New surveillance by the Campaign for Responsible Rodenticide Use (CRRU) UK has found genes for resistance to anticoagulant rodenticides in 78% of rats and 95% of house mice.
According to CRRU chairman Dr Alan Buckle, these include “small but troubling numbers” with two or more such genes, labelled ‘hybrid resistance’. In both rats and mice, the geographical distribution of both single-gene and hybrid-resistance continues to spread.
In UK house mice, for the first time, this latest annual study also identifies a new four-component ‘spretus’ resistance strain in Hertfordshire.
“This was acquired by house mice in Spain through interbreeding with the Iberian mouse species, Mus spretus,” said Dr Buckle. “The Hertfordshire spretus mice have almost certainly come in from the continent. More generally, London is now a clear hot-spot for mice with both single-gene and hybrid resistance.”
In rats during the two sampling years 2020 to 2022, the surveillance also found a proliferation of one resistance gene in particular, Y139C, to 30 widely spread new sites in England.
Dr Buckle said that this is one of the three most severe rat resistance genes, each of which is now being found in previously low incidence or resistance-free areas.
He warned that these latest findings suggest there are now few places in England with pest rodent populations wholly susceptible to anticoagulants.
Dr Buckle urged everyone involved to follow exactly a CRRU five-point action plan:
- Follow the CRRU Code of Best Practice guidelines;
- When using any rodenticide, apply the product’s label instructions in full;
- Monitor the effectiveness of control and always aim for 100% pest elimination;
- If resistance is suspected, look up the site’s likely status using Rodenticide Resistance Action Committee maps. If there is no data nearby the site in question, send tail samples to CRRU’s free service for resistance testing (bit.ly/3H4QCY3); and
- Where rodenticide resistance is found, follow in full the Rodenticide Resistance Action Group’s guidelines.
“It is expected soon that RRAG will publish updated advice for control of rodents with hybrid-resistance,” Dr Buckle added.