions for dipping out should be followed carefully.

To eradicate lice using a topical treatment, it is critical that effective levels of chemical are applied all over the sheep’s body. Poor application technique can leave areas of sub-lethal chemical in the fleece where lice that are more resistant can breed. The dilution rate indicated on product labels must be used. Increasing the dip concentration cannot counteract substandard application and should not be considered. Similarly, nothing other than a bacteriostat (a product that is used in some circumstances to inhibit the breeding of bacteria in the dip) should be mixed with registered products in a dip. Combining two or more products in a dip in an attempt to control lice that are resistant to one of the products makes no sense, is unnecessarily expensive, is likely to leave excessive residues in wool and must not occur.

With backline products, the backline strip must be applied exactly as described on the product label. For most products, this is right down the middle of the back and all of the way from poll to rump. If application is uneven or only part of the way along the backline, this can leave areas of low pesticide concentration where the more resistant lice can survive.

If shower dips are not functioning properly or if the sheep are not left in the dip for long enough and are not properly wet to the skin, there can be areas with no chemical or with only low levels of chemical where more resistant lice can survive and breed. Similarly, if sheep are not wet properly in plunge dips, areas of low chemical concentration can remain where lice that are more resistant can survive.

If using an oral drench, you must ensure that the correct dose is given for the sheep’s bodyweight, and that the dose is swallowed. It is advised to put a coloured mark on each treated animal for easy checking. For fluralaner, the manufacturer’s advice is that it is preferable and safer to give a possible second dose than to risk leaving a sheep untreated.

Untreated sheep in a mob will provide a source of ongoing infestation. While there are still protective levels of chemical on the treated sheep this will not be important as any lice that transfer will be killed. However, as chemicals break down on the treated sheep, lice that are more resistant will be able to transfer and survive to breed a more resistant population. In addition, sub-lethal amounts of synthetic pyrethroids can rub onto untreated sheep that are running with treated sheep and may contribute to resistance development.

• Therefore, it is critical to eliminate sources of lice:
• Ensure clean musters and that all sheep on the property are treated.
• Keep strays out of the mob.
• Quarantine newly purchased sheep.
• Avoid split shearings or at least keep mobs shorn at different times separate from each other.
• Avoid treating ewes with lambs at foot (unless the lambs are also treated) or pregnant ewes within 6 weeks of lambing.

Check the LiceBoss article Sheep lice – biosecurity can prevent introduction for information to assist in developing a lice biosecurity program to prevent new infestations.

Use chemicals from different chemical groups (Table 2) for consecutive treatments. Treating repeatedly with chemical from the same chemical group (even if it is a different active ingredient) exerts heavy pressure for the selection of resistance. Remember that there may be a number of different chemicals within a group; changing to another one of these within a group will not help to avoid resistance.

Treatment for flystrike exposes any lice present to chemical (see Table 1). Likewise, lice treatments can also select for resistance in sheep blowflies. If possible, use chemicals from different chemical groups for controlling lice and flystrike in the same year. The names of flystrike control products containing cyromazine and dicyclanil, which don’t affect lice, are listed in the LiceBoss Products Tool.

Long wool treatments do not eradicate lice and may allow the more resistant ones to survive and breed. Consult the LiceBoss Long Wool Tool to help determine if a long wool treatment is likely to be economically worthwhile before using a long wool product.

The exception to this is fluralaner, which will kill lice in sheep with any length of wool.

As long wool treatments don’t eradicate lice, all sheep will need to be treated again, after their next shearing. Using a chemical from a different group maximises the chance of killing any resistant lice left alive by the long wool treatment.

The exception to this is fluralaner, which will kill lice in sheep with any length of wool.