Rats, Rodenticides and Resistance, where now?

By Mike Swan, GWCT Head of Education

6 minute read

Young -rat

I have suspected it for a while, but confirmation popped into the inbox the other day. Four rat tail tips that I had sent to the Animal and Plant Health Agency (APHA) for genetic screening came back positive for second generation anticoagulant resistance.

From here on in, the two most used rat poisons, bromadiolone and difenacoum will be as good as useless on my shoot, and if we were daft enough to use them, we would simply be risking sending the chemicals up the food chain to the likes of our barn owls, kestrels and red kites, while having no significant impact on the rats.

So, where do we go from here? Well clearly rat control is essential on any shoot, so giving up is not an option, and this applies especially on a small wild bird shoot like mine, where I have the added risk of rats killing nesting hens and taking eggs. With all this in mind, I have always striven to minimise anticoagulant use, and keep rat numbers to a minimum, using all the other means at my disposal, but more of that later.

As far as poison baits are concerned, there is still an anti-coagulant option, if with a rather restricted application, and there is a new active substance on the horizon too. Until comparatively recently the anticoagulants brodifacoum, flocoumafen and difethialone have been available for indoor use only.

While they are not allowed for the sort of “open area” application around gamebird feeding stations that is the main “gamekeeping” use, the rules were changed a few years ago, so that, depending on the product label, they can be used “outdoors around buildings” to help control infestations in the buildings themselves. Since there is no resistance to these products, I should now find it easier to keep the rats down around the main farm and its houses, helping to prevent them from moving out onto the shoot.

Meanwhile, there is also a new poison option on its way called cholecalciferol. This is vitamin D3, and like many vitamins, a serious overdose can be highly toxic, acting by badly upsetting the calcium balance of the body, and causing all sorts of problems including kidney failure. At least one manufacturer promises a formulation for open area use “coming soon”, and there is the prospect of less non-target risk in terms of secondary poisoning, because of quick breakdown. Also, for larger animals, the amounts taken in when scavenging a cholecalciferol poisoned rat are likely to be small.

Those with long memories will remember that calciferol was used as a rat poison some decades ago, and that there were problems with bait shyness from animals that had taken a sub lethal dose, recovered and then refused to take more. This was with a different form of calciferol, and we are assured that there is much less risk with the new version. That said, I think that we will need to be scrupulous in following the label direction to remove all other food sources before use, so as to ensure that rats do take a lethal dose at the first feed.

But, all this talk of poisons is putting the cart before the horse. The main key is to prevent the problem in the first place, rather than be forced to deal with the rats when they get into the ascendancy. If, like me, you use feed hoppers, try to use a design that makes it hard for rats to scavenge the grub. My own tin drums, with slots in the base, hung up where rats cannot reach the feeding slots, mean that a rat hole near a feeder is now a rarity. However, I accept that these only really work for relatively low densities of pheasants and partridges, and may not make sense on bigger shoots. Even so, swapping to this approach through the hungry gap after the shooting season could well help suppress the rats come the breeding season.

If you scatter feed, try to do so only in daylight, so that there is no grub lying around at night when the rats come out from their siesta. Also, whatever the method, try always to feed away from likely rat harbourage. If you have a cover crop alongside a hedge or ditch, feed the outside edge, where the rats need to go further. Also, take the trouble to move your feeders a few yards each time you fill them. Rats, being creatures of habit, will take quite a while to relocate them, but your birds will do so straight away.

Providing our birds with food from cover crops is a great idea in theory, but its hard to legislate for who will eat it. Maize is especially at issue here, and its my opinion that the amount used by gamebirds is often remarkably small, not least because the cobs are up out of their reach, but where the rats can easily climb!

These days I almost always recommend using a late cobbing variety. These are often described as cobless in the catalogues, because they rarely set viable cobs in our climate. Being varieties bred for grain production, rather than cattle feed, they have the added advantage of stronger stems to prevent them falling over before harvest. They therefore stand well into the winter, offering better late winter cover for pheasants and redlegs.

Another part of my rat strategy going forward will be more trapping. With a new generation of highly humane traps in the form of the Perdix, DOC and Tulley, I am much happier that there will be virtually no miscaught animals that might be seen by passers-by; indeed, the ones who’s tails started all this were all very dead in the trap. If we thought about it most of us could think of plenty of places where we could trap quite unobtrusively, but do not at present.

Rat In A Trap (1 Of 1) (1)

There have been huge improvements in night vision and thermal imaging over the last decade, so with no need for lights, shooting rats at night with an air rifle has huge potential to help. And lastly, we should not forget gassing. Use of aluminium phosphide tablets in rat holes is specialist stuff, and you must have proper training, but it can be very helpful, especially where there are late winter build ups at feeding sites.  

For me this news of resistance in my own parish has brought dealing with rats on the shoot into sharp focus, but the truth is, too many of us have been a bit too complacent for too long. Gamekeepers have shown their mettle by embracing the need for training, and rodenticide stewardship in a very big way, but we still need to go further, and put what we know into better practice.

Have I got resistance?

If you are unsure about rodenticide resistance in your area, the Rodenticide Resistance Action Committee has an interactive map on its website that is well worth a look. You can zoom in on your location and find out what is known nearby. If you are far enough from a known site, The Campaign for Responsible Rodenticide Use (CRRU) is also still offering free tests by APHA, which is how I found out about my situation.

GWCT Rodenticide Courses

We run regular Rodenticide courses – if you would like more information please email advisory@gwct.org.uk.

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After Rodenticides

at 9:03 on 06/09/2021 by Nick vZ

I've long believed that poisons are not the right answer to any problem. Resistance and by-catch have always been problems. What's actually needed is a multi catch machine such as a gas / electric powered captive bolt gun which can recognise individual species. So it can recognise only the target species and deny entry to others. So it needs to recognise the difference not only between mice and rats but also a mouse and a shrew or a harvest mouse. A red from a grey squirrel. So it needs a camera and facial recognition software to arm it..

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