Rodenticide resistance

A new study by Reading University has identified ‘the massive extent of L120Q resistance across the whole of central southern England. Co-author, Dr Colin Prescott, explains that L120Q is the most severe form of rodenticide resistance identified to date, effective against first generation anticoagulants and one or more second generation.

‘Moreover, this doesn’t mean the rest of the UK can relax, because lack of sample availability means we just don’t have the data’, he says. ‘Another concern is that most rats with L120Q resistance carry the gene from both parents. Where this occurs, it suggests most or even all rats with some susceptibility have been eradicated by widespread use of resisted rodenticides, leaving a population of resistant pure-breds’.

The report was commissioned by the Campaign for Responsible Rodenticide Use (CRRU) under its stewardship regime remit to an HSE-led Government Oversight Group (GOG). CRRU chairman, Dr Alan Buckle, says the difficulty for pest controllers, of course, is knowing the resistance status of rats on their clients’ premises. ‘One effective course of action, but also involving extra work and cost, is to employ diagnostics’, he suggests. ‘Reading University, for example, offers paid-for resistance analysis of rat tissue samples, from which resistance status can be identified and control plans developed’. ‘This exemplifies how there is more to rodenticide stewardship than holding a certificate of competence. For stewardship to be judged a success by GOG, meaningful and lasting reductions in rodenticide residues carried by non-target wildlife are expected’.

Specifically and immediately, quoting advice from the UK Rodenticide Resistance Action Group, the report advises against using bromadiolone or difenacoum baits against rat populations known to carry the most severe forms of anticoagulant resistance. It also finds UK leading the world with the highest number of different genes for resistance in rats. Of nine identified, two more in addition to L120Q confer resistance to first generation anticoagulant rodenticides and one or more of the second generation group.

The report was requested by GOG because it is thought that new stewardship authorisations under the Biocidal Products Regulations, the promotion of best practice through the stewardship regime, and the emphasis in stewardship on the use of alternatives to anticoagulants, will all play a part in managing resistant infestations of rats and preventing their spread. It also shows for the first time the current spread of resistance mutations in Rattus norvegicus, the Brown or Norway Rat The report will be repeated annually as part of the UK Rodenticide Stewardship Regime’s monitoring procedures.