It does seem that rats are getting increasingly resistant to poisons. If you have or know someone with an air rifle, go up there at night with a torch, I find shooting rats by far the simplest and quickest solution to the problem.
Try changing the poison you use. I started off using the Rentokil one and the rats absolutely loved it - so much so that they ate their way into the container and wolfed the lot while I was on holiday because I'd stopped topping up the bait station. I switched to using Roban and the rats were gone within a week.
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It does seem that rats are getting increasingly resistant to poisons. If you have or know someone with an air rifle, go up there at night with a torch, I find shooting rats by far the simplest and quickest solution to the problem.
Try changing the poison you use. I started off using the Rentokil one and the rats absolutely loved it - so much so that they ate their way into the container and wolfed the lot while I was on holiday because I'd stopped topping up the bait station. I switched to using Roban and the rats were gone within a week.