Control in Resistance Foci

When working on sites where there is resistance, analysis of the specific resistance mutation present will permit the most effective rodent management strategy to be implemented. This strategy will include a requirement diligently to follow best practice guidelines and rodenticide label use recommendations. If in doubt, seek expert advice on the local circumstances either from manufacturers, distributors or from relevant government organisations.

If all other causes of treatment failure are ruled out, but confirmation of resistance by DNA-sequencing is not possible, or is not possible in a reasonable period of time, a sensible strategy is to apply an integrated approach to rodent pest management at the site (see also Rodent control), including if necessary the application of a product containing one of the most potent anticoagulants, brodifacoum, difethialone and flocoumafen or a non-anticoagulant.

When dealing with resistant Norway rats, and the nature of the resistance mutation present is known, recommendations for which anticoagulants rodenticides are effective against specific mutations are shown in Table 4 and Table 5. It is recommended to use products containing the respective compounds at the highest active substance concentration authorised for use against these rodent species.

It is important to understand that all known resistance mutations, in both rats and mice, are capable of effective control with applications of the most potent second-generation anticoagulants, brodifacoum, difethialone and flocoumafen, and that no practical resistance to any of these active substance is presently known.

It is more important than ever that complete eradication of rodents is achieved at resistance sites. This is best done using an integrated strategy which employs a combination of effective interventions, including the use of non-anticoagulant rodenticides, where these are available, as well as effective anticoagulants. Where residual rodent activity is identified after anticoagulant use, and this cannot be eradicated using other chemical interventions, apply intensive trapping to eliminate remaining rodents. Gassing or fumigation should be considered, provided it is carried out by personnel with appropriate expertise. New techniques of trapping and remote detection and monitoring should be considered to eradicate remaining infestations, and to prove the success of the control measure.

Where individual infestations are found to be resistant, or to contain resistant individuals, it is possible that the resistance extends beyond the treated site and onto neighbouring properties. Where there are indications that resistance may be more extensive than a single infestation, apply area or block control rodent programmes if possible. The area under such management should extend at least to the boundaries of the area of known resistance and ideally beyond. These programmes must be effectively coordinated and should encompass the procedures identified above. Carefully record the details of all measures employed so that useful local knowledge is accumulated about effective and ineffective resistance management strategies.

Do not use anticoagulant rodenticides as permanent baits as routine. Use permanent baits only where there is a clear and identified risk of immigration or introduction or where protection is afforded to high.

Table 4: Polymorphisms of the VKOR, and compounds recommended (+) to control these strains of the Norway rat (Rattus norvegicus), based on BCR data and field trials. Products containing rodenticides marked with (-) shall not be used to control respective strains.

VKOR Compounds recommended (+) and not recommended (-) for control
Strain First-generation anticoagulants Bromadiolone Difenacoum Brodifacoum Flocoumafen Difethialone
L120Q - - - + + +
L128Q - - + + + +
Y139C - - - + + +
Y139F - - + + + +
Y139S - + + + + +

Table 5: Polymorphisms of the VKOR proven to induce resistance to warfarin in the house mouse (Mus musculus), and compounds recommended to control them (+). Mouse strains resist those compounds are marked with (-).

VKOR Polymorphism Compounds recommended (+) and not recommended (-) for control
Strain First-generation anticoagulants Bromadiolone Difenacoum Brodifacoum Flocoumafen Difethialone
L120Q no data available + + +
L128S - - + + + +
Y139C - - + + + +
Y139S no data available + + +
R12W/A26S/ A48T/R61L (spretus-Introgression strain) - - - + + +